Electromagnetic waves in the terahertz (THz) frequency region have attracted increased attention and interest owing to emerging applications in radio astronomy, medical imaging, biological sensing, security screening, defense, material characterization, and communications, etc. The vast development of THz sources and detectors in the last two decades has turned the THz research into a rapidly growing technological field. Despite the large strides made in those fields, there is still a lack of progress in the development of high-performance tunable and reconfigurable THz circuits and components. Those devices are critical for the implementation of unique functionalities and superior system performance for THz applications such as advanced THz imaging and adaptive high-speed wireless communications at THz band. Demonstrations of such functional THz devices have been reported, but most of them suffer from limited achievable tunability, versatility, and performance. Besides, they are mostly based on free-space quasi-optical configurations, which make them difficult to integrate into practical and compact THz systems.
In this thesis, we explore and demonstrate the use of high-performance reconfigurable THz waveguide components based on spatially-resolved optical modulation technology to implement key tunable and reconfigurable circuits that are required in advanced THz sensing and adaptive THz wireless communication systems.The theories of such a novel optical control methodology using photogenerated free carriers in semiconductors are introduced first. An optically controlled WR-4.3 waveguide attenuator with record-breaking 60-dB range, 0.7-dB insertion loss, 350 kHz modulation speed, and high stability has been developed and demonstrated using the suggested optical control methodology for THz sensing, imaging, and communications. Furthermore, tunable and reconfigurable photo-induced electromagnetic band gap (PI-EBG) structures have been investigated, and the performance potential of the PI-EBG has been studied through simulations. An X-band tunable and reconfigurable microwave band-stop filter (BSF) based on the PI-EBG structures has been designed and simulated, showing large tunable center frequency range from 8-12 GHz and adjustable stop-band rejection and bandwidth. Two limiting factors, localized heating effects and finite lateral spatial resolution (due to carrier diffusion), that may affect the circuit performance in this technology have also been investigated and discussed. To verify the concept of using photogenerated free carriers to realize tunable and reconfigurable microwave circuits, a reconfigurable microwave circuit prototype based on photo-induced microstrip line structure has been experimentally demonstrated, with its design, simulation, fabrication, and characterization presented and discussed in details. In addition, on the basis of the microwave demonstration, two optically controlled reconfigurable THz waveguide filters based on photogeneration of EBG structures have been designed and simulated at WR-5.1 band. The first design features a pre-patterned EBG ground, and the functionality of the component can be reconfigured between a BSF with a center frequency at 175 GHz and a transmission line. The second design features an increased level of tunability and reconfigurability at THz frequencies by employing mesa array structured ground planes, showing a wide center frequency tunable range from 166-200 GHz and reconfigurable stop-band rejection and bandwidth. Mesa arrays have been designed and employed to overcome the limited lateral spatial resolution in naturally existing semiconductors for more effective optical spatial modulation of THz waves. The much-improved lateral spatial resolution enabled by the mesa array structures allows for the implementation of dynamically reconfigurable THz waveguide filters for a wide range of applications including constructing advanced THz sensing, imaging, and communication systems.
Finally, on the basis of the accomplishments in this thesis, future research directions including the development of optically controlled integrated RF switches for the implementation of more advanced tunable and reconfigurable THz waveguide circuits and photo-induced substrate integrated waveguides with the potential for realizing universally programmable THz passive circuits have been envisioned and briefly discussed.