Ultrafast Tilting of the Dispersion of a Photonic Crystal and Adiabatic Spectral Compression of Light Pulses

D. M. Beggs, T. F. Krauss, L. Kuipers, T. Kampfrath

Physical Review Letters 108 033902

We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photoniccrystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11%.We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide.

Ultracompact all-optical XOR logic gate in a slow-light silicon photonic crystal waveguide

C. Husko, T. D. Vo, B. Corcoran, J. Li, T. F. Krauss, B. J. Eggleton

Optics Express 19 (21) 20681-20690 (2011).

We demonstrate an ultracompact, chip-based, all-optical exclusive-OR (XOR) logic gate via slow-light enhanced four-wave mixing (FWM) in a silicon photonic crystal waveguide (PhCWG). We achieve error-free operation (<10−9) for 40 Gbit/s differential phase-shift keying (DPSK) signals with a 2.8 dB power penalty. Slowing the light to vg = c/32 enables a FWM conversion efficiency, η, of −30 dB for a 396 μm device. The nonlinear FWM process is enhanced by 20 dB compared to a relatively fast mode of vg = c/5. The XOR operation requires ≈ 41 mW, corresponding to a switching energy of 1 pJ/bit. We compare the slow-light PhCWG device performance with experimentally demonstrated XOR DPSK logic gates in other platforms and discuss scaling the device operation to higher bit-rates. The ultracompact structure suggests the potential for device integration.

Slow-light enhanced correlated photon pair generation in a silicon photonic crystal waveguide

C. Xiong, C. Monat, A. S. Clark, C. Grillet, G. D. Marshall, M. J. Steel, J. Li, L. Ofaolain, T. F. Krauss, J. G. Rarity, B. J. Eggleton

Optics Letters 36 (17) 3413-3415 (2011).

We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 μm. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip.

Slotted photonic crystal cavities with integrated microfluidics for biosensing applications

M. G. Scullion, A. Di Falco, T. F. Krauss

Biosensors And Bioelectronics 27 (1) 101-105 (2011).

We demonstrate the detection of dissolved avidin concentrations as low as 15 nM or 1 μg/ml using functionalized slotted photonic crystal cavities with integrated microfluidics. With a cavity sensing surface area of approximately 2.2 μm2, we are able to detect surface mass densities of order 60 pg/mm2 corresponding to a bound mass of approximately 100 ag. The ultra-compact size of the sensors makes them attractive for lab-on-a-chip applications where high densities of independent sensing elements are desired within a small area. The high sensitivity over an extremely small area is due to the strong modal overlap with the analyte enabled by the slotted waveguide cavity geometry that we employ. This strong overlap results in larger shifts in the cavity peak wavelength when compared to competing approaches.

Evidence of guided resonances in photonic quasicrystal slabs

A. Ricciardi, M. Pisco, A. Cutolo, A. Cusano, L. O'Faolain, T. F. Krauss, G. Castaldi, V. Galdi

Physics Review B 84 (8) 085135 (2011).

We report on the experimental evidence of Fano-type guided resonances (GRs) in aperiodically-ordered photonic quasicrystal slabs. With specific reference to the Ammann-Beenker (8-fold symmetric, quasiperiodic) octagonal tiling geometry, we present our experimental results on silicon-on-insulator devices operating at near-infrared wavelengths, and compare them with the full-wave numerical predictions based on periodic approximants. Our results indicate that spatial periodicity is not strictly required for the GR excitation, and may be effectively surrogated by weaker forms of long-range aperiodic order which intrinsically provide extra degrees of freedom (e.g., higher-order rotational symmetries, richer defect states and phase-matching conditions, etc.) to be exploited in the design and performance optimization of nanostructured dielectric slabs operating in the out-of-plane configuration. The essential spectral features may be qualitatively understood in terms of phase-matching conditions derived from approximate homogenized models, and turn out to be effectively captured by full-wave modeling based on suitably-sized periodic approximants.

Tight focusing with a binary microaxicon

V. V. Kotlyar, S. S. Stafeev, L. Ofaolain, V. A. Soifer

Optics Letters 36 (16) 3100-3102 (2011).

Using a near-field scanning microscope (NT-MDT) with a 100 nm aperture cantilever held 1 μm apart from a microaxicon of diameter 14 μm and period 800 nm, we measure a focal spot resulting from the illumination by a linearly polarized laser light of wavelength λ=532 nm, with its FWHM being equal to 0.58λ, and the depth of focus being 5.6λ. The rms deviation of the focal spot intensity from the calculated value is 6%. The focus intensity is five times larger than the maximal illumination beam intensity.

Third-harmonic generation in slow-light chalcogenide glass photonic crystal waveguides

C. Monat, M. Spurny, C. Grillet, L. O'Faolain, T. F. Krauss, B. J. Eggleton, D. Bulla, S. Madden, B. Luther-Davies

Optics Letters 36 (15) 2818-2820 (2011).

We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (∼2), we obtain a relatively large conversion efficiency (1.4×10−8/W2), which is 30× higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode.

Scaling of Raman amplification in realistic slow-light photonic crystal waveguides

I. H. Rey, Y. Lefevre, S. Schulz, N. Vermeulen, T. F. Krauss

Physical Review B 84 (3) 035306 (2011).

The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides.

Deliberate versus intrinsic disorder in photonic crystal nanocavities investigated by resonant light scattering

S. L. Portalupi, M. Galli, M. Belotti, L. C. Andreani, T. F. Krauss, L. O'Faolain

Physical Review B 84 045423

We report a study of photonic crystal nanocavities as a function of lattice disorder, which is implemented by deliberate radius variations of the holes forming the photonic crystal. Using cross-polarized laser light scattering, we demonstrate that lattice disorder affects the cavity mode symmetry, as is crucially evidenced by measuring scattering resonances for different sample orientations and explained by group-theoretical analysis together with calculations of the mode profile. We also quantify how the increase of lattice disorder leads to a reduction of the cavity Q factors and to a spread of both Q factors and resonance wavelengths. The trends as a function of disorder and for different radii distributions are well reproduced by theoretical calculations when both intentional (deliberate) and intrinsic (residual) disorder are taken into account. The results shed light on the effects of disorder on cavity resonances and on the interplay between intentional and intrinsic disorder, yielding reliable estimations of residual disorder (which ultimately affects technology limits) from optical measurements. Interestingly, the disorder values derived from the variation of the Q factor are lower than those derived from the wavelength spread: this suggests the occurrence of subtle interhole correlation effects that turn out to be beneficial for cavity Q factors.

Valve controlled fluorescence detection system for remote sensing applications

T. D. James, M. G. Scullion, P. C. Ashok, A. Di Falco, K. Dholakia, T. F. Krauss

Microfluidics And Nanofludics 1-8

We demonstrate a microfluidics-based fluorescence detection device where the filters, source, detector, and electronically controlled valves are embedded into a Polydimethylsiloxane (PDMS)-based microfluidic chip. The device reported here has been specifically designed for chlorophyll a fluorescence sensing in autonomous systems, such as oceanic applications. In contrast to a monolithic approach, the modular approach made the fabrication of this device simpler and cheaper. For fluorescence detection, an InGaN/GaN LED is used as the excitation source to specifically excite chlorophyll a; a metal-dielectric Fabry–Perot filter was used to extinguish out-of-band excitation. A simple Si photodiode is used as detector and provided with a thermally evaporated CdS emission filter to block the excitation source. This filter combination provides an excellent solution to the difficult problem of combining high-rejection excitation and emission filters in an integrated thin-film format. Furthermore, the metal-dielectric filter provides a much broader angular response than a comparable multilayer Bragg mirror, which is a key advantage in the integrated format. We use a novel paraffin wax-based valve design affords low power single-use actuation, between 0.5 and 1 J per actuation and withstands 0.6 bar differential pressure, which provides better performance than its previously reported counterparts. The remote valve-controlled operation of the fluorescence detection system is demonstrated, illustrating the measurement of a chlorophyll a solution, with a detection limit of 340 μM and subsequent valve-controlled flushing of the measurement reservoir.

Room-temperature emission at telecom wavelengths from silicon photonic crystal nanocavities

R. Lo Savio, S. L. Portalupi, D. Gerace, A. Shakoor, T. F. Krauss, L. O'Faolain, L. C. Andreani, M. Galli

Appl. Phys. Lett. 98 (20) 201106 (2011).

Strongly enhanced light emission at wavelengths between 1.3 and 1.6 μm is reported at room temperature in silicon photonic crystal (PhC) nanocavities with optimized out-coupling efficiency. Sharp peaks corresponding to the resonant modes of PhC nanocavities dominate the broad sub-bandgap emission from optically active defects in the crystalline Si membrane. We measure a 300-fold enhancement of the emission from the PhC nanocavity due to a combination of far-field enhancement and the Purcell effect. The cavity enhanced emission has a very weak temperature dependence, namely less than a factor of 2 reduction between 10 K and room temperature, which makes this approach suitable for the realization of efficient light sources as well as providing a quick and easy tool for the broadband optical characterization of silicon-on-insulator nanostructures.

Electrical conduction and optical properties of doped silicon-on-insulator photonic crystals

P. Cardile, G. Franzo, R. Lo Savio, M. Galli, T. F. Krauss, F. Priolo, L. O'Faolain

Appl. Phys. Lett. 98 (20) 203506 (2011).

We investigate the electrical properties of silicon-on-insulator (SOI) photonic crystals as a function of both doping level and air filling factor. The resistance trends can be clearly explained by the presence of a depletion region around the sidewalls of the holes that is caused by band pinning at the surface. To understand the trade-off between the carrier transport and the optical losses due to free electrons in the doped SOI, we also measured the resonant modes of L3 photonic crystal nanocavities and found that surprisingly high doping levels, up to 1018/cm3, are acceptable for practical devices with Q factors as high as 4×104.

Ultracompact 160 Gbaud all-optical demultiplexing exploiting slow light in an engineered silicon photonic crystal waveguide

B. Corcoran, M. D. Pelusi, C. Monat, J. Li, L. O'Faolain, T. F. Krauss, B. J. Eggleton

Optics Letters 36 (9) 1728-1730 (2011).

We demonstrate all-optical demultiplexing of a high-bandwidth, time-division multiplexed 160 Gbit/s signal to 10 Gbit/s channels, exploiting slow light enhanced four-wave mixing in a dispersion engineered, 96 μm long planar photonic crystal waveguide. We report error-free (bit error rate<10−9) operation of all 16 demultiplexed channels, with a power penalty of 2.2–2.4 dB, highlighting the potential of these structures as a platform for ultracompact all-optical nonlinear processes.

Slow-light and evanescent modes at interfaces in photonic crystal waveguides: optimal extraction from experimental near-field measurements

S. Ha, M. Spasenovic, A. A. Sukhorukov, T. P. White, C. Martijn De Sterke, L. Kuipers, T. F. Krauss, Y. S. Kivshar

Josa B 28 (4) 955-963 (2011).

We develop a systematic approach for simultaneous extraction of the dispersion relations and profiles of multiple modes in periodic waveguides though a special global optimization procedure applied to near-field electric field measurements in the waveguide plane. We apply this method to perform in-depth analysis of experimental data on wave propagation close to an interface between waveguide sections with different dispersion characteristics, and we successfully identify several modes contributing to the experimentally measured fields. We find clear evidence that when the group velocity is reduced across the interface, evanescent modes that facilitate the excitation of propagating slow-light waves appear, confirming previous theoretical predictions.

High Efficiency Interface for Coupling Into Slotted Photonic Crystal Waveguides

M. G. Scullion, T. F. Krauss, A. Di Falco

Photonics Journal, IEEE 3 (2) 203-208 (2011).

We investigate the mechanism of coupling light into slotted photonic crystal waveguides. We identify two alternative approaches for improving the coupling efficiency, based on engineered mode dispersion and on resonant mechanisms. For the optimized geometry, we calculate a loss figure of 0.3 dB over 100-nm bandwidth per interface and demonstrate a corresponding experimental value of 1.5 dB over 78-nm bandwidth per interface.

Luneburg lens in silicon photonics

A. Di Falco, S. C. Kehr, U. Leonhardt

Optics Express 19 (6) 5156-5162 (2011).

The Luneburg lens is an aberration-free lens that focuses light from all directions equally well. We fabricated and tested a Luneburg lens in silicon photonics. Such fully-integrated lenses may become the building blocks of compact Fourier optics on chips. Furthermore, our fabrication technique is sufficiently versatile for making perfect imaging devices on silicon platforms.

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

M. Spasenovic, T. P. White, S. Ha, A. A. Sukhorukov, T. Kampfrath, Y. S. Kivshar, C. Martijn De Sterke, T. F. Krauss, L. Kuipers

Optics Letters 36 (7) 1170-1172 (2011).

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is vg∼c/10 to a waveguide in which it is vg∼c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

Waveguide confined Raman spectroscopy for microfluidic interrogation

P. C. Ashok, G. P. Singh, H. A. Rendall, T. F. Krauss, K. Dholakia

Lab On A Chip 11 (7) 1262-1270 (2011).

We report the first implementation of the fiber based microfluidic Raman spectroscopic detection scheme, which can be scaled down to micrometre dimensions, allowing it to be combined with other microfluidic functional devices. This novel Raman spectroscopic detection scheme, which we termed as Waveguide Confined Raman Spectroscopy (WCRS), is achieved through embedding fibers on-chip in a geometry that confines the Raman excitation and collection region which ensures maximum Raman signal collection. This results in a microfluidic chip with completely alignment-free Raman spectroscopic detection scheme, which does not give any background from the substrate of the chip. These features allow a WCRS based microfluidic chip to be fabricated in polydimethylsiloxane (PDMS) which is a relatively cheap material but has inherent Raman signatures in fingerprint region. The effects of length, collection angle, and fiber core size on the collection efficiency and fluorescence background of WCRS were investigated. The ability of the device to predict the concentration was studied using urea as a model analyte. A major advantage of WCRS is its scalability that allows it to be combined with many existing microfluidic functional devices. The applicability of WCRS is demonstrated through two microfluidic applications: reaction monitoring in a microreactor and detection of analyte in a microdroplet based microfluidic system. The WCRS approach may lead to wider use of Raman spectroscopy based detection in microfluidics, and the development of portable, alignment-free microfluidic devices.

Mode structure of coupled L3 photonic crystal cavities

A. R. A. Chalcraft, S. Lam, B. D. Jones, D. Szymanski, R. Oulton, A. C. T. Thijssen, M. S. Skolnick, D. M. Whittaker, T. F. Krauss, A. M. Fox

Optics Express 19 (6) 5670-5675 (2011).

We investigate the energy splitting, quality factor and polarization of the fundamental modes of coupled L3 photonic crystal cavities. Four different geometries are evaluated theoretically, before experimentally investigating coupling in a direction at 30◦ to the line of the cavities. In this geometry, a smooth variation of the energy splitting with the cavity separation is predicted and observed, together with significant differences between the polarizations of the bonding and anti-bonding states. The controlled splitting of the coupled states is potentially useful for applications that require simultaneous resonant enhancement of two transitions.

Four-wave mixing in photonic crystal waveguides: slow light enhancement and limitations

J. Li, L. O'Faolain, I. H. Rey, T. F. Krauss

Optics Express 19 (5) 4458-4463 (2011).

We demonstrate continuous wave four-wave mixing in silicon photonic crystal waveguides of 396 μm length with a group index of ng = 30. The highest observed conversion efficiency is −24 dB for 90 mW coupled input pump power. The key question we address is whether the predicted fourth power dependence of the conversion efficiency on the slowdown factor (η ≈S4) can indeed be observed in this system, and how the conversion efficiency depends on device length in the presence of propagation losses. We find that the expected dependencies hold as long as both realistic losses and the variation of mode shape with slowdown factor are taken into account. Having achieved a good agreement between a simple analytical model and the experiment, we also predict structures that can achieve the same conversion efficiency as already observed in nanowires for the same input power, yet for a device length that is 50 times shorter.

Fabrication of low loss dispersion engineered chalcogenide photonic crystals

M. Spurny, L. O'Faolain, D. A. P. Bulla, B. Luther-Davis, T. F. Krauss

Optics Express 19 (3) 1991-1996 (2011).

We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge33As12Se55) glasses. The measured losses are as low as 21dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of ng = 40 in this relatively low refractive index contrast system.

Loss engineered slow light waveguides

L. O'Faolain, S. Schulz, D. M. Beggs, T. P. White, L. Spasenovic, L. Kuipers, F. Morichetti, A. Melloni, J. Mazoyer, P. Hugonin, P. Lalanne, T. F. Krauss

Optics Express 18 (26) 27627-27638 (2010).

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections (“extrinsic loss”) that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints.

Low-power continuous-wave generation of visible harmonics in silicon photonic crystal nanocavities

M. Galli, D. Gerace, K. Welna, T. F. Krauss, L. O'Faolain, G. Guizzetti, L. C. Andreani

Optics Express 18 (25) 26613-26624 (2010).

We present the first demonstration of frequency conversion by simultaneous second- and third-harmonic generation in a silicon photonic crystal nanocavity using continuous-wave optical excitation. We observe a bright dual wavelength emission in the blue/green (450-525 nm) and red (675-790 nm) visible windows with pump powers as low as few microwatts in the telecom bands, with conversion efficiencies of ∼10−5/W and ∼10/W2 for the second- and third-harmonic, respectively. Scaling behaviors as a function of pump power and cavity quality-factor are demonstrated for both second- and third order processes. Successful comparison of measured and calculated emission patterns indicates that third-harmonic is a bulk effect while second-harmonic is a surface-related effect at the sidewall holes boundaries. Our results are promising for obtaining practical low-power, continuous-wave and widely tunable multiple harmonic generation on a silicon chip.

Experimental evidence of guided-resonances in photonic crystals with aperiodically ordered supercells

A. Ricciardi, M. Pisco, I. Gallina, S. Campopiano, V. Galdi, L. O'Faolain, T. F. Krauss, A. Cusano

Optics Letters 35 (23) 3946-3948 (2010).

We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically ordered supercells. Using Ammann–Beenker (quasiperiodic, eightfold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-IR wavelengths (1300–1600 nm).Our results show a fairly good agreement between measurements and numerical predictions and pave the way for the development of new strategies (based on, e.g., the lattice symmetry breaking) for GR engineering.

