Novel methods in modulation and detection of terahertz waves for imaging applications

Takan, Taylan
Terahertz radiation, due to its unique properties offers a plethora of possibilities of applications in various domains including security & defense, communications, material characterization, product inspection and medical imaging. In all these fields, cost-effective, high-resolution, and high-speed terahertz imaging is a greatly desired technology. However due the inherent challenges associated with terahertz detection, building a terahertz imaging system that satisfies all these requirements necessitates the development of novel detectors, components, and imaging methods. In this thesis, we present the work done to advance terahertz detectors, modulators and imaging methods, towards the aim of developing a cost-effective, high-resolution, and high-speed terahertz imaging system. For the cost-effective detection of terahertz radiation, we investigate the use of neon indicator lamps as glow discharge detectors and achieve response comparable to commercially available high-cost detectors. Using a Schottky diode multiplied source and lock-in techniques behavior of glow discharge detectors up to 90 kHz modulation is investigated in the range from 0.260 THz to 0.380 THz. Besides, polarization sensitivity and transmittance of these detectors are also measured in this range. Overall, results obtained from these investigations suggest up to a twofold increase in detected signal as the modulation frequency increases from 1 kHz to 90 kHz. Furthermore, the measurements show that internal geometry of these detectors influences their interaction with terahertz radiation significantly and certain types of indicator lamps perform much better. For spatial modulation of terahertz waves, devices utilizing graphene and vanadium dioxide are characterized. Also, the coupling of metamaterials and frequency selective surfaces to these devices are studied. We present the experimental results for a graphene-based tunable THz cavity that provides %98 modulation at 0.370 THz. Also, a graphene-based spatial light modulator, capable of enabling broadband modulation of THz waves with voltage controlled patterns is investigated. Besides, we achieved frequency selectivity in the spatial light modulator by coupling it to frequency selective surfaces and metamaterials. Furthermore, a vanadium dioxide based metamaterial is shown experimentally to have frequency selectivity and temperature controlled transmissivity. Finally, we give experimental results of four different terahertz imaging systems which were set up to see objects hidden behind and inside walls and other visibly opaque obstructions. We show more than two times increase in speed compared to conventional pixel scanning methods, through the use of compressed sensing. Besides, we present a novel superimposed spatial light modulator that enables up to ten times reduction in size and increase in speed. Furthermore, by using the graphene-based spatial light modulator, we demonstrate an electrically controlled terahertz imaging system.