EU-FP6 FET “SPLASH”

Splash is a collaborative project between researchers in St Andrews, Glasgow, Milan, Paris and Amsterdam that is funded by the European Union under framework 6.  Our aim is to make optical switches and storage devices based on slow light that may be used in the next generation of all-optical data processing systems. Such systems will support the increasing speed and bandwidth needed to satisfy the raising demands of consumers for online video, music and other data.

Data is currently sent over the internet as pulses of light in optical fibres.  However, to get from place to place – to route or switch it – the data must be converted from optical signals to electrical ones, before being processed by electronic components and converted back to light.  Removing the bottleneck of this conversion has the potential to speed up internet connections and to reduce the power needed for operating  future internet systems.  In order to do this, compact and cheap optical switches are needed that can route optical signals without any conversion to electrical signals.  A related device, called a delay line, is also needed.  A delay line acts as a storage or memory element for the short time, or delay, that it takes for the signal to be routed to the correct place.

The problem with making all-optical components is that transparent materials interact only very weakly with light – they barely sense the light passing through them – because of the very fast speed that light travels at through the materials.  This means that devices need to be large and bulky, increasing the component costs, and also raising the energy needed to operate them.

A solution to the weak interaction problem is to slow the light down, so that the slower light has more time to interact with the material of the device, and the sensitivity of the material to the light increases.  The SPLASH project is named after this idea, namely SPLASH = “Slow Photon Light Activated SwitcH”.  The increase of the sensitivity created by the slow-light enhancement allows for smaller devices, which are cheaper to make, and which use less energy to run.

So, how does one slow light down?  Various ways exist, but in SPLASH we look at two different ways – photonic crystal technology and micro-ring resonator technology.  The unifying feature of these two different technologies is that they use interference of light waves in clever ways to make the light pass through them up to 100 times more slowly than it would normally.

By drawing on both photonic crystal and on microring technology, we can use the advantages of each approach in order to address the significant challenges of loss, dispersion and device footprint.

The core demonstrators of the project are:

a) Broadband slow light. Photonic crystal waveguides that combine sizeable slowdown factors (10-20) with broad bandwidth (500 GHz) and efficient (>90%) optical coupling

Systematic design of flat band slow light in photonic crystal waveguides

Optics Express, Vol. 16 Issue 9, pp.6227-6232 (2008)

Through suitable control of the relevant parameters, we are able to design slow light with varying group index but nearly constant group index - bandwidth product.

b) Tuneable coupler. By exploiting slow light phenomena in photonic crystals, the coupler will be very compact (10's of µm), yet require only refractive index changes of order 10-3 for complete switching. Tuning will be thermo-optic, electro-optic and all-optical.

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

Beggs, Daryl M; White, Thomas P; O'Faolain, Liam; Krauss, Thomas F
Optics Letters, Vol. 33 Issue 2, pp.147-149 (2008)

Miniaturised optical switch exploiting slow light effects in order to reduce the switching length 40-fold to 5 µm for a given refractive index shift of 4x10^-3

c) Tuneable delay line. Tuneable ring resonators will be used to continuously control the delay properties of a system of coupled resonators.

Error-free continuously-tunable delay at 10 Gbit/s in a reconfigurable on-chip delay-line

F. Morichetti, A. Melloni, C. Ferrari, and M. Martinelli, "Error-free continuously-tunable delay at 10 Gbit/s in a reconfigurable on-chip delay-line," Opt. Express 16, 8395-8405 (2008)

Schematic of tunable delay as implemented in ring resonators by colleagues in Milan, Italy.

d) Switchable optical storage. By tuning/detuning the elements of a coupled resonator delay line, optical information will be stored.

Structures will be realised in silicon on insulator technology to achieve the maximum compactness and maximum number of stored bits/unit area. The proposed programme is extremely timely, as the slow light opportunity needs to be seized now in order to be ready for exploitation on a 10-year horizon, when all-optical functionality will start to be implemented in systems and networks.
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Interaction CUDOS-Splash

This grant strengthens the linkeage between the Australian CUDOS consortium and SPLASH by supporting our joint work on photonic crystals in chalcogenide glasses.