Flexible metamaterials at visible wavelengths

A. Di Falco, M. Ploschner, T. F. Krauss

New Journal Of Physics 12 113006

We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.

Four-wave mixing in slow light engineered silicon photonic crystal waveguides

C. Monat, M. Ebnali-Heidari, C. Grillet, B. Corcoran, B. J. Eggleton, T. P. White, J. Li, T. F. Krauss

Optics Express 18 (22) 22915-22927 (2010).

We experimentally investigate four-wave mixing (FWM) in short (80 μm) dispersion-engineered slow light silicon photonic crystal waveguides. The pump, probe and idler signals all lie in a 14 nm wide low dispersion region with a near-constant group velocity of c/30. We measure an instantaneous conversion efficiency of up to −9dB between the idler and the continuous-wave probe, with 1W peak pump power and 6nm pump-probe detuning. This conversion efficiency is found to be considerably higher (>10 × ) than that of a Si nanowire with a group velocity ten times larger. In addition, we estimate the FWM bandwidth to be at least that of the flat band slow light window. These results, supported by numerical simulations, emphasize the importance of engineering the dispersion of PhC waveguides to exploit the slow light enhancement of FWM efficiency, even for short device lengths.

Dispersion-engineered slow light in photonic crystals: A comparison

S. Schulz, L. O'Faolain, D. M. Beggs, T. P. White, A. Melloni, T. F. Krauss

Journal Of Optics 12 (10) 104004 (2010).

We review the different types of dispersion engineered photonic crystal waveguides that have been developed for slow light applications. We introduce the group index bandwidth product (GBP) and the loss per delay in terms of dB ns − 1 as two key figures of merit to describe such structures and compare the different experimental realizations based on these figures. A key outcome of the comparison is that slow light based on photonic crystals performs as well or better than slow light based on coupled ring resonators.

Planar photonic crystal cavities with far-field optimization for high coupling efficiency and quality factor

S. L. Portalupi, M. Galli, C. Reardon, T. F. Krauss, L. O'Faolain, L. C. Andreani, D. Gerace

Optics Express 18 (15) 16064-16073 (2010).

Different types of planar photonic crystal cavities aimed at optimizing the far-field emission pattern are designed and experimentally assessed by resonant scattering measurements. We systematically investigate the interplay between achieving the highest possible quality (Q) factor and maximizing the in- and out-coupling efficiency into a narrow emission cone. Cavities operate at telecommunications wavelengths, i.e. around ∼ 1.55 mm, and are realized in silicon membranes. A strong modification of the far-field emission pattern, and therefore a substantial increase of the coupling efficiency in the vertical direction, is obtained by properly modifying the holes around L3, L5 and L7 type PhC cavities, as we predict theoretically and show experimentally. An optimal compromise yielding simultaneously a high Q-factor and a large coupling to the fundamental cavity mode is found for a L7-type cavity with a measured Q ≃ 62000, whose resonant scattering efficiency is improved by about two orders of magnitude with respect to the unmodified structure. These results are especially useful for prospective applications in light emitting devices, such as nano-lasers or single-photon sources, in which vertical in- and out-coupling of the electromagnetic field is necessarily required.

Statistical fluctuations of transmission in slow light photonic-crystal waveguides

S. Mazoyer, P. Lalanne, J. C. Rodier, J. P. Hugonin, M. Spasenovic, L. Kuipers, D. M. Beggs, T. F. Krauss

Optics Express 18 (14) 14654-14663 (2010).

We report statistical fluctuations for the transmissions of a series of photonic-crystal waveguides (PhCWs) that are supposedly identical and that only differ because of statistical structural fabrication-induced imperfections. For practical PhCW lengths offering tolerable −3dB attenuation with moderate group indices (ng≈60), the transmission spectra contains very narrow peaks (Q≈20,000) that vary from one waveguide to another. The physical origin of the peaks is explained by calculating the actual electromagnetic-field pattern inside the waveguide. The peaks that are observed in an intermediate regime between the ballistic and localization transports are responsible for a smearing of the local density of states, for a rapid broadening of the probability density function of the transmission, and bring a severe constraint on the effective use of slow light for on-chip optical information processing. The experimental results are quantitatively supported by theoretical results obtained with a coupled-Bloch-mode approach that takes into account multiple scattering and localization effects.

Broadband Mirrors in the Near-Infrared Based on Subwavelength Gratings in SOI

A. Ricciardi, S. Campopiano, A. Cusano, T. F. Krauss, L. O'Faolain

IEEE Photonics Journal 2 (5) 696-702 (2010).

We describe the design, fabrication, and characterization of high-reflectivity broadband mirrors operating in the near-infrared (700–1000 nm) wavelength range. The mirrors consist of 1-D and 2-D subwavelength resonant gratings (SWGs) fabricated on a silicon-on-insulator (SOI) wafer. A very good agreement between numerical and experimental results is obtained. The mirror response can be tailored by adjusting the geometrical parameters of the gratings, with the grating period as the main parameter. The optimized mirrors reflect strongly (> 95%) over a fractional optical bandwidth ∆λ/λ of about 12% and 7.5% for 1-D and 2-D gratings, respectively. The important and somewhat surprising feature of these gratings is that high reflectivities have been achieved, despite the fact that silicon exhibits significant absorption in this wavelength range.

Compact Optical Switches and Modulators Based on Dispersion Engineered Photonic Crystals

L. O'Faolain, D. M. Beggs, T. P. White, T. Kampfrath, L. Kuipers, T. F. Krauss

IEEE Photonics Journal 2 (3) 404-414 (2010).

We use slow-light photonic crystals to enhance optical switching and modulation in silicon. By using dispersion-engineered designs, a switch as short as 5 um was achieved, in which we have demonstrated rerouting of optical pulses on a 3-ps time scale through the absorption of a femtosecond pulse. We additionally demonstrate a modulator with a Mach–Zehnder interferometer (MZI) configuration with flat-band slow-light photonic crystal phase shifters that is designed to give a large group-index–bandwidth product. An extinction ratio in excess of 15 dB is obtained over the entire 11-nm bandwidth of the modulator.

Photonic Crystal Formed by the Imaginary Part of the Refractive Index

J. Li, B. Liang, Y. Liu, P. Zhang, J. Zhou, S. O. Klimonsky, A. S. Slesarev, Y. D. Tretyakov, L. O'Faolain, T. F. Krauss

Advanced Materials 22 1-4

A periodic array of dye-doped disks that form a 2D photonic lattice is fabricated and characterized. The array was generated by interference lithography, and the voids of the template were filled by SU8 without doping. This creates a photonic lattice with a uniform real part of the refractive index, but a periodically modulated imaginary part. The structure is characterized by diffraction measurements; the structure only diffracts light in the spectral range where the dye absorbs, hence acts like a truly imaginary index photonic crystal.

Ultrafast adiabatic manipulation of slow light in a photonic crystal

T. Kampfrath, D. M. Beggs, T. P. White, A. Melloni, T. F. Krauss, L. Kuipers

Phys. Rev. A 81 (4) 043837 (2010).

We demonstrate by experiment and theory that a light pulse propagating through a Si-based photonic-crystal waveguide is adiabatically blueshifted when the refractive index of the Si is reduced on a femtosecond time scale. Thanks to the use of slow-light modes, we are able to shift a 1.3-ps pulse at telecom frequencies by 0.3 THz with an efficiency as high as 80% in a waveguide as short as 19μm. An analytic theory reproduces the experimental data excellently, which shows that adiabatic dynamics are possible even on the femtosecond time scale as long as the external stimulus conserves the spatial symmetry of the system.

Optical signal processing on a silicon chip at 640Gb/s using slow-light

B. Corcoran, C. Monat, M. Pelusi, C. Grillet, T. P. White, L. O’faolain, T. F. Krauss, B. J. Eggleton, D. J. Moss

Optics Express 18 (8) 7770-7781 (2010).

We demonstrate optical performance monitoring of in-band optical signal to noise ratio (OSNR) and residual dispersion, at bit rates of 40Gb/s, 160Gb/s and 640Gb/s, using slow-light enhanced optical third harmonic generation (THG) in a compact (80µm) dispersion engineered 2D silicon photonic crystal waveguide. We show that there is no intrinsic degradation in the enhancement of the signal processing at 640Gb/s relative to that at 40Gb/s, and that this device should operate well above 1Tb/s. This work represents a record 16-fold increase in processing speed for a silicon device, and opens the door for slow light to play a key role in ultra-high bandwidth telecommunications systems.

Nonlinear loss dynamics in a silicon slow-light photonic crystal waveguide

B. Corcoran, C. Monat, D. Pudo, B. J. Eggleton, T. F. Krauss, D. J. Moss, L. O'Faolain, M. Pelusi, T. P. White

Optics Letters 35 (7) 1073-1075 (2010).

We directly investigate both experimentally and numerically the influence of optical nonlinear loss dynamics on a silicon waveguide based all-optical device. The dynamics of these nonlinear losses are explored through the analysis of optical limiting of an amplitude distorted 10 Gbit/s signal in a slow-light silicon photonic crystal waveguide. As the frequency of the distortion approaches the free-carrier recombination rate, freecarrier absorption reaches a steady state, leaving two-photon absorption the dominant dynamic nonlinear loss. Our results highlight the importance of engineering the free-carrier lifetime in silicon waveguides for high speed all-optical processing applications.

Tunable Delay Lines in Silicon Photonics: Coupled Resonators and Photonic Crystals, a Comparison

A. Melloni, A. Canciamilla, C. Ferrari, F. Morichetti, L. O'Faolain, T. F. Krauss, R. De La Rue, A. Samarelli, M. Sorel

IEEE Photonics Journal 2 (2) 181-194 (2010).

In this paper, we report a direct comparison between coupled resonator optical waveguides (CROWs) and photonic crystal waveguides (PhCWs), which have both been exploited as tunable delay lines. The two structures were fabricated on the same silicon-on-insulator (SOI) technological platform, with the same fabrication facilities and evaluated under the same signal bit-rate conditions. We compare the frequency- and time-domain response of the two structures; the physical mechanism underlying the tuning of the delay; the main limits induced by loss, dispersion, and structural disorder; and the impact of CROW and PhCW tunable delay lines on the transmission of data stream intensity and phase modulated up to 100 Gb/s. The main result of this study is that, in the considered domain of applications, CROWs and PhCWs behave much more similarly than one would expect. At data rates around 100 Gb/s, CROWs and PhCWs can be placed in competition. Lower data rates, where longer absolute delays are required and propagation loss becomes a critical issue, are the preferred domain of CROWs fabricated with large ring resonators, while at data rates in the terabit range, PhCWs remain the leading technology.

Investigation of phase matching for third-harmonic generation in silicon slow light photonic crystal waveguides using Fourier optics

C. Monat, C. Grillet, B. Corcoran, D. J. Moss, B. J. Eggleton, T. P. White, T. F. Krauss

Optics Express 18 (7) 6831-6840 (2010).

Using Fourier optics, we retrieve the wavevector dependence of the third-harmonic (green) light generated in a slow light silicon photonic crystal waveguide. We show that quasi-phase matching between the third-harmonic signal and the fundamental mode is provided in this geometry by coupling to the continuum of radiation modes above the light line. This process sustains third-harmonic generation with a relatively high efficiency and a substantial bandwidth limited only by the slow light window of the fundamental mode. The results give us insights into the physics of this nonlinear process in the presence of strong absorption and dispersion at visible wavelengths where bandstructure calculations are problematic. Since the characteristics (e.g. angular pattern) of the third-harmonic light primarily depend on the fundamental mode dispersion, they could be readily engineered.

Optical chromatography using a photonic crystal fiber with on-chip fluorescence excitation

P. Ashok, R. Marchington, T. Mthunzi, T. F. Krauss, K. Dholakia, N. A. Mortensen

Optics Express 18 (6) 6396-6407 (2010).

We describe the realization of integrated optical chromatography, in conjunction with on-chip fluorescence excitation, in a monolithically fabricated poly-dimethylsiloxane (PDMS) microfluidic chip. The unique endlessly-single-mode guiding property of the Photonic Crystal Fiber (PCF) facilitates simultaneous on-chip delivery of beams to perform optical sorting in conjunction with fluorescence excitation. We use soft lithography to define the chip and insert the specially capped PCF into it through a predefined fiber channel that is intrinsically aligned with the sorting channel. We compare the performance of the system to a standard ray optics model and use the system to demonstrate both size-driven and refractive index-driven separations of colloids. Finally we demonstrate a new technique of enhanced optofluidic separation of biological particles, by sorting of human kidney embryonic cells (HEK-293), internally tagged with fluorescing microspheres through phagocytocis, from those without microspheres and the separation purity is monitored using fluorescence imaging.

Optical forces near a nanoantenna

M. Ploschner, M. Mazilu, T. F. Krauss, K. Dholakia

Journal Of Nanophotonics 4 041570

The Maxwell stress tensor method is used to calculate the optical forces acting upon a glass nanosphere in the proximity of optically excited gold nanoantenna structures. The dependence of optical forces over a full range of excitation angles is explored: the total internal reflection excitation does not bring any particular advantage to trapping efficiency when compared to the normal incidence excitation. Our calculations show multiple trapping sites with similar trapping properties for the normal and the total internal reflection cases, respectively; furthermore, the convective heating probably dominates over any optical forces in such systems.

Temperature stabilization of optofluidic photonic crystal cavities (vol 94, 231114, 2009)

C. Karnutsch, C. L. C. Smith, A. Graham, S. Tomljenovic-Hanic, R. Mcphedran, B. J. Eggleton, L. O'Faolain, T. F. Krauss, S. S. Xiao, N. A. Mortensen

Applied Physics Letters 96 (7) 79901 (2010).

Slow Light Enhanced Nonlinear Optics in Silicon Photonic Crystal Waveguides

C. Monat, B. Corcoran, D. Pudo, M. Ebnali-Heidari, C. Grillet, M. D. Pelusi, D. J. Moss, B. J. Eggleton, T. P. White, L. O'Faolain, T. F. Krauss

IEEE Journal Of Selected Topics In Quantum Electronics 16 (1) 344-356 (2010).

We present a summary of our recent experiments showing how various nonlinear phenomena are enhanced due to slow light in silicon photonic crystal waveguides. These nonlinear processes include self-phase modulation (SPM), two-photon absorption (TPA), free-carrier related effects, and third-harmonic generation, the last effect being associated with the emission of green visible light, an unexpected phenomenon in silicon. These demonstrations exploit photonic crystal waveguides engineered to support slow modes with a range of group velocities as low as c/50 and, more crucially, with significantly reduced dispersion. We discuss the potential of slow light in photonic crystals for realizing compact nonlinear devices operating at low powers. In particular, we consider the application of SPM to all-optical regeneration, and experimentally investigate an original approach, where enhanced TPA and free-carrier absorption are used for partial regeneration of a high-bit rate data stream (10 Gb/s).

Strong High Order Diffraction of Guided Modes in Micro-Cavity Light-Emitting Diodes With Hexagonal Photonic Crystals

K. Bergenek, C. Wiesmann, H. Zull, C. Rumbolz, R. Wirth, N. Linder, K. Streubel, T. F. Krauss

IEEE Journal Of Quantum Electronics 45 (12) 1517-1523 (2009).

Photonic crystals (PhCs) have now been firmly established as an efficient means for light extraction from light emitting diodes (LEDs). We analyze the diffraction properties from thin GaN micro-cavity LEDs with hexagonal lattices that feature three guided TE modes only. In contrast to common design rules, we find that high order diffraction contributes significantly to the light extraction and increases the directionality of the emitted light. The implementation of the PhC leads to an enhancement in light extraction by a factor of up to 1.8 and the directionality of the light is greatly improved with a radiant intensity enhancement factor of 4.3, which can only be explained by the higher order diffraction that has been hitherto neglected. Furthermore, we show that higher order diffraction contributes significantly to the high azimuthal extraction uniformity we observe, suggesting that the use of quasi-crystal lattices is not necessary. We use a model including mode absorption where each in-plane angle of the guided modes is treated separately in order to explain the experimental results.

High-speed modulation of a compact silicon ring resonator based on a reverse-biased pn diode

F. Y. Gardes, A. Brimont, P. Sanchis, G. Rasigade, D. Marris-Morini, L. O'Faolain, F. Dong, J. M. Fedeli, P. Dumon, L. Vivien, T. F. Krauss, G. T. Reed, J. Marti

Optics Express 17 (24) 21986-21991 (2009).

High speed modulation based on a compact silicon ring resonator operating in depletion mode is demonstrated. The device exhibits an electrical small signal bandwidth of 19GHz. The device is therefore a candidate for highly compact, wide bandwidth modulators for a variety of applications. (C) 2009 Optical Society of America

Complete response characterization of ultrafast linear photonic devices

T. Kampfrath, D. M. Beggs, T. F. Krauss, L. Kuipers

Optics Letters 34 (21) 3418-3420 (2009).

We present a method to fully characterize linear photonic devices that change their properties on ultrashort time scales. When we feed the device with a broadband input pulse and detect the resulting output field for a sufficient number of arrival times of the input, the device response to any other input with smaller bandwidth can be extracted numerically, without the need for additional measurements. Our approach is based on the formalism of linear time-varying systems, and we experimentally demonstrate its feasibility for the example of an ultrafast nanophotonic switch. (C) 2009 Optical Society of America

Optical filter with very large stopband (approximate to 300 nm) based on a photonic-crystal vertical-directional coupler

M. Grande, L. O'Faolain, T. P. White, M. Spurny, A. D'Orazio, T. F. Krauss

Optics Letters 34 (21) 3292-3294 (2009).

We have designed, fabricated, and demonstrated a vertical directional coupler based on the coupling between a polymer waveguide and a W1 photonic crystal waveguide. The filters have a bandwidth of similar to 2 nm within a stopband of Delta lambda approximate to 300 nm and an on-chip insertion loss of 1 dB. This is the first ( to our knowledge) demonstration of a filter with such a large stopband that overcomes the bandwidth limitation of existing filters. (C) 2009 Optical Society of America

Disorder-induced incoherent scattering losses in photonic crystal waveguides: Bloch mode reshaping, multiple scattering, and breakdown of the Beer-Lambert law

M. Patterson, S. Hughes, S. Schulz, D. M. Beggs, T. P. White, L. O'Faolain, T. F. Krauss

Physical Review B 80 (19) 195305 (2009).

