Organic semiconductors have shown great potential as laser gain materials due to a combination of high available optical gain, solution processability and broad emission spectra. Only a thin film of material is required to achieve lasing leading to a great flexibility in the form and shape of organic lasers that range from microcavities such as microspheres and micro rings to distributed-feedback (DFB) lasers where the gain region is only a few hundred nanometres thick. The broad emission of these materials allows organic lasers to be highly tuneable across their emission spectrum while the ability to modify their optical properties through synthetic chemistry leads to laser devices that span the visible spectrum: a rainbow of organic lasers!

Our research extends over all aspects of organic semiconductor lasers, from fundamental research to device manufacturing and testing. We investigate new materials by performing complete photophysical characterisations and evaluate their potential as laser gain media. Resonator design is also important as it allows for optical feedback that matches the emission spectrum of different materials and can be tailored to match the needs of different applications. We combine these two aspects of laser design to fabricate and test complete devices, evaluating all operational aspects such as lasing threshold, efficiency and device lifetime. We also explore new and exciting aspects of organic lasers such as alternative optical pumping schemes and new applications that make use of the unique properties of this fascinating family of lasers. Below are some examples of new materials, fabrication techniques and novel devices that highlight the fascinating nature and great diversity of organic semiconductor lasers.

LED pumped polymer laser

The emission spectra of a corrugated fluorene copolymer waveguide pumped by an InGaN LED with variation of the LED different drive current. The threshold current is 152 A.

Electrically excited organic lasers are as yet an impossibility due to the low carrier mobilities in organic semiconductors. The low carrier mobilities lead to a number of problems, for example injected charge absorption loss and electrical contact loss, which both increase the pump energy required to reach the lasing threshold. Alternatively, by utilising the advantages of inorganic semiconductors, our group demonstrated a world first, a hybrid device with a polymer laser pumped by a high power InGaN light emitting diode (LED). The LED pumped polymer laser exhibits all the benefits of a direct electrically pumped laser (e.g. compact, cheap, current/voltage controlled etc.), thus reducing the size and the cost of the polymer laser system. Shown on the left is the emission spectra from the yellow emitting polymer laser under different drive currents applied to the LED. When the current is increased up to 148 amperes the photoluminescence increases linearly. A further increase of the drive current results in a peak appearing at 568 nm, close to the second dip of the spectrum where the Bragg scattering stop band is. The full-width-half-maximum of the peak is 1.1 nm, indicating that the device is lasing.

Encapsulation of polymer lasers

Normalised output energy as a function of total number of incident pulses for (a) the unencapsulated laser and (b) the encapsulated laser, both with pulse energy of 250 nJ/pulse.

A significant challenge for the application of the polymer lasers is their susceptibility to degradation in air due to the presence of oxygen and water, which cause instability of the polymer. Encapsulation provides an effective route to prolong the device lifetime. Encapsulated lasers can be realized by putting additional optically transparent and passive layers on top of the gain medium. These extra layers act as an atmospheric barrier to water and oxygen. Shown on the right is a comparison of the device lifetimes between an unencapsulated (a) and an encapsulated (b) laser in terms of the number of incident pump pulses. For the encapsulated device, when it is pumped at 250 nJ/pulse the lifetime of the encapsulated device has increased by a factor of >2500. The lifetime value is equivalent to the best reported unencapsulated lasers operating in vacuum.

Bisfluorene dendrimer lasers

Photoluminescence and amplified spontaneous emission (ASE) spectra of bisfluorene dendrimers. Inset: Molecular structures and film PLQY values.

Fluorescent dendrimers are a special category of organic semiconductors sharing their many features and advantages but also adding extra degrees of freedom in their design. They have a modular structure consisting of a light-emitting core onto which a number of branches (dendrons) are attached that allow the control of intermolecular interactions, while end groups are attached at the ends of the dendron to control the solubility of the materials in various solvents. Each part of these molecules can be separately designed, modified and attached to the rest of the molecule to affect only part of its properties. Bisfluorene dendrimers are blue-emitting materials with a bisfluorene core and various dendrons that combine their very attractive emission range with very high fluorescence efficiencies. We have performed extensive photophysical characterisation of this novel family of materials that allowed us to model the optical gain behaviour as a function of key material properties and to determine the best candidate to make an organic semiconductor laser. We have demonstrated low lasing threshold (4.5 microjoules per square centimeter) and excellent tunability (15 nm) in a solid-state blue-emitting bisfluorene laser based on one- and two-dimensional distributed feedback grating resonators.

Two-photon pumped bisfluorene lasers

Optical output and tunability for a two-photon pumped bisfluorene dendrimer laser.

Organic materials can have additional properties that allow for novel approaches to making laser devices. One such alternative approach revolves around shifting the pump wavelength for organic lasers from its normal regime of shorter wavelengths than the material's emission to longer wavelengths. This is called fluorescence up-conversion and can be implemented through two-photon absorption, a nonlinear absorption process by which two photons of lower energy (longer wavelength) can be used to bridge the energy gap of the material instead of one high-energy (short wavelength) photon. This is of particular interest for blue emitting materials, as their normal absorption band lies in the ultraviolet part of the spectrum for which there is a limited choice of pump sources. In the case of bisfluorene dendrimers, our studies show that there is substantial two-photon absorption near the red part of the spectrum. The combination of strong nonlinear absorption with high fluorescence efficiency allows us to demonstrate one of the world's very few two-photon pumped solid-state organic lasers, pumped at 590 nm and tunable blue emission (420 nm), highlighting some of the unique possibilities that organic semiconductors open up for laser devices.