Development of terahertz systems using quantum cascade lasers and photomixers

The terahertz (THz) region of the electromagnetic spectrum lies between the more established bands of microwave and infrared radiation. In the past few decades, this region has seen huge growth in the development of both THz sources and detectors for a growing number of potential applications including security, wireless communications, medical diagnostics and astronomy. This thesis makes use of three different methods of generation of THz radiation, these being, THz quantum cascade lasers (QCLs), THz time-domain spectroscopy (TDS) and terahertz photomixing.
In the first set of experiments, diffuse reflectance imaging of a range of powered samples has been demonstrated using a THz QCL. Imaging was done at four discrete frequencies in the range of 3–3.35 THz by electrically tuning the emission wavelength of the laser. Absorption coefficients of the samples was inferred using Kubelka–Munk model and was found to be in good agreement with the Beer–Lambert absorption coefficient obtained from broadband (0.3–6 THz) THz-TDS measurements.
In the second part of the work, photomixers were designed and fabricated on low-temperature-grown (LTG) GaAs substrates. Ex-situ annealing temperature of LTG GaAs was optimised for maximum bandwidth of the photomixers and the impact on recombination lifetime and resistivity of LTG GaAs was also studied.
The final set of experiments examined locking a THz QCL to an external stable source. This would allow access to both amplitude and phase information of the laser emission, which in turn would significantly improve the quality of the data obtained from QCL based imaging techniques, making them useful in many different applications. After investigates of various techniques to achieve this, photomixers driven at telecommunications wavelengths (~1550 nm) were successfully used to obtain injection locking a THz QCL.