Photo-Induced Tunable/Reconfigurable Terahertz Circuits and Components

In recent years, electromagnetic waves in the THz frequency region have attracted increased interest owing to their prospective applications in many important fields such as radio astronomy, chemical and biological sensing, medical imaging, security screening, and communication. The vast development of THz sources and detectors has turned the THz research into a rapidly growing technological field. However, for THz applications such as advanced imaging and adaptive high-speed wireless communication, tunable and reconfigurable THz circuits and components are needed. This kind of devices enables us to implement unique functionalities and superior system performance. Such functional THz devices are still scarce but in high demand for sophisticated circuits and systems needed in advanced THz sensing, imaging and communications.

In this thesis, a novel approach to realize tunable/reconfigurable THz circuits and systems based on optical THz spatial modulation (OTSM) using photo-induced free carriers in semiconductors has been investigated. Using this approach, tunable THzmodulators achieving a maximum modulation depth of ~50 dB in the WR1.5 frequency band (500-750 GHz) have been demonstrated. Furthermore, reconfigurable photo-induced aperture masks have been developed for advanced THz coded-aperture imaging, which may pave the way for realizing high resolution, real-time THz imaging systems. In addition, photo-induced Fresnel-zone-plate (FZP) antennas with the capability of dynamic beam steering and forming have been designed and demonstrated at and around 750 GHz. The above quasi-optical tunable/reconfigurable circuits and components have been successfully applied in THz beam mapping, antenna characterization and remote scanning imaging.

Despite the success in developing and demonstrating the above tunable/reconfigurable devices and components, it has been observed that both the circuit performance and highest achievable operation frequency are limited by the OTSM performance using naturally-existing semiconductor materials. To overcome these limitations, a novel approach using mesa-array structures has been proposed for improved OTSM with higher modulation depth, spatial resolution and modulation speed. Employing the mesa-array structure as a technology platform, THz mesh filters with universal tunability and reconfigurability have been simulated. In addition, photo-induced substrate-integrated waveguides (PI-SIWs) have been investigated for providing nearly unlimited possibilities to realize more advanced tunable/reconfigurable THz circuits with multiple functionalities.