Photonic crystal membrane in chalcogenide glass

CUDOS sub-meeting, Andrea Melloni and Thomas Krauss on a glacier in the Italian Alps, in March 2007. (Note the CUDOS sunhat)

Schematic of an highly integrated all optical regenerator.

Araknes Project

PROPHET - Postgraduate Research on Photonics as an Enabling Technology
Marie Curie ITN

Photonics, the generation and manipulation of light, is an important enabling technology for a diverse range of application areas; in 2006, the photonics industry in Europe accounted for revenues of €49 billion. PROPHET (Postgraduate Research on Photonics as an Enabling Technology) is an Initial Training Network funded by the EU Framework Programme 7 Marie Curie Actions, which aims to train the next generation of photonics researchers in the full range of skills required for a multi-disciplinary, industry-focused career in photonics.

The PROPHET network brings together a carefully-chosen, well-balanced consortium of 9 academic partners, 4 industry partners and 2 associated partners, with European and worldwide reputations as leaders in their fields. The network will train a cohort of 14 early stage researchers and 5 young experienced researchers in the full gamut of skills required for a career in photonics, including materials growth, device fabrication, characterisation, design, theory, and commercialisation. We here at the University of St Andrews will focus on three areas namely: communications, energy and the environment.

In communications, photonics has been the backbone of paradigm-changing advances such as fibre optics and broadband internet, and is set to continue this into the future with faster and higher capacity links. In PROPHET, we will focus on quantum dot mode-locked lasers at 1.3 µm and 1.55 µm for access networks, fibre-to-the-home (FTTH) and radio-over-fibre applications.

The generation, management and use of energy has always been important to society, and is even more relevant today as fossil fuel resources dwindle and the effects of global warming become apparent. Renewable energy sources are increasingly in demand, and the enormous amount of light and heat available from the sun holds great potential as one such supply. PROPHET will explore novel photonic nanostructures in photovoltaic cells to improve the efficiency and economy of solar energy collection.

OCT image of a retina, showing soft drusen,
an indication of age-related
macular degeneration

Monitoring the environment, whether it be the level of pollutants in the atmosphere, or gaseous components in industrial manufacturing processes, has been well served by photonics, utilising infrared absorption characteristics for gas sensing. PROPHET will look to develop novel mid-infrared laser sources for improved efficiency and sensitivity in the detection of a range of important gases.

Photonics is also well established as an important enabling technology for the life science and health care fields, dating back to when the first microscopes were employed to unlock the fascinating world of cells and bacteria. The technology has unique advantages as a non-invasive, benign imaging method for a variety of imaging applications. In PROPHET, we aim to realise new compact broadband fast-tunable laser sources which will be particularly suited to Optical Coherence Tomography applications.

For more information please see the PROPHET webpage.

Solar cells have an ever increasing
role to play on the renewable
energies agenda

A research consortium led by the University of Surrey (Graham Reed) and comprising St. Andrews (Thomas Krauss), Leeds (Robert Kelsall), Warwick (Evan Parker) and Southampton (Graham Ensell).

Silicon Photonics is a field that has seen rapid growth and dramatic changes in the past 5 years. According to the MIT Communications Technology Roadmap, which aims to establish a common architecture platform across market sectors with a potential $20B in annual revenue, silicon photonics is among the top ten emerging technologies.

This has in part been a consequence of the recent involvement of large semiconductor companies in the USA such as Intel and IBM, who have realised the enormous potential of the technology, as well as large investment in the field by DARPA in the USA under the Electronic and Photonic Integrated Circuit (EPIC) initiative. Significant investment in the technology has also followed in Japan, Korea, and to a lesser extent in the European Union (IMEC and LETI).