Through a combined theoretical and experimental study of disorder-induced incoherent scattering losses in slow-light photonic crystal slab waveguides, we show the importance of Bloch mode reshaping and multiple scattering. We describe a convenient and fully three-dimensional theoretical treatment of disorder-induced extrinsic scattering, including the calculation of backscatter and out-of-plane losses per unit cell, and the extrapolation of the unit-cell loss to the loss for an entire disordered waveguide. The theoretical predictions, which are also compared with recent measurements on dispersion engineered silicon waveguides, demonstrate the failure of the Beer-Lambert law due to multiple scattering. We also explain why the previously assumed group velocity scalings of disorder-induced loss break down in general.

Ultrafast all-optical switching in AlGaAs photonic crystal waveguide interferometers

D. M. Szymanski, B. D. Jones, M. S. Skolnick, A. M. Fox, D. O'Brien, T. F. Krauss, J. S. Roberts

Applied Physics Letters 95 (14) 141108 (2009).

We have demonstrated ultrafast all-optical switching with photonic crystals integrated into AlGaAs Mach-Zehnder interferometers. The nonlinearity is induced by optical excitation of carriers into one arm of the interferometer, and switching times as short as 3 ps are achieved by surface recombination at the air holes in the photonic crystal. The fast recombination times and high nonlinearities of the AlGaAs material make this design suitable for high speed all-optical switching applications. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3236542]

Temperature stabilization of optofluidic photonic crystal cavities

C. Karnutsch, C. L. C. Smith, A. Graham, S. Tomljenovic-Hanic, R. Mcphedran, B. J. Eggleton, L. O'Faolain, T. F. Krauss, S. S. Xiao, N. A. Mortensen

Applied Physics Letters 94 (23) 231114 (2009).

We present a principle for the temperature stabilization of photonic crystal (PhC) cavities based on optofluidics. We introduce an analytic method enabling a specific mode of a cavity to be made wavelength insensitive to changes in ambient temperature. Using this analysis, we experimentally demonstrate a PhC cavity with a quality factor of Q approximate to 15 000 that exhibits a temperature-independent resonance. Temperature-stable cavities constitute a major building block in the development of a large suite of applications from high-sensitivity sensor systems for chemical and biomedical applications to microlasers, optical filters, and switches.

Electro-optic modulation in slotted resonant photonic crystal heterostructures

J. H. Wulbern, J. Hampe, A. Petrov, M. Eich, J. D. Luo, A. K. Y. Jen, A. Di Falco, T. F. Krauss, J. Bruns

Applied Physics Letters 94 (24) 241107 (2009).

Two dimensional photonic crystal waveguides in high index materials enable integrated optical devices with an extremely small geometrical footprint on the scale of micrometers. Slotted waveguides are based on the guiding of light in low refractive index materials and a field enhancement in this particular region of the device. In this letter we experimentally demonstrate electro-optic modulation in slotted photonic crystal waveguides based on silicon-on-insulator substrates covered and infiltrated with nonlinear optical polymers. A photonic crystal heterostructure is used to create a cavity, while simultaneously serving as an electrical connection from the slot to the metal electrodes that carry the modulation signal.

Ultrafast rerouting of light via slow modes in a nanophotonic directional coupler

T. Kampfrath, D. M. Beggs, T. P. White, M. Burresi, D. Van Oosten, T. F. Krauss, L. Kuipers

Applied Physics Letters 94 (24) 241119 (2009).

We demonstrate that two coupled photonic-crystal waveguides can route two subsequently arriving light pulses to different output ports even though the pulses are only 3 ps apart. This rerouting of light is due to an ultrafast shift in the transmittance spectrum triggered by the generation of electrons and holes in the Si base material by a femtosecond laser pulse. The use of slow-light modes allows for a coupler length of only 5.2 mu m. Since these modes are not directly involved, the 3 ps dead time is solely determined by the duration of the input pulse rather than its transit time through the device.

Green light emission in silicon through slow-light enhanced third-harmonic generation in photonic-crystal waveguides

B. Corcoran, C. Monat, C. Grillet, D. J. Moss, B. J. Eggleton, T. P. White, L. O'Faolain, T. F. Krauss

Nature Photonics 3 (4) 206-210 (2009).

Slow light has attracted significant interest recently as a potential solution for optical delay lines and time-domain optical signal processing(1,2). Perhaps even more significant is the possibility of dramatically enhancing nonlinear optical effects(3,4) due to the spatial compression of optical energy(5-7). Two-dimensional silicon photonic-crystal waveguides have proven to be a powerful platform for realizing slow light, being compatible with on-chip integration and offering wide-bandwidth and dispersion-free propagation(2). Here, we report the slow-light enhancement of a nonlinear optical process in a two-dimensional silicon photonic-crystal waveguide. We observe visible third-harmonic-generation at a wavelength of 520 nm with only a few watts of peak power, and demonstrate strong third-harmonic-generation enhancement due to the reduced group velocity of the near-infrared pump signal. This demonstrates yet another unexpected nonlinear function realized in a CMOS-compatible silicon waveguide.

Integrated polymer microprisms for free space optical beam deflecting

C. Reardon, A. Di Falco, K. Welna, T. F. Krauss

Optics Express 17 (5) 3424 (2009).

We demonstrate beam deflection and multiple channel communication in free space optical communications using microprisms integrated directly onto an array of vertical cavity surface emitting lasers (VCSELs). The design and fabrication of such a transmitter is presented, and shown to achieve beam deflection of up to 10° in a planar configuration. A location discovery application, for use within a distributed network, is put forward and analysed.

Light scattering and Fano resonances in high-Q photonic crystal nanocavities

M. Galli, S. L. Portalupi, M. Belotti, L. C. Andreani, L. O'Faolain, T. F. Krauss

Applied Physics Letters 94 (7) 71101 (2009).

The authors show that light scattering from high-Q planar photonic crystal nanocavities can display Fano-like resonances corresponding to the excitation of localized cavity modes. By changing the scattering conditions, we are able to tune the observed lineshapes from strongly asymmetric and dispersivelike resonances to symmetric Lorentzians. Results are interpreted according to the Fano model of quantum interference between two coupled scattering channels. Combined measurements and line shape analysis on a series of silicon L3 nanocavities as a function of nearby hole displacement demonstrate that Q factors as high as 1.1x10(5) can be directly measured in these structures. Furthermore, a comparison with theoretically calculated Q factors allows to extract the rms deviation of hole radii due to weak disorder of the photonic lattice.

Slow light enhancement of nonlinear effects in silicon engineered photonic crystal waveguides

C. Monat, B. Corcoran, M. Ebnali-Heidari, C. Grillet, B. J. Eggleton, T. P. White, L. O'Faolain, T. F. Krauss

Optics Express 17 (4) 2944-2953 (2009).

We report nonlinear measurements on 80 mu m silicon photonic crystal waveguides that are designed to support dispersionless slow light with group velocities between c/20 and c/50. By launching picosecond pulses into the waveguides and comparing their output spectral signatures, we show how self phase modulation induced spectral broadening is enhanced due to slow light. Comparison of the measurements and numerical simulations of the pulse propagation elucidates the contribution of the various effects that determine the output pulse shape and the waveguide transfer function. In particular, both experimental and simulated results highlight the significant role of two photon absorption and free carriers in the silicon waveguides and their reinforcement in the slow light regime.. 2009 Optical Society of America

Chemical sensing in slotted photonic crystal heterostructure cavities

A. Di Falco, L. O'Faolain, T. F. Krauss

Applied Physics Letters 94 (6) 63503 (2009).

We fabricated slotted photonic crystal waveguides and cavities supporting resonant modes in air. Their peculiar geometry enables the detection of refractive index changes in a given analyte with high sensitivity because of the large overlap between the optical mode and the analyte. This yields a high figure of merit for the sensitivity of the device and we are able to report values of S=Delta lambda/Delta n over 1500. By applying a photonic crystal heterostructure to the slotted geometry, we are able to create high quality-factor cavities essential for realizing low detection limits up to Q=50 000.

Nanophotonic Polarization Diversity Demultiplexer Chip

F. Van Laere, T. Stomeo, C. Cambournac, M. Ayre, R. Brenot, H. Benisty, G. Roelkens, T. F. Krauss, D. Van Thourhout, R. Baets

Journal Of Lightwave Technology 27 (01-Apr) 417-425 (2009).

We demonstrate a very compact multifunctional photonic-crystal demultiplexer on high index contrast InP-membrane for coarse WDM applications. Polarization diversity is implemented using 2D-grating couplers. The performance of the device is evaluated using integrated p-i-n photodetectors. Polarization diversity from fiber to detector-without intermediate functional device-results in a minimal polarization dependent loss (PDL) of 0.2 dB. This value increases to 1.1 dB when including the photonic-crystal demultiplexer.

Ultrashort Photonic Crystal Optical Switch Actuated by a Microheater

D. M. Beggs, T. P. White, L. Cairns, L. O'Faolain, T. F. Krauss

IEEE Photonics Technology Letters 21 (01-Apr) 24-26 (2009).

We demonstrate a silicon photonic crystal (PhC) optical switch with a 20-mu s response time controlled by a thermo-optic microheater. The switch consists of a dispersion engineered PhC directional coupler that is only 4.9 mu m long. Optical and electrical isolation is provided by backfilling the holes and embedding the PhC in a silca cladding to produce a vertically symmetric structure that is more robust than a membrane geometry. No increase in optical loss is observed due to the silica. cladding, despite operating above the lightline; the insertion loss for airbridge and silica embedded structures are comparable at 1-2 dB.

Fourier space imaging of light localization at a photonic band-edge located below the light cone

N. Le Thomas, R. Houdre, D. M. Beggs, T. F. Krauss

Physical Review B 79 (3) 33305 (2009).

We observe light localization in a two-dimensional geometry, induced by residual disorder at a photonic band edge located below the light cone. The combination of a spectrally selective illumination and a grating assisted k-space imaging technique allows us to image the equifrequency surfaces associated with such a photonic band, with high accuracy and without aberrations. Thanks to this approach, the impact of the nonideal nature of real planar photonic crystals on the propagation properties of the Bloch wave at the band edge is decorrelated from the contribution of the intrinsic out-of-plane losses. As a by-product, our result demonstrates an immersion free effective numerical aperture as high as 3.5 in k space.

Directional light extraction from thin-film resonant cavity light-emitting diodes with a photonic crystal

K. Bergenek, C. Wiesmann, H. Zull, R. Wirth, P. Sundgren, N. Linder, K. Streubel, T. F. Krauss

Applied Physics Letters 93 (23) 231109 (2008).

We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.

Efficient slow-light coupling in a photonic crystal waveguide without transition region

T. P. White, L. C. Botten, C. Martijn De Sterke, K. B. Dossou, R. C. Mcphedran

Optics Letters 33 (22) 2644-2646 (2008).

We consider the coupling into a slow mode that appears near an inflection point in the band structure of a photonic crystal waveguide. Remarkably, the coupling into this slow mode, which has a group index ng >1000, can be essentially perfect without any transition region. We show that this efficient coupling occurs thanks to an evanescent mode in the slow medium, which has appreciable amplitude and helps satisfy the boundary conditions but does not transport any energy.

Silica-embedded silicon photonic crystal waveguides

T. P. White, L. O'Faolain, J. T. Li, L. C. Andreani, T. F. Krauss

Optics Express 16 (21) 17076-17081 (2008).

We report on the fabrication and characterization of silicon photonic crystal waveguides completely embedded in silica. These waveguides offer a robust alternative to air-membranes and are fully compatible with monolithic integration. Despite the reduced refractive index contrast compared to the air-membranes, these waveguides offer a considerable operating range of approximate to 10 nm in the 1550 nm window. While the reduced index contrast weakens the perturbations due to surface roughness, we measure losses of 35 +/- 3dB/cm compared to 12 +/- 3 dB/cm for nominally identical air-membranes. Numerical analysis reveals that the difference in loss results from the different mode distribution and group index of the respective waveguide modes. Radius disorder is used as a fitting parameter in the numerical simulations with the best fits found for disorder levels of 1.4 - 1.7 nm RMS, which attest to the high quality of our structures. (C) 2008 Optical Society of America

High-Q microfluidic cavities in silicon-based two-dimensional photonic crystal structures

U. Bog, C. L. C. Smith, M. W. Lee, S. Tomljenovic-Hanic, C. Grillet, C. Monat, L. O'Faolain, C. Karnutsch, T. F. Krauss, R. C. Mcphedran, B. J. Eggleton

Optics Letters 33 (19) 2206-2208 (2008).

We demonstrate postprocessed microfluidic double-heterostructure cavities in silicon-based photonic crystal slab waveguides. The cavity structure is realized by selective fluid infiltration of air holes using a glass microtip, resulting in a local change of the average refractive index of the photonic crystal. The microcavities are probed by evanescent coupling from a silica nanowire. An intrinsic quality factor of 57,000 has been derived from our measurements, representing what we believe to be the largest value observed in microfluidic photonic crystal cavities to date. (C) 2008 Optical Society of America

Reconfigurable microfluidic photonic crystal slab cavities

C. L. C. Smith, U. Bog, S. Tomljenovic-Hanic, M. W. Lee, D. K. C. Wu, L. O'Faolain, C. Monat, C. Grillet, T. F. Krauss, C. Karnutsch, R. C. Mcphedran, B. J. Eggleton

Optics Express 16 (20) 15887-15896 (2008).

We demonstrate the spectral and spatial reconfigurability of photonic crystal double-heterostructure cavities in silicon by microfluidic infiltration of selected air holes. The lengths of the microfluidic cavities are changed by adjusting the region of infiltrated holes in steps of several microns. We systematically investigate the spectral signature of these cavities, showing high Q-factor resonances for a broad range of cavity lengths. The fluid can be removed by immersing the device in toluene, offering complete reconfigurability. Our cavity writing technique allows for tolerances in the infiltration process and provides flexibility as it can be employed at any time after photonic crystal fabrication. (C) 2008 Optical Society of America

Influence of residual disorder on the anticrossing of Bloch modes probed in k space

N. Le Thomas, V. Zabelin, R. Houdre, M. V. Kotlyar, T. F. Krauss

Physical Review B 78 (12) 125301 (2008).

We retrieve the dispersion properties of photonic crystal waveguides near the band edge with high experimental accuracy. The dispersion diagram of the waveguide modes in the complex-valued plane is directly measured in the far field by using a Fourier space imaging technique. We show that the investigation of the modes in k space provides a clear signature of the transition between propagating, evanescent, and localized modes. It allows us to determine the impact of the structural disorder and of the dissipation on the group velocity of the propagating wave in the slow light regime.

Why do we need slow light?

T. F. Krauss

Nature Photonics 2 (8) 448-450 (2008).

The extreme speed at which light moves, and the fact that photons do not tend to interact with transparent matter, is of enormous benefit to mankind. It allows us to see deep into the Universe and to transmit data over long distances in optical fibres. So, why slow light down?.

Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals

K. Bergenek, C. Wiesmann, R. Wirth, L. O'Faolain, N. Linder, K. Streubel, T. F. Krauss

Applied Physics Letters 93 (4) 41105 (2008).

We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index n(eff) is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G=1.5 k(0), which is 47% better than an unstructured reference device. (C) 2008 American Institute of Physics.

Phase-sensitive Fourier space imaging of optical Bloch modes

N. Le Thomas, R. Houdre, M. V. Kotlyar, T. F. Krauss

Physical Review B 77 (24) 245323 (2008).

We present a phase-shifting holography method in k space that allows us to investigate the phase and amplitude of optical Bloch waves. The method is based on a Fourier space imaging technique combined with a Mach-Zehnder interferometer. Using a W3 photonic crystal waveguide as a model system, we determine, with high accuracy, both the intensity and the phase of the envelope function of the Bloch modes propagating in such waveguides. We show that local variation of the dielectric map as low as 20 nm can be clearly observed from the part of the angular spectrum of field located inside the light cone.

Integration of grating couplers with a compact photonic crystal demutiplexer on an InP membrane

T. Stomeo, F. Van Laere, M. Ayre, C. Cambournac, H. Benisty, D. Van Thourhout, R. Baets, T. F. Krauss

Optics Letters 33 (8) 884-886 (2008).

We demonstrate the integration of a 30% efficient grating coupler with a compact photonic crystal wavelength demultiplexer (DeMUX). The DeMUX has seven output channels that are spaced 10 nm apart and is aimed at coarse WDM applications. The integrated devices are realized on a high-index-contrast InP membrane using a simple benzocyclobutene wafer bonding technique. Cross talks of -10 to -12 dB for four channels 20 nm apart are obtained without optimization. (C) 2008 Optical Society of America.

Accurate determination of the functional hole size in photonic crystal slabs using optical methods

D. M. Beggs, L. O'Faolain, T. F. Krauss

Photonics And Nanostructures-fundamentals And Applications 6 (3-4) 213-218 (2008).

Control and repeatability in the fabrication of two-dimensional photonic crystal (PhC) slabs is becoming increasingly important as the technology matures towards practical applications. A key problem in this respect is the determination of the actual hole size in finished devices. We have developed an optical method for measuring the hole size in PhC slabs as an alternative to the inspection of scanning electron microscope (SEM) images. The optical method relies on determining the cut-off frequency of W1 PhC waveguides, which is easily measured and compared to calculations as a function of hole size. We show that the typical error in the measurement of hole diameter is approximately 2%, or 5 nm. This level of accuracy is a significant improvement over current methods, which rely on the inspection by SEM. SEM inspection can introduce large systematic errors because different electron detectors, and even different settings of the same detector, will provide differing contrasts between a hole and its edge. Such errors can be of the order of 20 nm, or as much as 5-10% of the absolute hole diameter. Furthermore, our method provides the functional or effective hole size, which determines the photonic function of the device, and which may be different from the physical hole size. (C) 2008 Elsevier B.V. All rights reserved.

