High-Performance Photo-Induced Tunable/Reconfigurable Terahertz Circuits Using Mesa-Array Structures

In this work, high-performance terahertz (THz) tunable/reconfigurable circuits/components using photo-patterned mesa array structures have been demonstrated for implementations in advanced imaging, sensing, and communications. In recent years, a variety of THz applications, from radio astronomy to medical imaging, have been explored and developed. Accelerated by significant progress in THz signal generation/detection techniques, growing interest has been drawn to more advanced THz applications. Tunable/reconfigurable circuits/components that can provide dynamic modulation of THz radiation for multi-band and multi-functional operation are needed in those applications. Several tuning approaches including mechanical, thermal, and electrical tuning, have been reported for realizing THz tunable/reconfigurable circuits. However, those approaches tend to show slow tuning responses, as well as limited tunability/reconfigurability due to the use of conventional pre-patterned circuits. This could be solved by using optically-generated free carriers in semiconductors to form photopatterned circuit structures and realize freely tunable/reconfigurable circuits without the need for mechanical tuning and prepatterned circuits/devices. However, the achievable resolution in unpatterned semiconductors is limited due to the carrier lateral diffusion. In this work, we propose and investigate a novel approach for realizing tunable and reconfigurable THz circuits on the basis of semiconducting micromachined mesa-array structures. The mesa-array structure consists of two-dimensional arrays of subwavelength and electrically isolated semiconductor mesas, thereby confining the free carriers within each mesa. Consequently, high-resolution photopatterns (e.g., with a spatial resolution on the order of 10 µm, compared to typical ~400 µm diffusion length in Si) can be generated for the implementation of high-performance tunable/reconfigurable circuits in the THz regime. In this work, the optical properties of the proposed mesa array structures have been investigated through theoretical calculation and full-wave electromagnetic simulations. A Si mesa array prototype structure was designed, fabricated, and tested. Measurements show that a modulation depth of ~20 dB was obtained in the frequency range of 740-750 GHz under a light intensity of ~12 W/cm2. The mesa array structure has been implemented in THz photo-induced coded aperture imaging (PI-CAI) to form high-fidelity tunable/reconfigurable aperture masks for subwavelength spatial resolution. This novel approach for PI-CAI with subwavelength spatial resolution is promising in advanced high-resolution THz imaging and sensing applications. In addition, mesa-array-based tunable/reconfigurable THz bandpass mesh filters have been demonstrated with insertion losses of 0.82-1.13 dB in the frequency range of 108-489 GHz. Other functionalities such as bandstop filtering can also be realized by optically changing photopatterns illuminated on the same mesa array structure. The wide tuning range and reconfigurability of the mesh filters demonstrate that the proposed approach is promising for developing tunable/reconfigurable circuits with multiple functionalities. Furthermore, tunable/reconfigurable THz sub-system components have also been designed and simulated using Si pillar arrays. SIW transmission line, as well as more advanced SIW-based circuits, including SIW bend, single-pole double-throw (SPDT) switch, and phase shifters have been demonstrated. The pillar-array-based universally tunable/reconfigurable SIW structures favorable candidates in more advanced THz sensing and adaptive telecommunication systems.