Room Temperature Terahertz Quantum Cascade Lasers on Silicon Substrates

The THz part of the electromagnetic spectrum has a number of potential applications which include oncology (skin cancer imaging), security imaging, THz bandwidth photonics, production monitoring and astronomy. The U.K. has been one of the pioneering countries in THz research but also in the exploitation of the technology with a number of companies including TeraView, QMC Instruments and Thruvision. At present most commercial imaging and spectroscopy systems use expensive femtosecond lasers with photoconductive antenna which fundamentally limits the power output to the microWatt level. Virtually all the applications referenced above require room temperature sources with over 10 mW of output power if parallel, fast, high performance imaging and/or spectroscopy systems are to be developed.While interband recombination of electrons and holes in Si and Ge are inefficient due to the indirect bandgap of the semiconductors, intersubband transitions provide an alternative path to a laser for low energy radiation such as THz frequencies. Intersubband unipolar lasers in the form of quantum cascade lasers have been demonstrated using III-V materials. Powers up to 248 mW at 10 K have been demonstrated at THz frequencies but due to polar optical phonon scattering and the associated reduction in intersubband lifetimes as the temperature is increased, such devices only operate at cryogenic temperatures. Previous work has been undertaken on p-type Si/SiGe quantum cascade lasers but due to large non-parabolicity and large effective mass (0.3 to 0.4 m_0) in the valence band, significant gain above 10 cm^-1 is difficult to engineer.In this proposal, we propose to use pure Ge quantum well designs and L-valley electrons for the first experimental demonstration of a n-type Si-based quantum cascade laser grown on top of a Si substrate. We demonstrate that the low effective of 0.118 m_0 and long non-polar lifetimes in the Ge/SiGe system potentially provide gain close to values demonstrated in GaAs THz quantum cascade lasers at 4 K and also potentially allow 300 K operation. Further the cheap and mature available Si process technology will allow at least a x100 reduction in the cost of THz quantum cascade lasers compared to GaAs devices. Such devices could be further developed into vertical cavity emitters (i.e. VCSELs) for parallel imaging applications or integrated with Si photonics to allow THz bandwidth telecoms. Finally we propose optically pumped structures which have the potential for broadband tunability, higher output powers and higher operating temperatures than THz quantum cascade lasers.This programme has brought together the modelling and design toolsets at Leeds University with the CVD growth expertise at Warwick University combined with the fabrication and measurement expertise of SiGe devices at Glasgow University to deliver internationally leading research. We have a number of industrial partners (AdvanceSis, Kelvin Nanotechnology and TeraView) who provide direct exploitation paths for the research. Successful room temperature quantum cascade lasers are an enabling technology for many new markets for THz applications including oncology (skin cancer imaging), security imaging, production monitoring, proteomics, drug discovery and astronomy.

Work has shown that THz has potential applications in medical imaging (oncology and dental areas), security imaging, explosives and narcotics detection and identification, drug discovery, proteomics, manufacturing quality control, gas sensing, pollution monitoring and astronomy. Present THz imaging systems cost >250k, use mirrors for optics and mechanically raster a single pixel using a <1 microWatt source. The success of this proposal would result in 300 K, high power (>10 mW) arrays of sources which could be used for parallel imaging systems. A tunable optically pumped source (using a MIR GaAs QCL pump) would also have a large number of applications where spectroscopic identification (molecular fingerprinting) is important. Such sources could be manufactured at extremely low cost compared to present photoconductive sources which require expensive femtosecond laser pumps and III-V QCLs. The achievement of fast, parallel imaging and spectroscopic systems would open up many new markets for THz systems and applications which previous have been unavailable due to slow, serial imaging and poor power-detector performance resulting in poor signal to noise. We will exploit successful results with the industrial partners AdvanceSis, Kelvin Nanotechnology and TeraView, providing IP and know-how to aid their economic growth and provide return to the U.K. investment in this research. Companies who purchase THz systems will benefit through new technology and increased productivity. The healthcare market requires far cheaper THz parallel imaging systems which could be significantly helped by the success of this proposal. At present all skin cancer detection requires an appointment with a consultant at a hospital and significant savings could be made if a safe and reliable diagnosis technique could be employed at GP surgeries. This is especially important as early detection is key for high survival rates. Also any 3D detection technique would significantly reduce trauma and patient time in hospital compared to present biopsy techniques thereby significantly reducing the costs for treatment for the NHS (see http://www.teraview.com/). Skin cancer imaging systems for GP surgeries would require a complete imaging system for less than 10k with source powers > 10 mW at room temperature for parallel imaging which could be achieved with the sources in this proposal. The success of this proposal could therefore have significant benefits to UK society by increasing the quality of living by fast detection of skin cancers and in doing so also significantly reduce the treatment costs in the NHS. From the security side, room temperature THz QCLs would be an enabling technology for the use of THz technology in portal security screening systems (i.e. walk through portals) for airports, civil buildings and public places. The technology would be enabling to allow safe screening through clothing at video rates for short stand-off distances compatible with portal systems. As the probability of terrorist attacks in the U.K. is higher than ever before, such fast detection using safe non-ionising radiation in public spaces could potentially save many lives. Also the detection of suicide bombers before detonation will not only save lives but also significant reduce the costs of first responders, the cost of medical care of victims and the cost of the clear up operation. While the Home Office through HOSDB is responsible for testing equipment, the potential beneficiaries in government include the Home Office (Police but also first responders at incidents i.e. Fire and Ambulance) and the Borders Agency. For the private U.K. airports, it is the operators who are responsible for security e.g. BAA. We will disseminate successful research to contacts in these agencies. In addition, any tunable source would be useful for spectroscopy systems for chemical and materials anaylsis. Hence the DH, NHS, HPA and DEFRA would all benefit through new analytical tools.