Photonic crystal waveguides for coarse-selectivity devices

M. Ayre, C. Cambournac, O. Khayam, H. Benisty, T. Stomeo, T. F. Krauss

Photonics And Nanostructures-fundamentals And Applications 6 (1) 19-25 (2008).

We discuss devices based on photonic crystal planar waveguides for use as wavelength-selective applications via the mini-stopband extraction mechanism. We present experimental data for the core of a compact demux system. We show that spatial and spectral behaviours are as intended from coupled-mode theory, as well as from the finite-difference time-domain approach. Finally, we propose architectures with large crosstalk and some inherent fabrication-related advantages achieved by duplicating the basic multimode waveguide and using it as a spectral filter. (c) 2007 Elsevier B.V. All rights reserved.

Systematic design of flat band slow light in photonic crystal waveguides

J. Li, T. P. White, L. O'Faolain, A. Gomez-Iglesias, T. F. Krauss

Optics Express 16 (9) 6227-6232 (2008).

We present a systematic procedure for designing flat bands of photonic crystal waveguides for slow light propagation. The procedure aims to maximize the group index - bandwidth product by changing the position of the first two rows of holes of W1 line defect photonic crystal waveguides. A nearly constant group index - bandwidth product is achieved for group indices of 30-90 and as an example, we experimentally demonstrate flat band slow light with nearly constant group indices of 32.5, 44 and 49 over 14 nm, 11 nm and 9.5 nm bandwidth around 1550 nm, respectively. (c) 2008 Optical Society of America.

Coupling length of silicon-on-insulator directional couplers probed by Fourier-space imaging

J. Jagerska, N. Le Thomas, R. Houdre, D. M. Beggs, D. O'Brien, T. F. Krauss

Applied Physics Letters 92 (15) 151106 (2008).

We use a Fourier-space imaging technique relying on outcoupling grating probes to study the coupled mode interaction and dispersion properties of guided modes in silicon-on-insulator codirectional couplers. Our approach allows us to measure the mode splitting inherent to coupled systems, determine the mode symmetry, and locally probe the coupling length with an accuracy of +/- 50 nm. A systematic study of directional couplers with different waveguide widths, coupling gaps, and e-beam exposure doses is reported in order to verify the results across a wider parameter space. (C) 2008 American Institute of Physics.

Optical vortex trap for resonant confinement of metal nanoparticles

M. Dienerowitz, M. Mazilu, P. J. Reece, T. F. Krauss, K. Dholakia

Optics Express 16 (7) 4991-4999 (2008).

The confinement and controlled movement of metal nanoparticles and nanorods is an emergent area within optical micromanipulation. In this letter we experimentally realise a novel trapping geometry near the plasmon resonance using an annular light field possessing a helical phasefront that confines the nanoparticle to the vortex core (dark) region. We interpret our data with a theoretical framework based upon the Maxwell stress tensor formulation to elucidate the total forces upon nanometric particles near the particle plasmon resonance. Rotation of the particle due to orbital angular momentum transfer is observed. This geometry may have several advantages for advanced manipulation of metal nanoparticles. (C) 2008 Optical Society of America.

Optical deflection and sorting of microparticles in a near-field optical geometry

R. F. Marchington, M. Mazilu, S. Kuriakose, V. Garces-Chavez, P. J. Reece, T. F. Krauss, M. Gu, K. Dholakia

Optics Express 16 (6) 3712-3726 (2008).

Near- field optical micromanipulation permits new possibilities for controlled motion of trapped objects. In this work, we report an original geometry for optically deflecting and sorting micro- objects employing a total internal reflection microscope system. A small beam of laser light is delivered off- axis through a total internal reflection objective which creates an elongated evanescent illumination of light at a glass/ water interface. Asymmetrical gradient and scattering forces from this light field are seen to deflect and sort polystyrene microparticles within a fluid flow. The speed of the deflected objects is dependent upon their intrinsic properties. We present a finite element method to calculate the optical forces for the evanescent waves. The numerical simulations are in good qualitative agreement with the experimental observations and elucidate features of the particle trajectory. In the size range of 1 mu m to 5 mu m in diameter, polystyrene spheres were found to be guided on average 2.9 +/- 0.7 faster than silica spheres. The velocity increased by 3.0 +/- 0.5 mu ms(-1) per mu m increase in diameter for polystyrene spheres and 0.7 +/- 0.2 mu ms(-1) per mu m for silica. We employ this size dependence for performing passive optical sorting within a microfluidic chip and is demonstrated in the accompanying video. (C) 2008 Optical Society of America.

Dispersion control and slow light in slotted photonic crystal waveguides

A. Di Falco, L. O'Faolain, T. F. Krauss

Appl. Phys. Lett. 92 083501

Slotted photonic crystal waveguides combine the ability to confine light in air with the dispersion control available from photonic crystals. Here, we study the dependence of their dispersion properties on geometry, especially the slot width, and demonstrate slow light operation with group indices in excess of 100.

Efficient polarization diversity grating couplers in bonded InP-membrane

F. Van Laere, T. Stomeo, D. Taillaert, G. Roelkens, D. Van Thourbout, T. F. Krauss, R. Baets

IEEE Photonics Technology Letters 20 (01-Apr) 318-320 (2008).

We report on the design, fabrication, and measurement of compact and efficient grating couplers between fiber and nanophotonic waveguides, in a polarization diversity configuration. This approach allows for polarization-independent integrated circuits. The gratings and waveguides are fabricated in a benzocyclobutene (BCB)-bonded InP-membrane, with high vertical refractive index contrast. The coupling loss of the gratings is -3.3 dB and the measured polarization-dependent loss is 0.79 dB.

Photonic crystal laser with mode selective mirrors

S. A. Moore, L. O'Faolain, T. P. White, T. F. Krauss

Optics Express 16 (2) 1365-1370 (2008).

The mini-stopband (MSB) of a W3 line-defect photonic crystal waveguide is used as a mirror for a GaAs based quantum-dot laser. Single mode, continuous-wave lasing is demonstrated for broad area lasers up to a current of 125 mA (2.7 x laser threshold), which demonstrates the high degree of mode selectivity of the MSB mirror. FDTD calculations indicate that optimisation of the mirror interface could lead to a further fourfold increase in reflectivity resulting in significantly reduced thresholds. (C) 2008 Optical Society of America.

Ultracompact and low-power optical switch based on silicon photonic crystals

D. M. Beggs, T. P. White, L. O'Faolain, T. F. Krauss

Optics Letters 33 (2) 147-149 (2008).

Switching light is one of the most fundamental functions of an optical circuit. As such, optical switches are a Major research topic in photonics, and many types of switches have been realized. Most optical switches operate by imposing a phase shift between two sections of the device to direct light from one port to another, or to switch it on and off, the major constraint being that typical refractive index changes are very small. Conventional solutions address this issue by making long devices, thus increasing the footprint, or by using resonant enhancement, thus reducing the bandwidth. We present a slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2 mu m. (c) 2008 Optical Society of America.

Polarized quantum dot emission from photonic crystal nanocavities studied under mode-resonant enhanced excitation

R. Oulton, B. D. Jones, S. Lam, A. R. A. Chalcraft, D. Szymanski, D. O'Brien, T. F. Krauss, D. Sanvitto, A. M. Fox, D. M. Whittaker, M. Hopkinson, M. S. Skolnick

Optics Express 15 (25) 17221-17230 (2007).

We study the linear polarization of the emission from single quantum dots embedded in an L3 defect nanocavity in a two-dimensional photonic crystal. By using narrow linewidth optical excitation in resonance with higher-order modes, we are able to achieve strong quantum dot emission intensity whilst reducing the background from quantum dots in the surrounding lattice. We find that all the dots observed emit very strongly linearly polarized light of the same orientation as the closest mode, despite the fact that these quantum dots may be spectrally detuned by several times the mode linewidth. We discuss the coupling mechanisms which may explain this behavior. (C) 2007 Optical Society of America.

Dependence of extrinsic loss on group velocity in photonic crystal waveguides

L. O'Faolain, T. P. White, D. O'Brien, X. D. Yuan, M. D. Settle, T. F. Krauss

Optics Express 15 (20) 13129-13138 (2007).

We examine the effects of disorder on propagation loss as a function of group velocity for W1 photonic crystal (PhC) waveguides. Disorder is deliberately and controllably introduced into the photonic crystal by pseudo-randomly displacing the holes of the photonic lattice. This allows us to clearly distinguish two types of loss. Away from the band-edge and for moderately slow light ( group velocity c/20-c/30) loss scales sublinearly with group velocity, whereas near the band-edge, reflection loss increases dramatically due to the random and local shift of the band-edge. The optical analysis also shows that the random fabrication errors of our structures, made on a standard e-beam lithography system, are below 1 nm root mean square. (C) 2007 Optical Society of America.

The resolution of optical traps created by light induced dielectrophoresis (LIDEP)

S. L. Neale, M. Mazilu, J. I. B. Wilson, K. Dholakia, T. F. Krauss

Optics Express 15 (20) 12619-12626 (2007).

Light induced dielectrophoresis ( LIDEP) is a variant of the dielectrophoresis ( DEP) mechanism that has been used for some time to manipulate particles in a microfluidic environment. Rather than relying on lithographically created contacts to generate the required electrical fields, the electrical contacts in LIDEP are created through the selective illumination of a photoconductor. The key question we address is how microscopic traps created via LIDEP compare to optical traps based on the gradient force, in terms of power required and trap stiffness achieved, as well as the size resolution of such a trap. We highlight the complex interplay between optical power and resolution with electrical parameters, such as the electrical resistance and applied AC Voltage. We show that for a spotsize of five micrometres and larger, particles can indeed be trapped with low power. We use trap stiffness per mW to compare LIDEP with an optical trap and show that our system is 470 +/- 94 times stiffer per mW than a conventional optical trap, with no loss of resolution. We also discuss the difficulties of achieving trapping at smaller spot sizes, and that the sub-micron resolution possible with gradient force trapping is very difficult to realise with LIDEP. (C) 2007 Optical Society of America.

Coupling into slow-mode photonic crystal waveguides

J. P. Hugonin, P. Lalanne, T. P. White, T. F. Krauss

Optics Letters 32 (18) 2638-2640 (2007).

We study efficient injectors for coupling light from z-invariant ridge waveguides into slow Bloch modes of single-row defect photonic crystal waveguides. Two-dimensional vectorial computations performed with a Bloch mode theory approach predict that very high efficiencies 90 percent can be achieved for injector lengths of only a few wavelengths in length, even for small group velocities in the range of c/100–c/400. This result suggests that photonic crystal devices operating with slow waves can be interfaced with classical waveguides without sacrificing compactness.

Tunable optical delay using photonic crystal heterostructure nanocavities

D. O'Brien, A. Gomez-Iglesias, M. D. Settle, A. Michaeli, M. Salib, T. F. Krauss

Physical Review B 76 (11) 115110 (2007).

We demonstrate tunable optical delay in a coupled resonator structure consisting of a chain of three heterostructure nanocavities. The group index of the device is measured using an interferometric technique, and the tuning is accomplished by the use of an optical pump that selectively heats one of the cavities in the array, altering the coupling and so changing the delay of the system. The speed of light in the device is varied over a range from c/75 to c/120 using effective pump powers below 100 mu W.

Compact polarization rotators for integrated polarization diversity in InP-based waveguides

D. M. Beggs, M. Midrio, T. F. Krauss

Optics Letters 32 (15) 2176-2178 (2007).

We present the design, fabrication, and operation of a polarization converter based on angled waveguides in the InP/InGaAsP material system. By combining design elements from mode evolution and birefringent devices, the total device length is kept short (less than 50 mu m) and the insertion efficiency high at 81%+/- 19%, which corresponds to an insertion loss of 1 dB. Devices operate broadband, i.e., the polarization conversion exceeds 15 dB over a 100 nm wavelength range. A polarization rotator with these specifications is a prime candidate for use in an integrated polarization diversity scheme. (C) 2007 Optical Society of America..

Silicon based organic semiconductor laser

A. E. Vasdekis, S. A. Moore, A. Ruseckas, T. F. Krauss, I. D. W. Samuel, G. A. Turnbull

Applied Physics Letters 91 (5) 51124 (2007).

The authors demonstrate silicon based visible lasers as potential optical interconnects by combining silicon processed resonators and solution processed light-emitting polymers. The high refractive index and absorption coefficient of silicon at these wavelengths were addressed by developing distributed Bragg reflector resonators on a silicon-on-insulator substrate. The performance of the hybrid structure was characterized and analyzed in comparison to an all-silica counterpart and mechanisms for controlling the number of longitudinal modes and for tuning the emission wavelength were explored. (c) 2007 American Institute of Physics.

Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry

A. Gomez-Iglesias, D. O'Brien, L. O'Faolain, A. Miller, T. F. Krauss

Applied Physics Letters 90 (26) 261107 (2007).

The authors report a direct, single-shot measurement of the group index profile of photonic crystal waveguides, combining spectral interferometry with Fourier transform analysis. This technique's versatility allows them to resolve subtle changes in dispersion and to quantify the slow light effect at the photonic crystal waveguide mode cutoff. For a waveguide 99 mu m long, they measure a group index up to 85, whereas for lengths of 397 and 695 mu m, they measure maximum values of 30 and 25, respectively. These results show the relationship between transmission characteristics and the maximum group delay observed in photonic crystals. (c) 2007 American Institute of Physics.

Mode structure of the L3 photonic crystal cavity

A. R. A. Chalcraft, S. Lam, D. O'Brien, T. F. Krauss, M. Sahin, D. Szymanski, D. Sanvitto, R. Oulton, M. S. Skolnick, A. M. Fox, D. M. Whittaker, H. Y. Liu, M. Hopkinson

Applied Physics Letters 90 (24) 241117 (2007).

The authors investigate the multiple confined modes of GaAs L3 photonic crystal air-bridge cavities, using single layers of InAs quantum dots as active internal light sources. Theoretical results for the energies, quality factors, and emission polarizations of the first five modes are compared to experimental data for cavities with lattice periods ranging from 240 to 270 nm. The authors also present in-plane field distributions for each mode. In addition to the well-known quality factor improvement of the fundamental mode, they show that outward displacement of the end-holes selectively redshifts modes with large end-hole-field overlaps, thus reordering the modes. (c) 2007 American Institute of Physics.

Slow light in photonic crystal waveguides

T. F. Krauss

Journal Of Physics D-applied Physics 40 (9) 2666-2670 (2007).

The physical principles behind the phenomenon of slow light in photonic crystal waveguides, as well as their practical limitations, are discussed and put into context. This includes the nature of slow light propagation, its bandwidth limitation, the scaling of linear and nonlinear interactions with the slowdown factor as well as issues such as losses, coupling into and the tuning of slow modes. Applications in all-optical signal processing appear to be the most promising outcome of the phenomena discussed.

Design and fabrication of high-efficiency fibre couplers for nanophotonic devices

D. M. Beggs, M. Ayre, D. F. G. Gallagher, T. F. Krauss

Microelectronic Engineering 84 (05-Aug) 1446-1449 (2007).

A dual waveguide heterostructure device that couples light from single mode optical fibres into nanophotonic devices with an overall modelled efficiency of 81% has been investigated. A four-stage taper design for the ridge waveguide which allows for a total device length of 610 mu m is described. The fabrication of the ridge waveguide using chemically-assisted ion-beam etching has been optimised. (c) 2007 Elsevier B.V. All rights reserved.

Compact slanted grating couplers between optical fiber and InP-InGaAsP waveguides

F. Van Laere, M. V. Kotlyar, D. Taillaert, D. Van Thourhout, T. F. Krauss, R. Baets

IEEE Photonics Technology Letters 19 (05-Aug) 396-398 (2007).

We present theoretical and experimental results for compact slanted gratings for vertical coupling between single-mode fiber and InP-InGaAsP waveguides. The maximum calculated coupling efficiency is 59%. We have measured a coupling efficiency of 16% for a 10-mu m-long slanted grating.

Dynamics of a two-state quantum dot laser with saturable absorber

E. A. Viktorov, M. A. Cataluna, L. O'Faolain, T. F. Krauss, W. Sibbett, E. U. Rafailov, P. Mandel

Applied Physics Letters 90 (12) 121113 (2007).

The authors study the regime of self-pulsations in a two-state quantum dot laser with saturable absorber. Experiments demonstrate and theory explains the appearance of self-pulsations at low relaxation oscillation frequency. The system exhibits a period doubling sequence of bifurcations leading to chaos. (c) 2007 American Institute of Physics.

Self-collimating photonic crystal polarization beam splitter

V. Zabelin, L. A. Dunbar, N. Le Thomas, R. Houdre, M. V. Kotlyar, L. O'Faolain, T. F. Krauss

Optics Letters 32 (5) 530-532 (2007).

We present theoretical and experimental results of a polarization splitter device that consists of a photonic crystal (PhC) slab, which exhibits a large reflection coefficient for TE and a high transmission coefficient for TM polarization. The slab is embedded in a PhC tile operating in the self-collimation mode. Embedding the polarization-discriminating slab in a PhC with identical lattice symmetry suppresses the in.-plane diffraction losses at the PhC-non-PhC interface. The optimization of the PhC-non-PhC interface is thereby decoupled from the optimization of the polarizing function. Transmissions as high as 35% for TM- and 30% for TE-polarized light are reported. (c) 2007 Optical Society of America.

Coupled photonic crystal heterostructure nanocavities

D. O'Brien, M. D. Settle, T. Karle, A. Michaeli, M. Salib, T. F. Krauss

Optics Express 15 (3) 1228-1233 (2007).

We show the first experimental demonstration of multiple heterostructure photonic crystal cavities being coupled together to form a chain of coupled resonators with up to ten cavities. This system allows us to engineer the group velocity of light over a wide range. Devices were fabricated using 193 nm deep UV lithography and standard silicon processing technology. Structures were analysed using both coupled resonator and photonic bandstructure theory, and we highlight the discrepancies arising from subtle imperfections of the fabricated structure. (c) 2007 Optical Society of America.

Flatband slow light in photonic crystals featuring spatial pulse compression and terahertz bandwidth

M. D. Settle, R. J. P. Engelen, M. Salib, A. Michaeli, L. Kuipers, T. F. Krauss

Optics Express 15 (1) 219-226 (2007).

