Hybrid Colloidal Quantum Dot Lasers for Conformable Photonics

We propose an innovative hybrid technology for mechanically-flexible lasers based on semiconductor colloidal quantum dots. These lasers will be suitable for integration in conformable photonic systems and have identified applications in (bio)-sensing.

Photonics on mechanically-flexible platforms (so called flexible or conformable photonics) is an exciting and potentially disruptive technology and applications in which it already plays a role include bendable displays, electronic paper and solar cells. It is an integral part of the field of plastic electronics, whose worldwide market will be worth several hundred billion pounds in 2025 according to expert analysts. In order to create the flexible photonic systems of the future and unleash the associated benefits, the availability of a laser technology compatible for embodiment in flexible formats is critical. Suitable solution-processed organic semiconductor (OS) and inorganic colloidal quantum dot (CQD) lasers have been the subject of intense research although only a minority rigorously studied mechanical flexibility. OS are carbon-based materials and CQDs are minuscule inorganic crystals with typical diameters below 10 nm, which benefit from a narrow, shape and size-dependent emission colour. Both OS and CQD can be processed from solution, are compatible with a wide range of materials including plastics and provide gain across the visible spectrum. Despite these attractive attributes, the deployment of these materials in practical laser systems has been hampered up to now due to specific material limitations.

In this context, we will create a mechanically-flexible laser technology based on hybrid CQD gain media that address these limitations. The approach combines the best of both OS and CQD materials into a new functional system with added light harvesting capabilities analogous to biological and biomimetic nanoantenna complexes.

More specifically our approach will build on the advantages of energy transfer effects, local field phenomena and enhanced material processability that nanocomposites confer. For this, different types of CQDs and OS will be blended or hybridised and optionally incorporated into transparent matrices. All-inorganic mixes of CQDs with heterogeneous sizes will also be studied. As opposed to existing soft laser gain media, these hybrid materials will relax tolerances on the excitation process and will enable a wide wavelength emission coverage with no compromise on the overall efficiency. Furthermore, by collecting and concentrating the excitation energy where it is needed, they will also lead to much improved CQD laser thresholds. In turn, the resulting nanocomposites will be patterned at the nanoscale to assemble low-threshold CQD lasers in a mechanically-flexible format.

The proposed lasers will be compatible with other categories of emerging conformable devices therefore paving the way for truly integrated conformable flexible systems. Ultimately they will be electrically-driven or more simply incorporated on top of flexible blue-emitting GaN lasers or light-emitting diodes (LEDs), yielding optically-pumped hybrid flexible lasers. Blue and violet emitting GaN devices on flexible platforms are the subject of recent intensive research in groups including our own and should be available in the near future. We note that such inorganic GaN sources emitting efficiently above 515 nm (blue-green) are challenging to develop due to inherent material constraints (the so-called 'green gap'). Henceforth, a hybrid integration approach may be regarded as essential for full visible region coverage as already demonstrated for rigid device formats. The flexible hybrid laser chips that we envision will find extensive applications. In particular, the structures are ideally suited for wearable (bio)-sensor systems, and we will take initial steps to demonstrate this capability.

This work is about creating a novel technology of mechanically-flexible lasers and is a step towards integrating the capabilities of photonics in flexible formats. Resulting devices, either stand-alone or as part of future high-level systems, will enable the deployment of photonic functionalities to non-planar surfaces. This will in turn create applications which would otherwise be impossible and will affect different fields as well as contributing to address broad societal challenges.

One such application, which directly springs to mind, is biology where media are clearly not planar but are curved and deformable. Flexible/conformable photonics holds the promise of high impact in this field by enabling novel techniques in order to study, monitor and probe living tissues. Such capabilities will bring advances in biological knowledge, control and medical monitoring, in turn bringing broader academic, commercial and societal benefits. In this programme, we have identified bio-sensing as a particular application that will benefit from our novel lasers. The soft laser materials of our devices will simplify functionalization for interaction with the biological world while allowing direct sensor positioning on biological surfaces/media. For example, wearable sensors that can be made to conform to body parts in the manner of a sticking plaster are envisaged. The plastic nature of the laser sensors also enables their direct implementation on existing glassware platforms such as those used by biologist in assays therefore extending the capability of existing biochemical measurement systems to new frontiers.

As we stated in the Case For Support, analyst sources estimate that the overall market for flexible electronics, which encompasses flexible photonics, will be worth a few hundreds billion pounds by 2025. The development of expertise and know-how in novel flexible photonic technology - to which the proposal will contribute - is therefore an important endeavour for the UK economy. Conformable photonics can also benefit lab-on-a chip applications for both life science and scientific instrumentation. These applications are equally set for growth in the next few years: a recent market analysis by Frost & Sullivan estimated that the European market for lab-on-chip and microfluidics will grow to around $1.5 billion in 2015. There are great opportunities for both planar and non-planar (i.e. conformable) plastic integrated light sources in such systems.

Although the above describes what we consider to be the main areas of impact, the research will also affect other fields such as energy by contributing to the engineering of light-emitting and light-harvesting soft-materials. It will additionally provide added competencies through the development of technical capabilities and know-how and through the training of individuals participating in the project. This research will therefore participate in addressing societal challenges while also bringing direct benefits to the UK's knowledge economy.