Exploring Short Wavelength Limits for High Performance Quantum Cascade Lasers

The realisation of high performance quantum cascade laser (QCL) sources at the short wavelength end of the 3-5 micron atmospheric transmission window is of major interest for a wide range of technological applications. Many of these are potentially of great significance for healthcare, security and the environment. However, conventional QCL materials systems such as InGaAs-AlInAs are fundamentally unsuitable for such short wavelength devices, as they do not have sufficiently deep quantum wells to support the high energy intersubband transitions required. Consequently, in recent years, attention has turned to alternative QCL materials systems based on III-V antimonides. At Sheffield we have established considerable expertise in the InGaAs-AlAsSb materials system. In addition to very deep quantum wells (~1.6 eV), this system provides lattice matched compatibility with InP-based waveguide and device fabrication technology. In this project we will develop short wavelength InGaAs-AlAsSb QCLs that will redefine the state of the art for semiconductor lasers in the 3-4 micron region, and provide unprecedented levels of performance and functionality for trace gas sensing and countermeasures applications. We will also exploit the potential of such deep QW devices for new developments in intersubband non-linear optics, in particular the demonstration of QCL operation at telecommunications wavelengths via intracavity second harmonic generation.

The 3-4 micron wavelength range is of key technological importance for a wide range of applications. As a consequence of the strong fundamental C-H stretch mode that occurs at around 3.3 microns, the detection of many important hydrocarbon species, for example ethane, methane, acetone, formaldehyde and butane has maximum sensitivity (potentially ~ parts per trillion) in this range. This leads to many potential applications in areas such as clinical diagnostics (breath analysis), process monitoring, control of outdoor and indoor pollution, and remote detection of oil and gas deposits. Many other examples exist, including numerous applications in the defence and security arena (IR countermeasures, explosives and weapons detection, etc) and free-space last mile telecommunications. A key aspect of our proposed work is that it will lead to the first truly high performance compact semiconductor sources in this wavelength range, which will facilitate major breakthroughs in the development of the above applications. The diversity of these applications means that there will be interest from a wide range of beneficiaries, including, for example, private companies with interests in trace gas monitoring, private and national transport providers, industrial manufacturers, defence contractors, the armed forces, law enforcement and border security, the medical sector, etc. Our work clearly therefore has the potential for very significant impact in economic terms and also in enhanced security and quality of life. In order to ensure that this impact is maximised, we will take the following steps (described in detail in the Impact Plan). 1) The project will be carried out in close collaboration with Cascade Technologies Ltd. In addition to providing invaluable feedback on the suitability of the devices we develop for sensing applications, Cascade will provide a direct route to market and provide links with major end users of the technology in the applications areas described above, with whom they have very strong existing interactions. 2) In collaboration with the EPSRC III-V National Centre we are currently putting into place plans for pilot production of single mode QCLs at longer wavelengths, where we have well-established capability to produce devices of commercial quality. This activity will naturally expand to encompass advances in our short wavelength QCL programme, providing a clear and direct route for technological exploitation. 3) We will exploit the strong reputation of R. Hogg (CI, Sheffield) and S. Sweeney (PI Surrey) in telecoms lasers and their links to the UK and international telecoms industry to ensure maximum exposure and impact for our work on telecoms wavelength SHG QCL emitters. 4) We will establish an annual UK-QCL workshop to promote academic-industrial interactions, thus maximising the industrial impact not only of the Sheffield work but also of UK research in mid IR/intersubband optoelectronics as a whole.