Paradoxically, slow light promises to increase the speed of telecommunications in novel photonic structures, such as coupled resonators [1] and photonic crystals [2,3]. Apart from signal delays, the key consequence of slowing light down is the enhancement of light-matter interactions. Linear effects such as refractive index modulation scale linearly with slowdown in photonic crystals [3], and nonlinear effects are expected to scale with its square [4]. By directly observing the spatial compression of an optical pulse, by factor 25, we confirm the mechanism underlying this square scaling law. The key advantage of photonic structures over other slow light concepts is the potentially large bandwidth, which is crucial for telecommunications [5]. Nevertheless, the slow light previously observed in photonic crystals [2,3,6,7] has been very dispersive and featured narrow bandwidth. We demonstrate slow light with a bandwidth of 2.5 THz and a delay- bandwidth product of 30, which is an order of magnitude larger than any reported so far.

Low-loss propagation in photonic crystal waveguides

L. O'Faolain, X. Yuan, D. Mcintyre, S. Thoms, H. Chong, R. M. De la Rue, T. F. Krauss

Electronics Letters 42 (25) 1454-1455 (2006).

The fabrication and characterisation of high-quality silicon membrane photonic crystals are reported. The etching process was carefully optimised to give holes with very smooth and vertical sidewalls, resulting in propagation, with a minimum loss of 4.1 +/- 0.9 dB/cm in a single line defect (W1) waveguide.

Cross-correlation timing jitter measurement of high power passively mode-locked two-section quantum-dot lasers

J. P. Tourrenc, S. O'Donoghue, M. T. Todaro, S. P. Hegarty, M. B. Flynn, G. Huyet, J. G. Mcinerney, L. O'Faolain, T. F. Krauss

IEEE Photonics Technology Letters 18 (21-24) 2317-2319 (2006).

High-power picosecond pulses with very low timing jitter have been produced by monolithic passively mode-locked quantum-dot lasers. 17 GHz trains of 1.3 ps near-Gaussian pulses were demonstrated at an emission wavelength of 1.285 mu m and peak power of 150 mW. Time domain measurements using optical cross correlation showed pulse-to-pulse timing jitter as low as 20 fs/pulse cycle.

Ultrafast monolithic semiconductor-polymer Kerr-lens saturable absorber

M. B. Flynn, T. F. Krauss

Journal Of The Optical Society Of America B-optical Physics 23 (11) 2291-2294 (2006).

We propose an ultrafast Kerr-lens saturable absorber, which is suitable for incorporating monolithically into semiconductor waveguides. We propose suitable materials and geometries for device fabrication. We study the saturable absorption using numerical methods and consider the effect of such a hard Kerr-lens configuration on a monolithic passively mode-locked semiconductor diode device and the possibility of producing pulses of the order of hundreds of femtoseconds. (c) 2006 Optical Society of America.

Ultrafast reflectivity modulation in AlxGa1-xAs/InyAlxGa1-x-yAs multiple quantum well photonic crystal waveguides

A. Z. Garcia-Deniz, P. Murzyn, A. M. Fox, D. O. Kundys, J. P. R. Wells, M. Whittaker, M. S. Skolnick, T. F. Krauss, J. S. Roberts

Physical Review B 74 (16) 165327 (2006).

We report an ultrafast optical tuning of the reflectivity of AlxGa1-xAs/InyAlxGa1-x-yAs multiple quantum well photonic crystal waveguides using a reflection geometry, pump-probe technique. Nonlinear shifts of the photonic resonances between 800 and 900 nm are demonstrated with a measured response time of similar to 300-400 fs. The nonlinear shift is attributed to virtual carriers, with the response time determined by the pump pulse duration. This demonstrates ultrafast photonic switching in a high-contrast photonic structure based on III-V quantum wells.

Diode pumped distributed Bragg reflector lasers based on a dye-to-polymer energy transfer blend

A. E. Vasdekis, G. Tsiminis, J. C. Ribierre, L. O'Faolain, T. F. Krauss, G. A. Turnbull, I. D. W. Samuel

Optics Express 14 (20) 9211-9216 (2006).

We report the demonstration of a compact, all-solid-state polymer laser system comprising of a Gallium Nitride ( GaN) semiconductor diode laser as the pump source. The polymer laser was configured as a surface emitting, distributed Bragg reflector laser ( DBR), based on a novel energy transfer blend of Coumarin 102 and the conjugated polymer poly( 2-methoxy-5-( 2'-ethylhexyloxy)-1,4-phenylene vinylene). In this configuration, diode pumping was possible both due to the improved quality of the resonators and the improved harvesting of the diode laser light. (c) 2006 Optical Society of America.

Reduced surface sidewall recombination and diffusion in quantum-dot lasers

S. A. Moore, L. O'Faolain, M. A. Cataluna, M. B. Flynn, M. V. Kotlyar, T. F. Krauss

IEEE Photonics Technology Letters 18 (17-20) 1861-1863 (2006).

We examine the surface recombination rate in quantum-dot semiconductor lasers and determine the diffusion length (1.0 mu m) and, for the first time, provide a value for surface recombination velocity (5 X 10(4) cm/s) in quantum-dot material. As a result of strong carrier confinement in the dots, these values are much lower than in comparable quantum-well lasers (5 X 10(5) cm/s and 5 mu m, respectively) allowing the creation of narrow (2-3 mu m wide) lasers with comparable threshold currents to those of broad area devices.

Monolithic integration of microfluidic channels and semiconductor lasers

S. J. Cran-Mcgreehin, K. Dholakia, T. F. Krauss

Optics Express 14 (17) 7723-7729 (2006).

We present a fabrication method for the monolithic integration of microfluidic channels into semiconductor laser material. Lasers are designed to couple directly into the microfluidic channel, allowing submerged particles pass through the output beams of the lasers. The interaction between particles in the channel and the lasers, operated in either forward or reverse bias, allows for particle detection, and the optical forces can be used to trap and move particles. Both interrogation and manipulation are made more amenable for lab-on-a-chip applications through monolithic integration. The devices are very small, they require no external optical components, have perfect intrinsic alignment, and can be created with virtually any planar configuration of lasers in order to perform a variety of tasks. Their operation requires no optical expertise and only low electrical power, thus making them suitable for computer interfacing and automation. Insulating the pn junctions from the fluid is the key challenge, which is overcome by using photo-definable SU8-2000 polymer. (c) Optical Society of America.

Control of the nonlinear carrier response time of AlGaAs photonic crystal waveguides by sample design

P. Murzyn, A. Z. Garcia-Deniz, D. O. Kundys, A. M. Fox, J. P. R. Wells, D. M. Whittaker, M. S. Skolnick, T. F. Krauss, J. S. Roberts

Applied Physics Letters 88 (14) 141104 (2006).

We have used reflection geometry pump-probe spectroscopy to investigate the free carrier response time of AlGaAs high refractive index contrast one-dimensional photonic crystal waveguides. We have observed pump-induced shifts of photonic resonances in the near infrared spectral region, and have studied the dependence of the decay time on the sample parameters. We find that the response time can be varied from 8 to 33 ps by changing the structure period and etch depth. This, combined with the large changes observed in the reflectivity, demonstrates excellent potential for application as ultrafast photonic switches with a controllable recovery time.

Compact and efficient fibre-to-waveguide grating couplers in InP-membrane

F. Van Laere, M. Ayre, D. Taillaert, D. Van Thourhout, T. F. Krauss, R. Baets

Electronics Letters 42 (6) 343-345 (2006).

The design and fabrication of compact grating couplers in InP-membrane waveguides for coupling to standard singlemode fibre is presented. A wafer bonding technique is applied to achieve a high vertical index contrast. Coupling efficiencies of 30% were measured on first fabricated couplers.

Low loss silicon on insulator photonic crystal waveguides made by 193nm optical lithography

M. Settle, M. Salib, A. Michaeli, T. F. Krauss

Optics Express 14 (6) 2440-2445 (2006).

We show the successful fabrication and operation of photonic crystal waveguides on SOI, with lower silicon dioxide cladding remaining, using 193 nm DUV lithography. We demonstrate that 193 nm lithography gives more process latitude, allowing a wider range of periods and hole diameters to be printed, as well as reducing the optical proximity effect to a minimum. The smallest period/hole size variation printed successfully was 280 nm and 150 nm, which is very promising for ambitious future designs. Lowest losses obtained were 14.2 +/- 2.0 dB/ cm for a W1 waveguide in a 400 nm lattice with an r/a of 0.25 at a frequency of 0.257 a/lambda, which approaches the best losses reported for air-bridge type W1s. (c) 2006 Optical Society of America.

Compact and integrated 2-D photonic crystal super-prism filter-device for wavelength demultiplexing applications

A. S. Jugessur, A. Bakhtazad, A. G. Kirk, L. Wu, T. F. Krauss, R. M. De La Rue

Optics Express 14 (4) 1632-1642 (2006).

A two-dimensional photonic crystal (PhC) super-prism integrated with one-dimensional photonic crystal microcavity filters has been designed using the plane wave expansion (PWE) and 2-D Finite Difference Time Domain (FDTD) methods based on Silicon-on-Insulator (SOI) technology. The super-prism operates as a coarse spatial filter with an average response bandwidth of 60 nm, while the 1-D PhC microcavity filters operate as narrow band-pass transmission filters with an average filter response line-width of 10 nm. This work demonstrates the simultaneous operation of two photonic devices for de-multiplexing applications on a single platform that could be useful in future Photonic Crystal Integrated Circuits (PCICs). (c) 2006 Optical Society of America.

Fabrication of photonic crystals using a spin-coated hydrogen silsesquioxane hard mask

L. O'Faolain, M. V. Kotlyar, N. Tripathi, R. Wilson, T. F. Krauss

Journal Of Vacuum Science & Technology B 24 (1) 336-339 (2006).

We present a method for creating a hard mask for the dry etching of microphotonic structures and devices. We demonstrate that spin-on glass [hydrogen silsesquioxane (HSQ)] has sufficient dry etch resistance to allow the creation of high-quality, deeply etched photonic crystals. Furthermore, HSQ is a more favorable hard mask for the creation of active devices than plasma-enhanced chemical-vapor deposition (PECVD) silica, as less damage is incurred. It is also an economic and convenient replacement for PEVCD in photonic crystal fabrication. We examine this method and show that it can create photonic crystals of equivalent quality to those created using PEVCD masking. (c) 2006 American Vacuum Society.

Integrated monolithic optical manipulation

S. Cran-Mcgreehin, T. F. Krauss, K. Dholakia

Lab On A Chip 6 (9) 1122-1124 (2006).

We present a new approach to optical manipulation that integrates microfluidic channels directly onto semiconductor laser material creating a compact integrated optical trap that requires no alignment and is wholly portable.

Ultrafast nonlinear response of AlGaAs/InAlGaAs MQW photonic crystal waveguides

P. Murzyn, A. Z. Garcia-Deniz, A. M. Fox, J. P. R. Wells, D. M. Whittaker, M. S. Skolnick, T. F. Krauss, J. S. Roberts

Physica Status Solidi A-applications And Materials Science 202 (14) 2653-2656 (2005).

We report ultrafast optical tuning of the reflection properties of MQW AlGaAs/lnAlGaAs 1D photonic crystal waveguides using a reflection geometry, pump-probe technique. Very fast pump-induced shifts of photonic resonances between 800-900 nm by up to 18 nm nanometers are demonstrated. This is followed by a fast decay which varies from 5 to 30 ps depending on the sample dimensions. Together with the maximum reflectivity change of Delta R/R > 30%, this demonstrates excellent potential for application of photonic crystals as ultrafast nonlinear switches with decay times depending on the structure period.

Effect of gain localization in circular-grating distributed feedback lasers

G. A. Turnbull, A. Carleton, A. Tahraouhi, T. F. Krauss, I. D. W. Samuel, G. F. Barlow, K. A. Shore

Applied Physics Letters 87 (20) 201101 (2005).

We explore the influence of gain localization on the lasing performance of circular-grating distributed feedback (CDFB) lasers. The effect is studied in an optically pumped CDFB laser resonator based on a waveguide of the conjugated polymer poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-phenylene vinylene]. Variations in lasing threshold and slope efficiency are determined as a function of the radius of the optical excitation. The experimental lasing results are compared with predictions from a theoretical analysis based on an adaptation of the transfer matrix method. We find that a strong localization of the gain near the center of the CDFB laser can lead to both a substantial reduction in threshold and increase in output efficiency. As the excitation radius changes from a 90 to a 15 mu m radius, the threshold energy decreases from 5.3 to 0.29 nJ, and the surface-emitted output efficiency increases by an order of magnitude. A simple model is developed that confirms that the significant reduction in threshold can be explained by an enhanced overlap of the population inversion with the resonant mode. (C) 2005 American Institute of Physics.

Low tuning current semiconductor coupled-cavity lasers incorporating Bragg reflectors

D. H. Brown, M. B. Flynn, L. O'Faolain, T. F. Krauss

IEEE Photonics Technology Letters 17 (11) 2262-2264 (2005).

Edge-emitting coupled-cavity semiconductor lasers employing a deeply etched Bragg reflector at one facet have been realized. The sidemode suppression ratio (SMSR) and wavelength tuning were investigated as a function of low level current injection. A single-mode tuning range of 3 nm centered around 977 nm displaying an SMSR better than 20 dB was obtained for tuning currents in the milliampere range.

Influence of grating characteristics on the operation of circular-grating distributed-feedback polymer lasers

G. A. Turnbull, A. Carleton, G. F. Barlow, A. Tahraouhi, T. F. Krauss, K. A. Shore, I. D. W. Samuel

Journal Of Applied Physics 98 (2) 23105 (2005).

We explore the influence of grating characteristics on the lasing performance of polymer circular-grating distributed-feedback lasers. A range of circular-grating sizes and profiles were fabricated on a single silica substrate, which was coated with a thin film of the conjugated polymer poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-phenylene vinylene]. Variations in lasing threshold and surface-emitted slope efficiency were determined as a function of grating outer diameter and duty cycle. The experimental lasing results are compared with predictions from a theoretical analysis based on an adaptation of the transfer matrix method. We find that an outer diameter of at least 200 mu m is required to minimize the threshold and optimize the surface-emitted slope efficiency. A groove-to-period duty cycle of similar to 25% gives the lowest lasing thresholds by optimizing the in-plane feedback. We also find that the structure of the polymer-air surface varies substantially with substrate duty cycle, which has implications for optimum device design. (c) 2005 American Institute of Physics.

Experimental verification of numerically optimized photonic crystal injector, Y-splitter, and bend

M. Ayre, T. J. Karle, L. J. Wu, T. Davies, T. F. Krauss

IEEE Journal On Selected Areas In Communications 23 (7) 1390-1395 (2005).

We present the experimental measurement of a photonic crystal (PhC) device comprising an injector, Y-splitter, and 60 degrees bend. The complete device consists of a 9-mu m-long injector tapering down from 5 pin into a triangular-lattice-of-holes single-line defect waveguide with period a = 430 mn and 36.2% air filling factor (corresponding to a radius over period (r/a) ratio of 0.30), an optimized Y-junction, 60 degrees bend and output injectors, with a total device footprint of 30 mu m. This is etched into a GaAs/AlGaAs heterostructure using chlorine/argon chemically assisted ion beam etching (CAME). An erbium-doped fiber amplifier (EDFA)-based source and Fabry-Perot technique are used to characterize the device. The device displays a bandwidth of approximately 110 nm in the 1.55 mu m window, and a transmission of 70% relative to the same length of 5-mu m-wide waveguide. This is compared with three-dimensional finite-difference time-domain (3-D FDTD) results, which have a bandwidth and transmission of 120 nm and 75%, respectively. The highlight of this paper is the close agreement of the numerically optimized complete microcircuit with its experimental equivalent, and the significant improvement in bandwidth over previous work on Y-junctions.

All-optical control of microfluidic components using form birefringence

S. L. Neale, M. P. Macdonald, K. Dholakia, T. F. Krauss

Nature Materials 4 (7) 530-533 (2005).

Reflection and emission of Brillouin zone edge states for active photonic crystal waveguides

A. D. Bristow, A. Garcia-Deniz, A. M. Fox, D. M. Whittaker, M. S. Skolnick, T. F. Krauss, M. Hopkinson

Journal Of Optics A-pure And Applied Optics 7 (6) S270-S275 (2005).

A one-dimensional photonic crystal is patterned into an active planar waveguide containing multiple InGaAs quantum wells. External coupling reflectivity is used to map out the photonic band structure, revealing a clear anti-crossing at similar to 1.37 eV in the 45 degrees TM polarized spectrum. The band structure is compared to and confirmed by scattering-matrix calculations. Photoluminescence measurements are performed in the same geometry on the patterned and unpatterned regions of the sample. The latter shows conventional quantum-well emission and GaAs emission with increasing pump power. In contrast, the spectra from the patterned sample contain a sharp mode from the dielectric-like band of the anti-crossing, corresponding to the Brillouin zone edge. Power-dependent excitation shows this mode to be strongly super-linear, indicative of band edge lasing.

Compact polarization converter in InP-based material

M. V. Kotlyar, L. Bolla, M. Midrio, L. O'Faolain, T. F. Krauss

Optics Express 13 (13) 5040-5045 (2005).

We present a polarization converter using one-dimensional grating principles. The device is based on slanted slots etched deeply into an InP/InGaAsP heterostructure. Almost complete polarization conversion, with a 14 dB extinction ratio, is observed for a device less than 2 mu m long. (C) 2005 Optical Society of America

Local probing of Bloch mode dispersion in a photonic crystal waveguide

R. J. P. Engelen, T. J. Karle, H. Gersen, J. P. Korterik, T. F. Krauss, L. Kuipers, N. F. Van Hulst

Optics Express 13 (12) 4457-4464 (2005).

The local dispersion relation of a photonic crystal waveguide is directly determined by phase-sensitive near-field microscopy. We readily demonstrate the propagation of Bloch waves by probing the band diagram also beyond the first Brillouin zone. Both TE and TM polarized modes were distinguished in the experimental band diagram. Only the TE polarized defect mode has a distinctive Bloch wave character. The anomalous dispersion of this defect guided mode is demonstrated by local measurements of the group velocity. The measured dispersion relation and measured group velocities are both in good agreement with theoretical calculations. (C) 2005 Optical Society of America.

