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

Coherent three-dimensional terahertz imaging through self-mixing in a quantum cascade laser

We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in a quantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the amplitude and phase parameters of the reflected field. From the phase, we reconstruct the depth of the sample surface, and we show that the amplitude can be related to the sample reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL. (C) 2013 AIP Publishing LLC

Generation of continuous wave terahertz frequency radiation from metal-organic chemical vapour deposition grown Fe-doped InGaAs and InGaAsP

We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAsmaterial found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and materialdoping level were also studied

Diffuse-Reflectance Spectroscopy Using a Frequency-Switchable Terahertz Quantum Cascade Laser

We demonstrate diffuse-reflectance (DR) spectroscopy of powders using a discretely-tunable terahertz-frequency quantum cascade laser (THz QCL) with a heterogeneous active region. DR signatures were obtained at frequencies of 3.06, 3.21, 3.28, and 3.35 THz, and the relative absorption coefficients were inferred at each frequency using a Kubelka-Munk (KM) scattering model. The spectral lineshapes reproduce the absolute Beer-Lambert (BL) absorption spectra of a range of materials, which were also measured using conventional transmission-mode THz time-domain spectroscopy. It is shown that the DR technique works reliably for materials that include pharmaceutical compounds and foodstuffs, with BL absorption coefficients in the range 2-10 mm(-1). This method is potentially suitable for automated material identification, without any requirement for a priori knowledge of the refractive index or scattering properties of the sampled material