The generic device functions we envisage are as follows: Optical modulation; coupling from fibre to sub-micron silicon waveguides; interfacing of optical signals within sub micron waveguides; optical filtering; optical/electronic integration; optical detection; optical amplification. In each of these areas we propose to design, fabricate, and test devices that will improve the current state of the art. Subsequently we will integrate these optical devices with electronics to further improve the state of the art in optical/electronic integration in silicon

Monolithic integration of optical traps and microfluidic channels

The vision of creating true “Lab-on-a-chip” (LOC) devices has fascinated both researchers and clinicians for some time. A family of devices that offers the same biochemical functionality as arrays of test tubes or expensive and complicated machines operated by skilled personnel is very attractive indeed. It simplifies biochemical testing and allows conducting many processes that are currently done in a remote laboratory at the point of care.

Since the LOC concept was first developed in the late 1980’s and early 1990’s, much progress has been made and many devices that perform such reactions in a microfluidic environment are now commercially available. One vital component that is yet missing from this toolkit is the ability to manipulate and control individual particles, such as cells, in an integrated environment

Schematic of the monolithically integrated trap featuring microfluidic channels etched into active semiconductor laser material.

Optical trapping of bioparticles, which was first demonstrated in the 1980’s and which is now widely used in biomedical research, offers this ability.Optical trapping has proven to be a vital building block for the manipulation of cells and other bioparticles because of its unique capability to exercise control on the micro- and nanoscale. Optical trapping would thus ideally complement the LOC toolkit. The large majority of optical traps, however, are created with externally supplied laser sources, thus requiring bulky optics.

Realisation of a bank of lasers feeding into a microfluidic channel insulated with SU-8

In order to address this issue, we have recently demonstrated therealisation of a monolithically integrated optical trap, which integrates microfluidic channels directly into the semiconductor material, thus allowing laser sources to be an intrinsic part of the circuit. Building on this success, the aim of the present proposal is to fully explore the capabilities of the integrated optical trap concept and to broaden its appeal to a wider range of applications.

OSIRIS - Optical signal regeneration in slow light silicon waveguides

In modern optical communication systems, signal degradation in the optical channel caused by dispersion, nonlinearity and noise in the fibre is a critical issue. This issue is currently addressed with electronic devices using optical-electronic-optical (OEO) conversion, which are inefficient and expensive.

Low power all-optical data processing devices, such as all-optical regenerators, offer an alternative solution. Optical regeneration can be performed using the four-wave mixing effect and can be used to reshape, re-amplify and retime the signal in the optical domain, which removes the need for OEO conversion and will not effect the data rate.

OSIRIS will develop a multichannel regenerator based on four-wave mixing. The regenerator will be based on a photonic crystal waveguide that exploits slow light effects. Ideally, the FWM effect will be enhanced by the slowdown factor of the crystal. This significant effect allows the reduction of the device length or the pump power, or both.

The work will be based on specifically engineered slow light waveguides that have been already been developed within the group. The goal of this project will be to demonstrate efficient optical regeneration in an engineered slow light photonic crystal waveguide of only 100's of micrometres in length, using only mW's of optical power.

Optical pulse regeneration using four-wave mixing in an engineered slow light silicon photonic
crystal waveguide.

Multichannel FWM and engineered slow light PhC waveguides based regenerator.

LECSIN - Lasing of Erbium in Crystalline Silicon Photonic Nanostructures

LECSIN Logo

One of the bottlenecks for the widespread application of Silicon Photonics, and for the merging of electronic and optical functions on the same chip, is the lack of efficient light sources in Silicon. The material itself is known to be a poor light emitter because of its indirect fundamental band gap. By doping silicon with erbium ions, it is possible to obtain a radiative transition in the 4f shell of the rare earth at ~1.54 µm. This system is very promising for obtaining controlled (and, possibly, electrically driven) light emission at telecom wavelengths and is especially interesting at low temperature, where several narrow emission lines are observed due to the crystal-field splitting. Moreover, the excitation cross section for Er in crystalline Si is particularly high being achieved through carrier recombination processes, thus yielding a good emission efficiency.

