Terahertz Detection Using Enhanced Two-Step Absorption in Photoconductive Metasurfaces Gated at λ = 1.55 µm

Superior ultrafast photoconductive properties make low temperature grown (LT) GaAs one of the best materials for photoconductive terahertz (THz) detection. However, the large bandgap of LT-GaAs limits its operation to gating at wavelengths shorter than 870 nm. Here, it is demonstrated for the first time that nanostructuring the LT-GaAs into a highly absorbing metasurface enables THz photoconductive detection with a pulsed laser at λ = 1.55 µm. The very weak sub-bandgap absorption mediated by midgap states in LT-GaAs is strongly enhanced using the concept of perfect absorption via degenerate critical coupling. Integrated with a THz antenna, the LT-GaAs metasurface enables high sensitivity THz detection with a high dynamic range of 60 dB and large bandwidth up to 4.5 THz. The LT-GaAs metasurface has the potential to serve as a universal ultrafast switching element for THz applications, enabling low-cost, turn-key THz systems for a variety of real-world applications.