Coherent detection and manipulation of terahertz quantum cascade lasers

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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.