Photonically driven pseudo-continuous and broadband THz phase shifting using spatially resolved photoconductivity modulation

We report what we believe to be a novel and unique approach for achieving high-performance and broadband THz phase shifting based on spatially-resolved photoconductivity modulation (SRPM). By changing the illumination area on a hybrid Au-Ge mesa-array (AGMA) structure in front of an indium tin oxide (ITO) layer for local photoconductivity modulation, the phase difference between the incident- and reflected-waves can be tuned nearly continuously with extremely low reflection loss. For a prototype demonstration, a photonically-driven THz phase shifting device based on the WR-5.1 (140-220 GHz) waveguide configuration was designed, modeled and simulated. To achieve phase tuning in the range of 0° to -180° at 180 GHz (band center frequency), a mesa-array consisting of 12 × 6 unit cells (each 105 µm × 105 µm) was designed, and a distance d of 250 µm between the AGMA and ITO was used. The SRPM is accomplished using computer-generated light patterns from a closely-coupled micro-LED array for through-ITO illumination, without the need for any biasing circuitry. Full wave simulation results have shown that pseudo-continuous and broadband phase shifting can be achieved in the entire WR-5.1 band, and a shifting range of 0° to -180° at 180 GHz can be realized as designed. In addition, by using light patterns of different combinations of vertical strips, a fine phase tuning step as small as ∼0.05° can be demonstrated. For all phase tuning states, the simulated reflection loss is generally less than 1 dB with low loss variation. The proposed technology for high-performance THz phase modulation is promising and powerful, while offering far more design flexibility and frequency scalability than the current state-of-the-art since it requires no biasing wires thus eliminating parasitic-related performance degradation. Therefore, this technology is suitable for the development of large-scale THz phased-arrays, reconfigurable reflectarrays, and tunable metasurfaces for dynamic beam steering/forming required in next generation (6G or beyond) wireless communications.