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Title: Imaging using an electronically tunable terahertz quantum cascade laser
Author: Saat, Nor Kamilah Binti
ISNI:       0000 0004 2742 6001
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2011
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This thesis studies THz frequency imaging using different electrically tunable THz QCLs as the source. The THz QCL is based on a three-well resonant phonon active region design and uses a single metal waveguide. Using a heterogeneous cascade scheme, obtained by changing the growth rate of Ga during epitaxial process, the thickness of the GaAs is reduced progressively by 10% throughout the active region. As a result, the emission frequency can be varied from 3.05 to 3.35 THz by switching the bias of the QCL. Two types of imaging system are demonstrated. First, dual frequency imaging is demonstrated by electrically tuning the QCL bias, and a transmission image of the high explosive PETN and other polycrystalline samples (i.e. lactose monohydrate, glucose monohydrate, and sucrose) at frequencies of 3.05 THz and 3.24 THz are obtained in a single raster scan of a sample. By taking the difference of the natural logarithms of the transmission, coefficients obtained at each frequency, difference- attenuation-coefficient images for the samples can be obtained. The difference- intensity imaging may also be found directly by combining amplitude modulation of the QCL bias, and hence output frequency, with lock-in detection. Owing to their unique molecular-specific absorption spectra, the samples measured could be readily distinguished using this methodology. Second, multi-frequency imaging is demonstrated by electrically tuning a heterogeneous active region QCL at five different biases, producing five different output frequencies. Using a scanning resolution target, the imaging system has been shown to be independent of the radiation wavelength, which has been explained through measurement of the far-field emission profile of the QCL. Using the imaging system, the attenuation coefficients of polycrystalline samples have been determined at five frequencies and these results compared with those obtained using THz frequency time-domain spectroscopy and Fourier transform spectroscopy
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available