Electrooptic tuning of InP-based microphotonic Fabry-Perot filters

M. V. Kotlyar, L. O'Faolain, A. B. Krysa, T. F. Krauss

Journal Of Lightwave Technology 23 (6) 2169-2174 (2005).

This paper presents the experimental results for compact (20- and 40-mu m-long) electrooptically tuned deeply etched Fabry-Perot (F-P) filters in InP-based material. Both the quantum-confined Stark effect (QCSE) and carrier-injection (C-I) effects were implemented to achieve tunability of these microcavity filters. Red and blue shifts of the transmission peaks in the order of 1 to 2 nm were observed for both effects, and the limitations on C-I due to thermal effect are clearly demonstrated and discussed. The advantages and disadvantages of both tuning mechanisms are highlighted.

Integrated chirp compensation in a monolithic passively mode-locked semiconductor diode laser

M. B. Flynn, L. O'Faolain, T. F. Krauss

Applied Physics Letters 86 (22) 221104 (2005).

We present traveling wave numerical simulations of passively mode-locked diode lasers that incorporate chirp compensation monolithically using a Fabry-Perot interferometer, which provides a negative group delay. Numerical results show pulse compression of up to 65%. Time-frequency domain study of the pulses reveal a significant reduction in chirp across the pulse. Limitations of this simple linear chirp compensation scheme are examined. (c) 2005 American Institute of Physics.

Dual lattice photonic-crystal beam splitters

L. J. Wu, M. Mazilu, J. F. Gallet, T. F. Krauss

Applied Physics Letters 86 (21) 211106 (2005).

Light propagation in photonic crystals (PhC) is both sensitive to incident angle and wavelength. By combining two different PhC lattices, we utilize this effect to demonstrate a wavelength-dependent beam splitter with enhanced angular separation. The first lattice acts as a superprism that separates the incoming light according to wavelength, whereas the second lattice acts as an angular amplifier. We obtain 90 degrees angular separation for two wavelengths separated by 70 nm (1300 nm regime) in a structure that is less than 10 mu m long. (c) 2005 American Institute of Physics.

Control at the quantum level

T. F. Krauss

Science 308 (5725) 1122-1123 (2005).

Direct observation of Bloch harmonics and negative phase velocity in photonic crystal waveguides

H. Gersen, T. J. Karle, R. J. P. Engelen, W. Bogaerts, J. P. Korterik, N. F. Van Hulst, T. F. Krauss, L. Kuipers

Physical Review Letters 94 (12) 123901 (2005).

The eigenfield distribution and the band structure of a photonic crystal waveguide have been measured with a phase-sensitive near-field scanning optical microscope. Bloch modes, which consist of more than one spatial frequency, are visualized in the waveguide. In the band structure, multiple Brillouin zones due to zone folding are observed, in which positive and negative dispersion is seen. The negative slopes are shown to correspond to a negative phase velocity but a positive group velocity. The lateral mode profile for modes separated by one reciprocal lattice vector is found to be different.

Kerr-effect-induced passive Q switching of a monolithic semiconductor diode laser

M. B. Flynn, L. O'Faolain, T. F. Krauss

Journal Of The Optical Society Of America B-optical Physics 22 (4) 792-795 (2005).

We present a novel (to our knowledge) method to passively Q switch monolithic semiconductor diode lasers using a Bragg reflector filled with a polymer exhibiting the Kerr effect. We present numerical modeling of such devices that display Q switching. We numerically characterize the pulsed laser output in terms of repetition frequency, pulse power, and pulse energy. We also examine the influence of spontaneous emission strength on pulse dynamics. Advantages of this technique include higher repetition rates and very short pulses.(c) 2005 Optical Society of America.

Electrically tunable multiquantum-well InGaAsP-InGaAsP microphotonic filter

M. V. Kotlyar, L. O'Faolain, A. B. Krysa, T. F. Krauss

IEEE Photonics Technology Letters 17 (4) 837-839 (2005).

Planar waveguide-based Fabry-Perot microcavities of 20- and 40-mu m length have been studied. The cavity resonance has been tuned via carrier injection by a maximum of 1.9 nm at a wavelength of 1.3 mu m with only 6 mW of tuning power. The interplay between electrical and thermal tuning is highlighted.

Reprocessing of thermally oxidized aluminum arsenide (AlAs) in epitaxial multilayers without delamination

L. Hobbs, I. Eddie, G. Erwin, A. C. Bryce, R. M. De la Rue, J. S. Roberts, T. F. Krauss, D. W. Mccomb, M. Mackenzie

Journal Of Electronic Materials 34 (3) 232-239 (2005).

Annealing or processing of AlAs that has been subjected to a wet thermal oxidation process can result in severe delamination of material at the oxidation front. This paper reports a procedure for preventing this delamination and presents a possible cause for the delamination.

Real-space observation of ultraslow light in photonic crystal waveguides

H. Gersen, T. J. Karle, R. J. P. Engelen, W. Bogaerts, J. P. Korterik, N. F. Van Hulst, T. F. Krauss, L. Kuipers

Physical Review Letters 94 (7) 73903 (2005).

We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least 3 ps, movement of this field is hardly discernible: its group velocity would be at most c/1000. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.

Ultrafast nonlinear tuning of the reflection properties of AlGaAs photonic crystal waveguides by two-photon absorption

A. D. Bristow, D. O. Kundys, A. Z. Garcia-Deniz, J. P. R. Wells, A. M. Fox, M. S. Skolnick, D. M. Whittaker, A. Tahraoui, T. F. Krauss, J. S. Roberts

Journal Of Applied Physics 96 (9) 4729-4734 (2004).

We have studied the power-dependent wavelength shift of photonic coupling resonances of a two-dimensional photonic crystal waveguide by reflection geometry pump-probe measurements. The quadratic response is indicative of two-photon induced carrier creation, which alters the refractive index of the semiconductor core of the photonic lattice. A free-carrier model is used to simulate the phenomenon, giving values of the change in refractive index per unit carrier density that satisfactorily compare to values calculated for bulk AlGaAs under similar conditions. Time-resolved spectra are also presented, showing relaxation times of <10 ps which are consistent with surface recombination times in the patterned waveguide. (C) 2004 American Institute of Physics.

A Kerr mode-locked semiconductor laser: Design and theory

L. O'Faolain, M. B. Flynn, R. Wilson, T. F. Krauss

IEEE Journal Of Selected Topics In Quantum Electronics 10 (5) 1063-1069 (2004).

We propose a novel type of semiconductor laser mode-locked by the Kerr-effect induced refractive index change of a polymer. Such a device is shown to have the potential to produce dramatically shorter pulses than those devices currently in use.

An experimental and numerical study of Q-switched mode-locking in monolithic semiconductor diode lasers

M. B. Flynn, L. O'Faolain, T. F. Krauss

IEEE Journal Of Quantum Electronics 40 (8) 1008-1013 (2004).

We present experimental results of Q-switched mode-locking (QML) of a monolithic two-section semiconductor laser. We demonstrate tuning of the Q-switched envelope repetition rate with pumping current over the range of 0.4 to 1.6 GHz. Detailed numerical modeling is used to study a range of similar devices and to investigate the mechanisms and conditions for QML to occur. We also discuss design conditions for increasing the tuning up to 4 GHz.

High-aspect-ratio chemically assisted ion-beam etching for photonic crystals using a high beam voltage-current ratio

M. V. Kotlyar, L. O'Faolain, R. Wilson, T. F. Krauss

Journal Of Vacuum Science & Technology B 22 (4) 1788-1791 (2004).

We investigate etching conditions for photonic crystals (PhCs) in InGaAsP/InP and AlGaAs/GaAs using a new regime of CAME operation. We show that the beam voltage-current ratio is critical in obtaining high material/mask selectivity. For one-dimensional PhCs, i.e., air slots, selectivities of 22:1 and 50:1 were achieved in InP and GaAs, respectively, using a very high beam voltage (about 1500 V) and a low beam current (about 10 mA). Etched features were observed to be very smooth, i.e., edge roughness was low. Two-dimensional PhCs were etched in InGaAsP/InP under similar conditions achieving selectivities up to 27:1 and 34:1 for hole diameters of 170 and 270 nm, respectively. (C) 2004 American Vacuum Society.

Planar photonic crystal polarization splitter

L. J. Wu, M. Mazilu, J. F. Gallet, T. F. Krauss, A. Jugessur, R. M. De La Rue

Optics Letters 29 (14) 1620-1622 (2004).

The differential dispersion relation for the E and H modes (TM-like and TE-like, respectively) in planar photonic crystals is used to control the polarization-dependent propagation of light. E- and H-polarized beams were separated by 10degrees after propagating through a 20-mum-long planar photonic crystal in the wavelength range from 1250 to 1300 nm. The plane-wave expansion calculation matches well with the experimental results. This result represents the first demonstration, to our knowledge, of a polarization splitter realized in a planar photonic crystal configuration in the near-infrared wavelength range operating solely in transmission mode. (C) 2004 Optical Society of America.

Low-loss photonic crystal defect waveguides in InP

M. V. Kotlyar, T. Karle, M. D. Settle, L. O'Faolain, T. F. Krauss

Applied Physics Letters 84 (18) 3588-3590 (2004).

We have fabricated high-quality planar photonic crystal defect waveguides in InP/InGaAsP material. Using Fourier analysis of the Fabry-Perot fringes obtained in transmission, we derive the propagation losses. Values as small as 1.8 dB/mm for waveguides consisting of three rows of missing holes (W3) were measured. We believe that the reduction in losses is due to the high quality of etching carried out using a high beam voltage-current ratio regime of chemically assisted ion-beam etching. (C) 2004 American Institute of Physics.

Observation of pulse compression in photonic crystal coupled cavity waveguides

T. J. Karle, Y. J. Chai, C. N. Morgan, I. H. White, T. F. Krauss

Journal Of Lightwave Technology 22 (2) 514-519 (2004).

We demonstrate the compression of picosecond pulses using the large group velocity dispersion available from planar photonic crystal (PhC) coupled cavity waveguides (CCWs). A maximum pulsewidth reduction of 40%, from 1.91 to 1.17 ps, is achieved, in transmission through an 8-mum-long planar PhC waveguide. The equivalent dispersion value is >10(6) times larger than that available from standard single-mode fiber, at the pulse center wavelength of 1536 nm. The device performance is analyzed with the aid of both two-dimensional (2-D) eigenmode expansion and 2-D finite-difference time-domain (FDTD) models. The models included the material dispersion of the GaAs/AlGaAs heterostructure, into which the PhCs are etched, and show remarkable agreement with experiment.

Photonic crystals - Cavities without leaks

T. F. Krauss

Nature Materials 2 (12) 777-778 (2003).

Photonic crystal microcavities have been less able to trap light than their microsphere and microdisk counterparts. A new design concept shows how to stop the leaks.

Omnidirectional and compact light extraction from Archimedean photonic lattices

M. Rattier, H. Benisty, E. Schwoonb, C. .. Weisbuch, T. F. Krauss, C. J. M. Smith, R. Houdre, U. Oesterle

Appl. Phys. Lett. 83 (7) 1283-1285 (2003).

We address the issue of extracting light from a waveguide towards air in a compact way for randomly oriented guided waves. The goal is to enhance the extraction efficiency of light-emitting diodes while retaining planar processing. For incidence-angle-independent extraction, preferred lattice designs appear to possess a ring-shaped Fourier transform. We demonstrate this property for an Archimedean lattice. This system is the outer part of a resonant-cavity light-emitting diode. Data suggest that 40% extraction efficiency is at hand in a planar top-emitting device retaining its substrate.

Ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides

A. D. Bristow, J. P. R. Wells, W. H. Fan, A. M. Fox, M. S. Skolnick, D. M. Whittaker, A. Tahraoui, T. F. Krauss, J. S. Roberts

Applied Physics Letters 83 (5) 851-853 (2003).

We have used femtosecond pump-probe spectroscopy to study the ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides in the near-infrared spectral region. The modulation of the reflectivity spectra due to the refractive index change produced by photogenerated carriers was measured. We observed an instantaneous pump-induced shift in the wavelength of a photonic resonance at 882 nm with a fast decay time of approximate to8 ps. The magnitude of the reflectivity change was very large at wavelengths close to the photonic resonance, with a maximum value of DeltaR/R>30% at 877 nm. These results confirm the excellent potential of photonic crystal waveguides in ultrafast nonlinear switching applications. (C) 2003 American Institute of Physics.

Defect states and commensurability in dual-period AlxGa1-xAs photonic crystal waveguides

A. D. Bristow, D. M. Whittaker, V. N. Astratov, M. S. Skolnick, A. Tahraoui, T. F. Krauss, M. Hopkinson, M. P. Croucher, G. A. Gehring

Physical Review B 68 (3) 33303 (2003).

AlxGa1-xAs waveguides with one-dimensional dual-period patterning, consisting of a short period lattice of semiconductor stripes with periodically placed defects, are investigated by surface coupling reflectivity. Defect states, characterized by flat dispersions and enhanced optical density in the defect region, are observed within an energy gap arising from the short period modulation of the structure. When the defect width is incommensurate with the short period, strong additional features due to band folding are observed. The experimental spectra and dispersions are found to be in very good agreement with the predictions of scattering-matrix calculations.

Efficient photonic crystal Y-junctions

R. Wilson, T. J. Karle, I. Moerman, T. F. Krauss

Journal Of Optics A-pure And Applied Optics 5 (4) S76-S80 (2003).

A highly efficient Y-junction based on a planar photonic crystal (PhC) platform is presented. The PhC consists of a triangular array of holes etched into a GaAs/AlGaAs heterostructure, with a typical period of 322 nm and similar to35% fill factor. The Y-junction has smaller holes positioned at the centre of the junction, giving rise to very uniform splitting and high transmission. The performance is very encouraging, with experimental transmission of approximately 40% for each arm of the Y-splitter relative to a comparable single-defect PhC waveguide.

Coupled-mode theory and propagation losses in photonic crystal waveguides

S. Olivier, H. Benisty, C. Weisbuch, C. J. M. Smith, T. F. Krauss, R. Houdre

Optics Express 11 (13) 1490-1496 (2003).

Mode coupling phenomena, manifested by transmission mini-stopbands, occur in two-dimensional photonic crystal channel waveguides. The huge difference in the group velocities of the coupled modes is a new feature with respect to the classical Bragg reflection occurring, e. g., in distributed feedback lasers. We show that an adequate ansatz of the classical coupled-mode theory remarkably well accounts for this new phenomenon. The fit of experimental transmission data from GaAs-based photonic crystal waveguides then leads to an accurate determination of the propagation losses of both fundamental and higher, low-group-velocity modes. (C) 2003 Optical Society of America.

Beam steering in planar-photonic crystals: From superprism to supercollimator

L. J. Wu, M. Mazilu, T. F. Krauss

Journal Of Lightwave Technology 21 (2) 561-566 (2003).

We utilize the anomalous dispersion of planar-photonic crystals (PhCs) near the dielectric band edge to control the wavelength-dependent propagation of light. Light beams with up to 20degrees divergence were collimated over a 25-nm (1285 nm to 1310 nm) bandwidth using a triangular lattice. The super-prism phenomenon is demonstrated in the same configuration, simply by tuning the wavelength. Sources of loss are discussed. Both the plane-wave expansion calculation and finite-difference time-domain simulation match well with the experimental results. This is the first experimental demonstration of self-collimating phenomena in a PhC configuration.

Two-mode fringes in planar photonic crystal waveguides with constrictions: a probe that is sensitive to propagation losses

E. Schwoob, H. Benisty, S. Olivier, C. Weisbuch, C. J. M. Smith, T. F. Krauss, R. Houdre, U. Oesterle

Journal Of The Optical Society Of America B-optical Physics 19 (10) 2403-2412 (2002).

We analyze the transmission of planar photonic crystal channel waveguides, each of which consists of three missing rows in a triangular lattice of air holes and modified at both ends by constrictions. The structures are fabricated into a GaAs/AlGaAs heterostructure in which an internal source consisting of three layers of quantum dots is embedded. The constrictions induce peculiar spectral features that are used to improve the sensitivity of transmission measurements to propagation losses. Two effects are pointed out: (i) The constrictions act as mirrors, inducing Fabry-Perot fringes on the transmitted spectra, (ii) and the constrictions also induce a mode-mixing process, mostly between the fundamental and the third transverse modes of the waveguides. Using the visibility of the resultant two-mode fringes observed on the transmitted spectra, we extract a quantitative value for propagation losses at lambda = 1 mum: alpha(1) = 25 cm(-1) (1 dB/100 mum) for the fundamental mode. (C) 2002 Optical Society of America.

Planar photonic crystal coupled cavity waveguides

T. J. Karle, D. H. Brown, R. Wilson, M. Steer, T. F. Krauss

IEEE Journal Of Selected Topics In Quantum Electronics 8 (4) 909-918 (2002).

We present absolute transmission measurements of coupled cavity waveguides defined within planar photonic crystals. We investigate a range of cavity types and also vary the spacing between cavities. Modal analysis of the individual cavities reveals the symmetries that determine the coupling between adjacent cavities. Enhanced transmission is demonstrated by modifying the photonic crystal lattice. We highlight the need for correct impedance matching at the waveguide input in order to improve the transmission.

Improved 60 degrees bend transmission of submicron-width waveguides defined in two-dimensional photonic crystals

S. Olivier, H. Benisty, C. Weisbuch, C. J. M. Smith, T. F. Krauss, R. Houdre, U. Oesterle

Journal Of Lightwave Technology 20 (7) 1198-1203 (2002).

We compare quantitatively the transmission properties of various 60degrees bends carved into a photonic crystal based on a two-dimensional triangular lattice of holes perforating a GaAs-based heterostructure. The bends are inserted into channel waveguides defined by three missing rows in the photonic crystal. Their design is inspired by some ideas from classical integrated optics. We show experimentally that in some cases the transmission of the bent waveguide is fairly high, up to 70%, within a bandwidth of 3%, e.g., 30 nm at 1 mum, sufficient to contemplate wavelength-division-multiplexing applications. The observed performance opens the opportunity to implement a variety of optical functions in view of future photonic crystal integrated circuits for which low-loss bends constitute an essential building block.

