Coherent detection and manipulation of terahertz quantum cascade lasers

The terahertz (THz) region of the electromagnetic spectrum spans the frequency range between microwaves and the mid-infrared. Historically, this is the most illusive and least-explored region of the spectrum, predominantly owing to the lack of suitable laboratory sources of THz frequency radiation, particularly high-power, compact, room-temperature solid-state devices. Nevertheless, over the past decade, THz frequency radiation has attracted much interest for the development of new imaging and spectroscopy technologies, owing to its ability to discriminate samples chemically, to identify changes in crystalline structure, and to penetrate dry materials enabling sub-surface or concealed sample investigation.

One of the most significant recent developments within the field of THz photonics has been the THz quantum cascade laser (TQCL). These high-power compact semiconductor sources have opened up a host of new opportunities in the field of THz photonics and have attracted significant research interest world-wide. However, there is the need to develop techniques for measurement of the phase of the radiation field emitted from TQCLs, thereby providing a complementary technology to currently established incoherent detection schemes. Furthermore, there is a need to explore fully the advances that can be made through control and manipulation of the phase of the THz field emitted by TQCLs.
My vision is to initiate a range of research programmes with the aim of probing, manipulating and utilising the coherent nature of TQCL radiation. This will lay the foundations for a wealth of research opportunities in THz photonics, as well as facilitating the exploitation of THz technology for fundamental science and also for real-world applications.

I will develop both optical and electronic techniques for coherent detection/measurement of the field emitted by TQCLs. One means of achieving optical coherent detection is through the up-conversion of the phase and amplitude of the THz field into the near-infrared band with an electro-optic (EO) crystal. This approach will also allow the large field amplitudes and narrow line-widths of TQCLs to be exploited, enabling QCL radiation to be sampled using a broad-area EO crystal and a standard optical CCD. This will open up a significant range of opportunities for exploiting well developed visible/near infrared detector and CCD technologies within THz science. In parallel, I will develop coherent detection techniques by down-conversion of the THz field to radio frequencies. I will accomplish this through heterodyne phase-locking the fields from two TQCLs using a Schottky diode.

I will investigate coherent detection using self-mixing in TQCLs. This method relies on sensing junction voltage perturbations induced by feedback of the radiation field into the TQCL cavity, enabling coherent detection of the field using a single TQCL device as both source and detector. Using this approach, linewidth narrowing in TQCLs will be investigated, as well as techniques for three-dimensional 'detector-less' imaging and tomography.

I will also establish a programme concentrating on the radio-frequency control and manipulation of the THz field through the use of dynamic and static gratings, generated and controlled via the interaction of surface acoustic waves (SAWs) with TQCL devices. This approach will be used to provide a non-contact means to apply a potential modulation to TQCL devices, thereby providing a distributed feedback mechanism for the THz wave. As part of this I will develop TQCLs with reduced active regions thicknesses and TQCL mesa structures.

The combination of all these technologies will be combined to demonstrate the first 2D phase-sensitive THz tomography system using QCLs, the first full-field imaging system combining TQCLs and commercial CCD technology, and high-resolution THz gas spectroscopy.

The potential impact of the proposed programme is far-reaching and would encompass academic, economic and societal aspects.

Academics, both in the UK and internationally, will benefit in the short-medium term (1-5 years) through the scientific advancements and technological developments accomplished by the proposed research. Specifically, these include: Achieving greater understanding of semiconductor lasers and detectors, the development of terahertz (THz) quantum cascade laser (TQCL) technology, the development of THz systems applicable to a wide range of research areas across the physical, chemical and biological sciences, the development of novel coherent detection and measurement techniques for TQCLs, as well as facilitating the exploitation of well-developed visible/near-infrared technologies in THz science. In addition, this programme will lead to longer-term economic impacts through the training of PhD researchers and undergraduate students in semiconductor fabrication techniques, laser photonics, and system development skills that will be directly transferable to careers in high-technology industries including telecommunications. This programme will also enhance the reputation of the University of Leeds as an international research institute, thereby improving the UK's competitiveness.

However, the impact of this programme goes far beyond this. Owing to the unique properties of THz radiation, THz systems have many potential application areas outside of academia including pharmaceutical process monitoring, airport security screening, chemical sensing, industrial inspection, non-destructive testing, and medical imaging. As such, high-technology engineering companies in the UK will benefit in the medium-long term (5-10 years) through the development of THz imaging, tomography and spectroscopy systems based on TQCL technology within this proposed programme of work. An example of such a company is Teraview, who supply Ti:sapphire-based spectroscopy systems to the pharmaceutical sector, where they are used to characterise polymorphs of drugs during development and production cycles. The technological developments from my programme would therefore have a positive impact on the UK economy through the creation and growth of such companies, and the associated creation of wealth, and also by attracting R&D investment into the UK.

The THz systems developed within this fellowship will also have potential long-term impact (>10 years) both to the public sector as well as society as a whole. For example, development of airport security screening systems (both for detection of concealed weapons and sensing of chemical substances including explosives) would have significant positive implications to national security, thereby also improving public well-being. In this respect, benefactors would also include UK governmental agencies (e.g. Home Office, HMGCC). Likewise, improved production cycles of pharmaceuticals would both improve the effectiveness of public services (the NHS) and also general quality of life and health/well-being through improved drug treatments. Other potential impacts to society, in the long-term, include the development of non-invasive medical imaging techniques as well as environmental monitoring systems, the latter of which could also have profound implications to the long-term protection of the global environment.

Through examples such as these, where societal benefits are immediately tangible, the public awareness, appreciation and understanding of science/technology will also benefit.