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Title: The development of a continuous-wave terahertz imaging system
Author: Gregory, I. S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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Scientific progress in the terahertz (0.1-10 THz) region of the electromagnetic spectrum has been impeded by a lack of coherent sources and detectors. Recent developments have seen the emergence of novel optoelectronic devices, which have generated considerable interest. One such approach is the coherent emission and detection of continuous-wave (cw) THz radiation by mixing two cw visible or near-infrared lasers in a semiconductor. The photomixer efficiency is dependent upon the carrier dynamics of the semiconductor material, in addition to the electronic properties of the antenna and electrodes. This thesis describes work undertaken to develop and characterise these components. Low-temperature-grown (LT)-GaAs is used for the photoconductor, with the necessary ultrashort (100 fs) carrier lifetimes achieved by a new approach to the post-growth annealing. This success is explained using a novel semi-quantitative model to describe the trapping of charge carriers by defects in the material. The photomixer and antenna designs are independently optimised using both finite element simulations and experimental characterisation at frequencies up to 2 THz. Detailed measurements of the polarisation state of the THz radiation allow a better understanding of the role of the photomixer and antenna at different frequencies. Antenna theory and equivalent electrical circuits are applied to assess the impedance match for optimum power transfer to the antenna, and resonant designs are shown to improve both emission and detection, particularly when coupled using a choked feed. Simulations show that the observed power enhancement at resonance is associated with improved directionality of the far-field radiation pattern. THz radiation has many potential applications, including imaging. Most established imaging systems require a femtosecond pulsed laser, but its high bulk and cost might limit commercial market penetration. In contrast, tunable cw diode lasers are small and inexpensive, and cw-imaging systems might help to generate large volume commercial THz products. For the first time, technology based on diode lasers is combined with a phase-sensitive, room-temperature homodyne detection scheme to demonstrate a system that is compact, robust, genuinely turn-key and of low cost. The unsurpassed performance of the present system, enabled by the innovations presented here, is illustrated with a variety of application examples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available