Y junctions in photonic crystal channel waveguides: high transmission and impedance matching

S. Boscolo, M. Midrio, T. F. Krauss

Optics Letters 27 (12) 1001-1003 (2002).

We investigate the efficiency of transmission through photonic crystal Y junctions and show the importance of matching mode symmetries. Furthermore, we show that by adding tuning holes to the input waveguide it is possible to achieve almost perfect impedance matching, leading ideally to unitary transmission through the junction. The model system is based on a triangular photonic lattice of holes in dielectrics to reflect experimental reality. (C) 2002 Optical Society of America.

Propagation of ultrashort nonlinear pulses through two-dimensional AlGaAs high-contrast photonic crystal waveguides

M. D. Rahn, A. M. Fox, M. S. Skolnick, T. F. Krauss

Journal Of The Optical Society Of America B-optical Physics 19 (4) 716-721 (2002).

The propagation of 120-fs laser pulses through nonlinear waveguides with embedded high-contrast, two-dimensional photonic crystals was studied. Each AlGaAs waveguide fabricated upon a GaAs substrates contained a region of deeply etched air holes in a triangular lattice to form the photonic crystal. In transmission, a photonic bandgap was formed with a short-wavelength photonic bandedge at 925 nm from a 270-nm period lattice in the Gamma-K orientation. Pulse propagation was highly nonlinear, with both strong optical limiting and spectral narrowing. These effects arose from the waveguide rather than from the photonic crystal. Nonlinear effects were simulated theoretically, with good agreement with the data, by consideration of the effects of two-photon absorption and self-phase modulation on chirped incident pulses. (C) 2002 Optical Society of America.

Toward Ultrahigh-Efficiency aluminum oxide microcavity light-emitting diodes: Guided mode extraction by photonic crystals

M. Rattier, H. Benisty, R. P. Stanley, J. F. Carlin, R. Houdre, U. Oesterle, C. J. M. Smith, C. Weisbuch, T. F. Krauss

IEEE Journal Of Selected Topics In Quantum Electronics 8 (2) 238-247 (2002).

In this paper, we present an improved version of microcavity light-emitting diodes, relying on the use of a low-index material, aluminum oxide. Our work addresses in particular the injection scheme required by the insulating nature of this oxide. The device we fabricated demonstrated efficiencies up to 28% in air, using only planar technology. In these structures, most of the emission is guided. We further propose to include photonic crystals to extract this guided light. The design of the photonic crystals are discussed and substantiated by photoluminescence-based experiments.

High extraction efficiency, laterally injected, light emitting diodes combining microcavities and photonic crystals

M. Rattier, T. F. Krauss, J. F. Carlin, R. Stanley, U. Oesterle, R. Houdre, C. J. M. Smith, R. M. De La Rue, H. Benisty, C. Weisbuch

Optical And Quantum Electronics 34 (01-Mar) 79-89 (2002).

The use of photonic crystals (PCs) for realistic light emitting diodes (LEDs) is discussed, given the constraints of planar semiconductor technology. A viable route for the fabrication of high-efficiency high-brightness electrically injected LEDs is presented. The starting point is a top-emitting microcavity using a single Alox Bragg mirror. The active area is surrounded by two-dimensional PCs, namely arrays of air rods etched through the top layers; injection of the electrons is achieved through the crystals. Design rules for PCs as efficient out-couplers are detailed. The building blocks are assessed experimentally, and we show that promising results are at hand.

Ultrashort in-plane semiconductor microlasers with high-reflectivity microstructured mirrors

L. Raffaele, R. M. De La Rue, T. F. Krauss

Optical And Quantum Electronics 34 (01-Mar) 101-111 (2002).

We present very compact, as short as 20 mum long, low-threshold in-plane semiconductor lasers operating at a wavelength of 980 nm, in which microstructured mirrors have been formed at both cavity ends by deep reactive ion etching (RIE). The back mirror consists of a seven-period third order Bragg reflector with a measured reflectivity of similar to 95%. The front mirror has a similar configuration, but consists of three periods with a lower reflectivity (similar to 80%) in order to allow output coupling. Lasing has been achieved from 20 mum long and 8 mum wide devices exhibiting a current threshold of 7 mA. These are among the shortest in-plane Fabry-Perot electrically pumped lasers demonstrated to date. Design issues are discussed, along with experimental data from which values for the reflectivity of the mirrors are derived. State-of-the-art electron beam lithography (EBL) and high-aspect-ratio RIE have been used for device fabrication, while additional strategies are proposed for the further improvement of the device performance.

Diffraction and transmission of light in low-refractive index Penrose-tiled photonic quasicrystals

M. A. Kaliteevski, S. Brand, R. A. Abram, T. F. Krauss, P. Millar, R. M. De la Rue

Journal Of Physics-condensed Matter 13 (46) 10459-10470 (2001).

We report the measurements of the diffraction pattern of a two-dimensional Penrose-tiled photonic quasicrystal, obtained by etching air cylinders in a silica substrate, and the modelling of the light propagation and dispersion relations of photons inside such a structure. The calculated transmission spectra exhibit dips whose positions are insensitive to the direction of propagation and whose depth increases with increasing structure length. An approach is developed for the calculation of the dispersion relations which is based on a set of reciprocal vectors defined by the diffraction pattern. The dispersion curves exhibit gap-like features at frequencies corresponding to the dips in the transmission spectra.

Coupled defects in photonic crystals

A. L. Reynolds, U. Peschel, F. Lederer, P. J. Roberts, T. F. Krauss, P. J. I. De Maagt

IEEE Transactions On Microwave Theory And Techniques 49 (10) 1860-1867 (2001).

We present a theoretical and numerical description of coupled defects in photonic-bandgap crystals, expandable to cover a wide range of applications. Based on a weak interaction approach, explicit expressions are derived for defect interaction. The basis is formed by a system of coupled ordinary differential equations for the field amplitudes for individual defects. The actual configuration of the defects (chain, lattice, bend, or anything else) enters the equations as a linear coupling between neighboring defects. The strength of this method is that many solutions of this system are known analytically; the band structure as well as the transmission response of a defect chain, or of a defect lattice, can be determined. The results for the superlattice of defects are compared with widely accepted numerical methods, the transfer matrix method, and finite-difference time domain.

3D control of light in waveguide-based two-dimensional photonic crystals

C. Weisbuch, H. Benisty, S. Olivier, M. Rattier, C. J. M. Smith, T. F. Krauss

Ieice Transactions On Electronics E84C (5) 660-668 (2001).

Photonic crystals have seen major advances in the past few years in the optical range. The association of inplane waveguiding and two-dimensional photonic crystals (PCs) in thin-slab or waveguide structures leads to good 3D confinement with easy fabrication. Such structures, much easier to fabricate than 3D PCs open many exciting opportunities in optoelectronic devices and integrated optics. We present experiments on a variety of structures and devices, as well as modelling tools, which show that 2D PCs etched through waveguides supported by substrates are a viable route to high-performance PC-based photonic integrated circuits (PICs). In particular, they exhibit low out of-plane diffraction losses. Low-loss waveguides, high finesse microcavities, and their mutual coupling are demonstrated.

3D control of light in waveguide-based two-dimensional photonic crystals

C. Weisbuch, H. Benisty, S. Olivier, M. Rattier, C. J. M. Smith, T. F. Krauss

Ieice Transactions On Communications E84B (5) 1286-1294 (2001).

Photonic crystals have seen major advances in the past few years in the optical range. The association of inplane waveguiding and two-dimensional photonic crystals (PCs) in thin-slab or waveguide structures leads to good 3D confinement with easy fabrication. Such structures. much easier to fabricate than 3D PCs open many exciting opportunities in optoelectronic devices and integrated optics. We present experiments on a variety of structures and devices. as well as modelling tools, which show that 2D PCs etched through waveguides supported by substrates arp a viable route to high-performance PC-based photonic integrated circuits (PICs). In particular, they exhibit low outer-plane diffraction losses. Low-loss waveguides, high finesse microcavities. and their mutual coupling are demonstrated.

Enhanced phonon-assisted absorption in single InAs/GaAs quantum dots

A. Lemaitre, A. D. Ashmore, J. J. Finley, D. J. Mowbray, M. S. Skolnick, M. Hopkinson, T. F. Krauss

Physical Review B 63 (16) 161309 (2001).

Exciton-longitudinal optic-phonon coupling in InAs/GaAs quantum dots is investigated by means of single-dot spectroscopy. Photoluminescence spectra in the excitonic ground-state region exhibit a series of new emission lines which we ascribe to single exciton recombination perturbed by charged defects close to the dot. Compared to unperturbed excitonic recombination, the resulting dipole in these complexes leads to enhanced coupling to LO phonons in photoluminescence excitation spectra. Evidence for resonant enhancement of phonon-assisted processes ill absorption is also presented.

Tuneable distributed feedback lasing in MEH-PPV films

G. A. Turnbull, T. F. Krauss, W. L. Barnes, I. D. W. Samuel

Synthetic Metals 121 (01-Mar) 1757-1758 (2001).

Tuning of distributed feedback lasing is demonstrated in a thin film of the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene). By using an array of different grating periods on a single substrate, the laser emission was systematically tuned from 612 to 632 nm The Bragg scattered emission from the film features a dip at the Bragg wavelength; lasing occurs at the long-wavelength edge of this stopband.

Excitation and relaxation mechanisms in single In(Ga)As quantum dots

J. J. Finley, A. Lemaitri, A. D. Ashmore, D. J. Mowbray, M. S. Skolnick, M. Hopkinson, T. F. Krauss

Physica Status Solidi B-basic Research 224 (2) 373-378 (2001).

Spatially resolved spectroscopy is used to investigate excitation and relaxation mechanisms in individual sell-assembled In(Ga)As quantum dots. For low excitation levels (similar to1 e-h pair in the dot), both charged (X*) and neutral (X) single exciton species are observed simultaneously in the Cl-photoluminescence (mu PL) spectrum. The charge status of X* and X is unambiguously identified using mu PL-excitation (mu PLE) spectroscopy. At higher excitation levels, additional spectral features appear, red-shifted from X, arising from the recombination of few exciton complexes (2X, 3X). Relaxation mechanisms are investigated using mu PL-excitation spectroscopy. For X, the mu PLE spectrum exhibits sharp resonances, which are attributed to direct absorption of excitonic states, and broader features due to phonon-assisted absorption, whilst X* is observed only for excitation above the wetting layer energy.

Mini-stopbands of a one-dimensional system: The channel waveguide in a two-dimensional photonic crystal

S. Olivier, M. Rattier, H. Benisty, C. Weisbuch, C. J. M. Smith, R. M. De la Rue, T. F. Krauss, U. Oesterle, R. Houdre

Physical Review B 63 (11) 113311 (2001).

We show that a channel waveguide in a two-dimensional photonic crystal channel can be considered as a periodic one-dimensional system. Waveguides were fabricated into a GaAs/AlxGa1-xAs waveguide heterostructure and the waveguide transmission was measured by detecting the guided photoluminescence of embedded InAs quantum dots excited near the guide entrance. Mini-stopbands related to anticrossings in the dispersion were observed and the spectral location and width of these transmission stopbands is found to agree well with calculated values.

Coupled guide and cavity in a two-dimensional photonic crystal

C. J. M. Smith, R. M. De la Rue, M. Rattier, S. Olivier, H. Benisty, C. Weisbuch, T. F. Krauss, R. Houdre, U. Oesterle

Applied Physics Letters 78 (11) 1487-1489 (2001).

We demonstrate, in a planar two-dimensional (2D) configuration, in the optical regime a clear association of two photonic crystal elements and the ability to produce a low-loss coupled system. A channel waveguide is brought to between two and five crystal rows (450 to 1126 nm) from a 2D microcavity fabricated in a GaAs/AlGaAs waveguide. We probe these two elements individually and explore their interaction. (C) 2001 American Institute of Physics.

Charged and neutral exciton complexes in individual self-assembled In(Ga)As quantum dots

J. J. Finley, A. D. Ashmore, A. Lemaitre, D. J. Mowbray, M. S. Skolnick, I. E. Itskevich, P. A. Maksym, M. Hopkinson, T. F. Krauss

Physical Review B 63 (7) 73307 (2001).

Charged (X*) and neutral (X) exciton recombination is reported in the photoluminescence spectra of single In(Ga)As quantum dots. Photoluminescence excitation (PLE) spectra show that the charged excitons are created only for excitation in the barrier or cladding layers of the structure, consistent with their charged character, whereas the neutral excitons in addition show well-defined excitation features for resonant excitation of the dots. The PLE spectra for X and X* exhibit a clear anticorrelation in the region of the wetting layer transition, showing that they compete for photocreated carriers.

Performance of waveguide-based two-dimensional photonic-crystal mirrors studied with Fabry-Perot resonators

M. Rattier, H. Benisty, C. J. M. Smith, A. Beraud, D. Cassagne, C. Jouanin, T. F. Krauss, C. Weisbuch

IEEE Journal Of Quantum Electronics 37 (2) 237-243 (2001).

As a step toward the use of photonic crystals in optoelectronic devices, we present a thorough study of 2-D photonic-crystal mirrors etched into a GaAs/AlGaAs planar waveguide. Fabry-Perot resonators are fabricated to deduce the reflectivity, transmission, losses, as well as the penetration lengths of these mirrors. The guided photoluminescence of InAs quantum dots embedded in GaAs is used to obtain the transmission spectra of those cavities, The varying thickness between the mirrors allows a scan across the whole bandgap spectral range. Quality factors (up to 200) and peak transmissions (up to 0.3) are measured showing that mirrors of four rows of holes have 88% reflectivity, 6% transmission and 6% losses. Losses are also related to a two-dimensional transfer matrix method calculation including a recently introduced scheme to account for losses.

Low-loss channel waveguides with two-dimensional photonic crystal boundaries

J. M. Smith, R. M. De La Rue, T. F. Krauss, H. Benisty, S. Olivier, M. Rattier, C. Weisbuch, R. Houdre, U. Oesterle

Appl. Phys. Lett. 77 (18) 2813-2815 (2000).

We have used transmission measurements to estimate the propagation loss of submicron channels defined in two-dimensional photonic crystals patterned into a Ga(Al)As waveguide. The measured propagation loss of the fundamental mode is indistinguishable from the material absorption, setting an upper limit of 50 cm^1 (2 dB per 100 um). We also find that, provided the etching is deep enough, propagation losses of these photonic crystal waveguides are lower than those of ridge waveguides etched in the same run.

Two-dimensional Penrose-tiled photonic quasicrystals: diffraction of light and fractal density of modes

M. A. Kaliteevski, S. Brand, R. A. Abram, T. F. Krauss, R. M. De La Rue, P. Millar

Journal Of Modern Optics 47 (11) 1771-1778 (2000).

We report measurements of the diffraction pattern of a two-dimensional photonic quasicrystal structure and use the set of plane waves defined by the diffraction pattern as the basis of a theoretical approach to calculate the photonic band structure of the system. An important feature of the model is that it retains the essence of the rotational and inflational properties of the quasicrystal at all levels of approximation: properties lost in approximate models which artificially introduce elements of periodicity. The calculated density of modes of the quasicrystals is found to display a weakly depleted region analogous to the bandgap that occurs in a periodic system. The calculated transmission spectra for different polarizations and directions of propagation show features that correlate with the behaviour of the density of modes.

Spectral features associated with nonlinear pulse compression in Bragg gratings

N. G. R. Broderick, P. Millar, J. S. Richardson, J. S. Aitchison, R. M. De La Rue, T. F. Krauss

Optics Letters 25 (10) 740-742 (2000).

We report, for the f irst time to our knowledge, direct spectral measurements of nonlinear spectral broadening caused by nonlinear propagation through Bragg gratings written on integrated AlGaAs waveguides. The spectral broadening is associated with pulse compression from 400 to 80 ps. The high nonlinearity of AlGaAs enables high-repetition-rate, low-peak-power sources to be used, facilitating easy spectral measurements.

Surface recombination measurements on III-V candidate materials for nanostructure light-emitting diodes

M. Boroditsky, I. Gontijo, M. Jackson, R. Vrijen, E. Yablonovitch, T. F. Krauss, C. C. Cheng, A. Scherer, R. Bhat, M. Krames

Journal Of Applied Physics 87 (7) 3497-3504 (2000).

Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes.

Heavy Photon Dispersions in Photonic Crystal Waveguides

V. N. Astratov, R. M. Stevenson, I. S. Culshaw, D. M. Whittaker, M. S. Skolnick, T. F. Krauss, R. M. De La Rue

Applied Physics Letters 77 (2) 178-180 (2000).

Heavy photon dispersion curves exhibiting group velocities suppressed by two orders of magnitude are measured directly for deeply etched AlGaAs waveguide structures by means of surface coupling techniques. It is shown that due to the wave vector-selective nature of surface coupling, such techniques permit the excitation of modes of specific, known dispersion in photonic crystal waveguides. Coupling to regions of very strong anomalous dispersion is demonstrated, with potential to be developed into a method for excitation of gap solitons.

Spontaneous Emission Extraction and Purcell Enhancement from Thin-Film 2-D Photonic Crystals

M. Boroditsky, R. Vrijen, T. F. Krauss, R. Coccioli, R. Bhat, E. Yablonovitch

Journal Of Lightwave Technology 17 (11) 2096-2112 (1999).

Electromagnetic band structure can produce either an enhancement or a suppression of spontaneous emission from two-dimensional (2-D) photonic crystal thin films. We believe that such effects might be important for light emitting diodes. Our experiments were based on thin-film InGaAs/InP 2-D photonic crystals at ambient temperature, but the concepts would apply equally to InGaN thin films, for example. We show that the magnitude of Purcell enhancement factor, Fp~2, for spatially extended band modes, is similar to that for a tiny mode in a three-dimensional (3-D) nanocavity. Nonetheless, light extraction enhancement that arises from Zone folding or Bragg scattering of the photonic bands is probably the more important effect, and an external quantum efficiency >50% is possible. Angle resolved photoluminescence from inside the photonic crystal gives a direct spectral readout of the internal 2-D photonic band dispersion. The tradeoffs for employing various photonic crystal structures in high efficiency light-emitting diodes are analyzed.