Much progress has been made in recent years in the control of radiation-matter interaction in III-V semiconductors through the use of micro- and nanocavities. A particularly interesting phenomenon is the Purcell effect, namely the reduction of radiative lifetime due to the interaction of the excited "atom" (usually a quantum dot) with a high Q cavity mode. The radiative decay rate then scales as Q/V, where Q is the quality factor and V is the mode volume. Ultra-high Q factors with low mode volumes were demonstrated for photonic crystal nanocavities in Si membranes, however those results were obtained in passive Si systems and only few studies of radiation-matter interaction in active Si-based photonic nanostructures have been reported up to now.

The project focuses on the control of radiative emission of Erbium ions in photonic crystal nanostructures made of crystalline Silicon, with the final goal of achieving laser emission around 1.54 micron wavelength. To this purpose, photonic crystal waveguides and nanocavities will be fabricated in Si membranes doped with Er3+ ions. Nanocavity structures that are resonant with the narrow emission lines of Er3+ at low temperature will tailor the radiative dynamics and enhance optical gain. Micro-photoluminescence experiments under suitable pumping conditions will allow studying the radiative emission of Er3+ ions, towards achieving net gain and lasing threshold. Theoretical studies of Er3+ emission coupled to nanocavity modes will allow exploring cavity quantum electrodynamic effects.

The LECSIN (Lasing of Erbium in Crystalline Silicon Photonic Nanostructures) project builds on a new European partnership by four groups at Catania, St Andrews, Pavia, and Grenoble, with complementary expertise in Silicon photonics, nano-technonology and nano-photonics, theoretical simulation and design, quantum optics. It is organized in two workpackages and eight tasks as follows:

WP1: ERBIUM-OXYGEN CLUSTERS IN PHOTONIC NANOSTRUCTURES

• Task 1.1: Erbium-Oxygen Nanoclusters

• Task 1.2: Photonic Nanostructures

• Task 1.3: Active Properties of Er-Doped Silicon Nanostructures

• Task 1.4: QED of Er in Si Photonic Nanostructures

WP2: AMPLIFICATION, LASING AND COLLECTIVE EFFECTS

• Task 2.1: Amplification in Silicon-on-Insulator Waveguides

• Task 2.2: Optical Pumped Lasing and Superradiance

• Task 2.3: Electrically Driven Lasing

• Task 2.4: Theory of Low-Threshold Lasing and Superradiance

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Speckled Computing EPSRC

Speckled Computing offers a radically new concept in information technology that has the potential to revolutionise the way we communicate and exchange information. Specks will be minute (around 1 mm3) semiconductor grains that can sense and compute locally and communicate wirelessly. Each speck will be autonomous, with its own captive, renewable energy source. Thousands of specks, scattered or sprayed on the person or surfaces, will collaborate as programmable computational networks called Specknets.

Overview of Specknet Project

Computing with Specknets will enable linkages between the material and digital worlds with a finer degree of spatial and temporal resolution than hitherto possible; this will be both fundamental and enabling to the goal of truly ubiquitous computing.

Our group is responsible for the optical communication between Specks, using vertical cavity lasers (VCSELs) as active elements.

For more information see the SPECKNET project website below.

Array of microemitters connecting to neighbouring specks

Metaflex - EPSRC

Metamaterials (MMs) are man made materials with unusual electromagnetic properties that are not typically found in Nature. They are the key to achieving such extraordinary properties as invisibility cloaks and perfect lenses. At present, they are bulky and confined to laboratories. If they were flexible, they could become much more versatile and practical. This project is working towards a novel concept for flexible MMs (Metaflex) that will turn current cloaking devices from suits of armour into true cloaks.

Concept for Metaflex that will turn current cloaking devices from suits of armour into true cloaks

Interest in Metaflex arises from diverse theoretical and experimental projects in photonic structures and nanofabrication and from the knowledge gained throughout these projects, including the physics and applications of MMs. This project contains many exciting scientific challenges, which offer the possibility of developing the extraordinary properties of MMs for every-day life applications that were unimaginable only a few years ago.

Recently we have managed to demonstrate that it is possible to fabricate Metaflex with a fishnet meta-atom, with a resonance in the visible. This could be the first step towards three-dimensional flexible metamaterials at visible frequencies.

A single layer of Metaflex placed on a commercial disposable contact lens