Perfectly absorbing photoconductive metasurfaces for terahertz applications

Optoelectronic devices, including terahertz (THz) photoconductive detectors and emitters, require efficient optical absorption and ultrafast photoconductivity switching. To overcome material limitations of standard photoconductors, here we demonstrate perfectly absorbing all-dielectric photoconductive metasurfaces made of interconnecting nanoscale GaAs channels. The metasurface supports two degenerate Mie modes - the electric and magnetic dipoles - which are critically coupled to the incident 800 nm excitation to achieve full absorption. The combination of perfect absorption, photoconductivity and wavelength tunability makes the metasurfaces ideal for terahertz photoconductive detectors that use pump beams in the near-infrared spectral range. In this application, the metasurface replaces the bulk, sub-optimal active region in the gap of a THz antenna with an ultra-thin (160 nm), highly absorbing photoconductive layer.