Understanding the rich physics of light propagation in slow photonic crystal waveguides

T. F. Krauss, L. O'Faolain, S. Schulz, D. M. Beggs, F. Morichetti, A. Canciamilla, M. Torregiani, A. Melloni, S. Mazoyer, P. Lalanne, A. Samarelli, M. Sorel, R. De La Rue

Proceedings Of The Society Of Photo-optical Instrumentation Engineers (spie) 7612 7612L

We study propagation losses in slow light photonic crystal waveguides and show that dispersion engineering can reduce the loss. We develop an improved understanding of why and how this occurs and develop an new approach to modeling these devices that provides new design insights.

Nanometre control and determination of hole size in photonic crystal slabs

D. M. Beggs, L. O'Faolain, T. F. Krauss

Spie Proceedings Vol. 7713

All photonic crystal slab designs rely on the accurate control of the sizes of the holes during fabrication - a small error of just a few nanometers can mean a shift of tens of nanometers in the operating wavelength of the device. A key problem is the measurement of the hole size. We have developed an optical method which determines the functional hole diameter with a typical accuracy of 5 nm. We also suggest a way for the nanometre control of the hole diameter.

Ultrafast adiabatic frequency conversion using slow-light in photonic crystal waveguides

D. M. Beggs, T. Kampfrath, L. Kuipers, T. F. Krauss

Spie Proceedings Vol. 7713

By performing an ultrafast pump-probe experiment, we demonstrate the adiabatic frequency conversion of a telecommunications pulse in a silicon photonic crystal waveguide. By using slow-light modes to spatially compress the pulse, a 1.3 ps long pulse is blue-shifted by 0.3 THz with 80% efficiency in a waveguide just 19 ìm long. We also present the results of an adiabatic model of the process, which agrees excellently with the experimentally measured data.

How does slow light propagate in a real photonic-crystal waveguides?

S. Mazoyer, J. P. Hugonin, P. Lalanne, D. M. Beggs, T. F. Krauss

Spie Proceedings Vol. 7713

We report ensemble-average transport characteristics obtained for a series of photonic-crystal waveguides that are supposedly identical and that only differ because of statistical structural fabrication-induced imperfections. In particular, we evidence that, in addition to a smearing of the local density of states, the probability density function of the transmission rapidly broadens in the slow light regime even for group indices as small as ng¡Ö20 and for practical situations offering tolerable -3dB losses. This brings a severe constraint on the effective use of slow light for on-chip optical information processing. The experimental results are quantitatively supported by theoretical results obtained with a coupled-Bloch-mode approach that takes into account multiple scattering and localization effects.

Tunable optical delay line in engineered slow light photonic crystal waveguides

I. H. Rey, D. M. Beggs, T. Kampfrath, L. Kuipers, T. F. Krauss

15th European Conference On Integrated Optics (ecio 2010)

We propose a new design for compact tunable optical delay lines, based on adiabatic wavelength conversion and group velocity dispersion, in engineered silicon slow light photonic crystal waveguides. For a 3nm adiabatic wavelength shift, we obtain a simulated change in group index from ng=25 to ng=124.

Slow-light photonic crystal switches and modulators

D. M. Beggs, T. P. White, T. Kampfrath, L. Kuipers, T. F. Krauss

Spie Proceedings Vol. 7606

We discuss the performance of slow-light enhanced optical switches and modulators fabricated in silicon. The switch is based on photonic crystal waveguides in a directional coupler geometry, and the dispersion of the device is engineered to allow a switching length as short as 5 ìm and rerouting of optical signals within 3 ps. The 3 ps switching time is demonstrated using free carriers in the silicon generated by the absorption of a femtosecond pump pulse. The modulator is based on a Mach-Zehnder interferometer configuration, with photonic crystal waveguides in each arm to act as phase-shifters. A flat-band slow-light region has been engineered in the phase-shifters to provide an extinction ratio in excess of 15 dB over the entire 11 nm bandwidth of the modulator device.

Generic packaging concepts in the frame of network of excellence ePIXnet

T. Tekin, L. Zimmermann, H. Schroder, P. Dumon, W. Bogaerts, J. V. Galan, P. Sanchis, L. O'Faolain, D. M. Beggs

Spie Proceedings Vol. 7604

Generic packaging concepts for silicon photonics have been developed in the frame of EU-funded Network of Excellence ePIXnet (FP6). Three approaches for Silicon photonic packaging will be presented within this paper. Two concepts provide solutions for fiber array coupling to high-index contrast photonic wire waveguide gratings. Third concept is the integration of inverted taper-based fiber coupling structure with silicon etched V-grooves. Using standardized SOI chip designs and commercial available assembly parts, the packaging concepts allow for small footprint or flexible use in an R&D environment. The work presented here has resulted from cooperation within the European Network of Excellence ePIXnet.

Optical beam-steering for wireless sensor networks

C. Reardon, A. Di Falco, K. Welna, T. F. Krauss

IEEE Photonics Society - The 22nd Annual Meeting Of The IEEE Photonics Society (leos 09)

We demonstrate beam-steering with vertical-cavity surface-emitting lasers (VCSELs) for wireless communication in sensor networks. We demonstrate static beam-steering using integrated microprisms and introduce a dynamic beam-steering technique based on coupled VCSEL arrays.

Ultrafast Re-routing of Slow Light in a Nanophotonic Directional Coupler

T. Kampfrath, D. M. Beggs, T. P. White

Information: Conference On Lasers And Electro-optics/quantum Electronics And Laser Science Conference (cleo/qels 2009) 1-5 866-867

Impact of the residual disorder on the slow light regime in 1D and 2D photonic crystals structures

N. Le Thomas, R. Houdre, M. V. Kotylar

Information: Conference On Lasers And Electro-optics/quantum Electronics And Laser Science Conference (cleo/qels 2009) 1-5 2080-2081

Slow light in dispersion-engineered photonic crystal waveguides

T. F. Krauss

Information: Conference On Lasers And Electro-optics/quantum Electronics And Laser Science Conference (cleo/qels 2009) 1-5 2953-2954

Ultra-compact optical switch based on photonic crystal waveguides

T. F. Krauss

Information: Conference On Lasers And Electro-optics/quantum Electronics And Laser Science Conference (cleo/qels 2009) 1-5 2430-2432

Demonstration of an integrated optical switch in a silicon photonic crystal directional coupler

D. M. Beggs, T. P. White, L. Cairns, L. O'Faolain, T. F. Krauss

Physica E-low-dimensional Systems & Nanostructures 41 (6) 1111-1114 (2009).

We demonstrate a silicon photonic crystal (PhC) directional coupler optical switch. By engineering the supermodes of the coupled system to provide a slow-light region, the switch has a total length of just 4.9 mu m. Thermo-optic switching with a 20 mu s response time via an integrated nickel microheater is shown. The proposed switch uses a silica overlayer and infilling of the etched silicon PhC, rather than the more usual membrane slab geometry. The silica infilling and overlayer provides a vertically symmetric structure and is a more robust solution compared to the membrane geometry. We also show that there is no apparent increase in optical loss due to the silica overlayer, despite operating above the lightline: the insertion loss for the air-bridge and silica-embedded structures are comparable (at 1-2 dB). (C) 2008 Elsevier B.V. All rights reserved

Accurate determination of hole sizes in photonic crystal slabs using an optical measurement

D. M. Beggs, L. O'Faolain, T. F. Krauss

Physica E-low-dimensional Systems & Nanostructures 41 (6) 1115-1117 (2009).

Control and repeatability in fabrication of two-dimensional photonic crystal (PhC) slabs will become increasingly important as the technology matures into real device applications. A related problem is the determination of hole sizes in final etched devices. We have developed ail optical method of measuring the hole size in PhC slabs as ail alternative to the inspection of scanning electron microscope (SEM) images. The optical method relies on W1 PhC waveguides by patterning and fabricating reference W1 waveguides, the cut-off frequency of the waveguiding defect state can be measured and compared to calculations of this frequency as a function of hole size. Such calculations are relatively straightforward, and such in-situ transmission measurements are relatively cheap and fast. We show that the typical error in the measurement of hole radius is approximately 2%, or just 2-3 nm, and that this error is dominated by the uncertainty in the silicon slab thickness. Such performance is a significant improvement on current methods, which rely oil the inspection via SEM. Not only is this slow and expensive, but there can be a large systematic error involved in the measurement. Different detectors, and even different settings of the same detector, will provide different contrasts between a hole and its edge, leading to different apparent hole sizes. Such errors in the absolute hole size can be of the order of 10 nm, which is as much as 5-10% for a PhC. (C) 2008 Elsevier B.V. All rights reserved.

Fabrication of integrated polymer microprisms with VCSELs for location discovery based on free space optical beam steering

C. Reardon, A. Di Falco, K. Welna, T. F. Krauss

Semiconductor And Integrated Optoelectronics (sioe 09)

We report on the fabrication of integrated microprisms, using greyscale electron-beam lithography. The microprisms are integrated onto an array of vertical cavity surface emitting lasers (VCSELs) and used for beam steering and multiple channel communication in free space optical (FSO) communications.

Slotted Photonic Crystal waveguides and cavities for slow light and sensing applications

A. Di Falco, L. O'Faolain, T. F. Krauss

2008 5th IEEE International Conference On Group Iv Photonics 228-230

We present experimental evidence of fight guiding and confinement in suspended slotted Photonic Crystal waveguides and cavities, where light is confined in extremely small air volumes, for slow fight and chemical sensing applications.

Slow light and low loss propagation in SOI photonic crystal waveguides

T. F. Krauss

2007 4th IEEE International Conference On Group Iv Photonics 235-237

Slow light in photonic crystal waveguides offers field and phase enhancement for photonic functionality. We demonstrate broadband operation (2.5 THz) of slow light devices and low loss (4 dB/cm) propagation of e-beam and DUV fabricated structures and discuss their dependence on disorder.

Design and fabrication of high-efficiency fibre couplers for nanophotonic devices

D. M. Beggs, M. Ayre, D. F. G. Gallagher, T. F. Krauss

Microelectronic Engineering 84 1446-1449

A dual waveguide heterostructure device that couples light from single mode optical fibres into nanophotonic devices with an overall modelled efficiency of 81 percent has been investigated. A four-stage taper design for the ridge waveguide which allows for a total device length of 610 um is described. The fabrication of the ridge waveguide using chemically-assisted ion-beam etching has been optimised.

Guest editorial - Nanotechnologies for communications

C. Jagadish, D. G. Deppe, S. Noda, T. F. Krauss, O. J. Painter

IEEE Journal On Selected Areas In Communications 23 (7) 1305-1307 (2005).

Novel optical micromanipulation and rotation in microfluidic environments

K. Dholakia, M. Macdonald, S. Neale, S. Mcgreehin, L. Paterson, T. F. Krauss

2005 IEEE Leos Annual Meeting Conference Proceedings (leos) 374-375

Optical micro manipulation and microfluidics offer an exciting opportunities for future studies. In this paper I will discuss work on integrated optical traps, actuating and rotating microfluidic components using form birefringence and studies of optical sorting.

Propagation of optical pulses in photonic crystal waveguides

Y. J. Chai, C. N. Morgan, R. V. Penty, I. H. White, T. J. Karle, T. F. Krauss

Iee Proceedings-optoelectronics 151 (2) 109-113 (2004).

The dispersion properties of planar photonic crystal (PhC) coupled cavity waveguides are investigated. Both pulse broadening and compression have been demonstrated by propagating various chirped sub-picosecond pulses through the waveguides. The PhC waveguide exhibits large positive group-velocity dispersion (GVD), which is opposed to the negative GVD in a single-mode fibre (SMF). A maximum pulse width reduction of 40%, from 1.91 ps to 1.17 ps, is achieved, after transmission through an 8 mum long planar PhC waveguide. The equivalent dispersion value is >10(6) times larger than that available from standard single-mode fibre at the pulse centre wavelength of 1536 nm.

Passively mode-locked monolithic semiconductor diode lasers incorporating dispersion compensation

M. B. Flynn, L. O'Faolain, T. F. Krauss

2004 IEEE Leos Annual Meeting Conference Proceedings, Vols 1 And 2 679-680

Coupled cavity tunable semiconductor diode lasers incorporating multi-layer mirrors

M. B. Flynn, D. H. Brown, T. F. Krauss

2004 IEEE Leos Annual Meeting Conference Proceedings, Vols 1 And 2 25-26

We present numerical and experimental results of coupled cavity tunable semiconductor lasers incorporating multi-layer mirrors. We also describe the fabrication techniques employed. We obtain enhanced single mode tunability with high side mode suppression.

Square lattice photonic crystal collimator

L. Wu, M. Mazilu, J. -. F. Gallet, T. F. Krauss

Photonics And Nanostructures 1 31-36

Planar photonic crystal waveguide devices for integrated optics

T. F. Krauss

Physica Status Solidi A-applied Research 197 (3) 688-702 (2003).

The concept of photonic crystal waveguides and related device applications in integrated optics are discussed, as well as some of the fabrication technology required to realise these structures in practise. Photonic crystal waveguides interact with light on a wavelength scale, which leads to a novel class of miniaturised devices that display designer dispersion.

Coupled cavity lasers incorporating Bragg mirrors

D. H. Brown, M. B. Flynn, L. O'Faolain, W. Sibbett, T. F. Krauss

2003 IEEE Leos Annual Meeting Conference Proceedings, Vols 1 And 2 583-584

We report results on a coupled cavity laser incorporating a photonic bandgap mirror. This enhances fabrication tolerances over standard cleaving. We present numerical results and describe preliminary experiments.

Introduction to the feature section on photonic crystal structures and applications

T. F. Krauss, T. Baba

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 724-725 (2002).

Cascaded photonic crystal guides and cavities: Spectral studies and their impact on integrated optics design

S. Olivier, C. J. M. Smith, H. Benisty, C. Weisbuch, T. F. Krauss, R. Houdre, U. Oesterle

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 816-824 (2002).

We report on the investigation of cavities coupled to channel waveguides defined in a triangular-lattice photonic crystal etched into a GaAs/AlGaAs heterostructure waveguide. We make use of the internal probe technique by selectively exciting the photoluminescence of InAs dots embedded in the planar waveguide. We collect at the nearby cleaved edge the fundamental mode transmitted through the channel waveguides. From the study of a number of basic cases, we exemplify how one may deduce the main guideline design rules for given cavity-guide interaction mechanisms and then for the implementations of functions such as bending and filtering. In particular, the role of the so-called ministopband (MSB) feature of the waveguide and the nature of the cavity modes involved are outlined.

Transmission and reflection analysis of functional coupled cavity components

U. Peschel, A. L. Reynolds, B. Arredondo, F. Lederer, P. J. Roberts, T. F. Krauss, P. J. I. De Maagt

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 830-836 (2002).

This paper contributes to the ongoing discussion within the photonic crystal community by providing essential insight into the limiting conditions of the coupled cavity waveguiding mechanism. A theoretical and numerical description of coupled defects in PBG crystals is applied to quantify the conditions under which reflections occur within coupled cavity photonic crystal systems. We present an analysis of coupled cavity systems that form a straight and bent waveguide, a Y-shaped symmetric power splitter, and a waveguide incorporating two bends. The method is based on a weak interaction approach; the actual configuration of the defects (chain, lattice, bend, splitter, or anything else) enters the equations as a linear coupling between neighboring defects. The strength of this method is that many solutions of this system are known analytically, and that the band structure as well as the transmission and reflection response of the system can be determined.

Polarization conversion in the reflectivity properties of photonic crystal waveguides

A. D. Bristow, V. N. Astratov, R. Shimada, I. S. Culshaw, M. S. Skolnick, D. M. Whittaker, A. Tahraoui, T. F. Krauss

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 880-884 (2002).

Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.

Superprism phenomena in planar photonic crystals

L. J. Wu, M. Mazilu, T. Karle, T. F. Krauss

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 915-918 (2002).

We utilize the anomalous dispersion of planar photonic crystals near the dielectric band edge to control the wavelength-dependent propagation of light. We typically observe an angular swing of up to 10degrees as the input wavelength is changed from 1290 urn to 1310 mn, which signifies an angular dispersion of 0.5 degree/nm. Such a strong angular dispersion is of the order required for wavelength-division multiplexing systems. This is the first demonstration of the superprism effect in a planar configuration with a small lattice.

An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers

D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, R. Baets

Third Workshop On Photonic And Electromagnetic Crystal Structures (pecs Iii) 38 (7) 949-955 (2002).

We have designed and fabricated an out-of-plane coupler for butt-coupling from fiber to compact planar waveguides. The coupler is based on a short second-order grating or photonic crystal, etched in a waveguide with a low-index oxide cladding. The coupler is optimized using mode expansion-based simulations. Simulations using a 2-D model show that up to 74% coupling efficiency between single-mode fiber and a 240-nm-thick GaAs-AlOx waveguide is possible. We have measured 19% coupling efficiency on test structures.

Propagation of picosecond pulses through photonic crystal waveguides at C-band region

T. J. Karle, Y. J. Chai, C. N. Morgan, I. H. White, T. F. Krauss

2002 Ieee/leos Annual Meeting Conference Proceedings, Vols 1 And 2 881-882

High repetition rate picosecond optical pulses are transmitted through a photonic crystal coupled cavity waveguide for the first time. Strong pulse shaping effects are observed at the edge of the waveguide transmission band.

Photonic crystal based integration

T. F. Krauss, L. Wu, R. Wilson, T. Karle

2002 Ieee/leos Annual Meeting Conference Proceedings, Vols 1 And 2 572-573

Photonic crystals provide a new platform for miniaturised integrated optical circuits and offer new functionalities in dispersion engineering and WDM components.

Photonic crystal integrated optics

T. F. Krauss

Cleo(r)/pacific Rim 2001, Vol I, Technical Digest 282-283

The photonic crystal platform provides many exciting opportunities for novel integrated optical circuits. In order to realise this promise, issues such as propagation losses, spectral channel separation, integration and active functionality must be addressed.