Absorption Enhancement of Transmission-Mode InGaAs Photocathodes Based on Micro-Nanostructures

InGaAs photocathodes operating in the 0.9–1.7 μm spectral band are fundamentally limited by optical reflection losses and incomplete light absorption in thin active layers. In this work, we constructed periodic micro-nanostructured InGaAs photocathode models using finite-difference time-domain (FDTD) simulations, and systematically compared five micro-nanostructures (square pillars, circular pillars, square truncated pyramids, hexagonal prisms, and square pyramids) against a planar reference structure. At a wavelength of 1.55 μm, coupled scans of lateral dimension and height yielded maximum absorptance values of 0.749 for square pillars (0.6 μm side length, 0.9 μm height) and 0.682 for circular pillars (0.9 μm diameter, 0.9 μm height), corresponding to enhancements of 35.4% and 23.3%, respectively, relative to the planar reference. Circular pillars exhibited the lowest reflectance (0.072); however, their higher forward transmittance caused lower overall absorptance than that of square pillars. We then coupled the FDTD generation profiles to a steady-state drift-diffusion analysis of carrier collection. Under normalized illumination and a 5 V bias, the integrated generation increased by 29.2%. The modeled internal generation-rate-weighted collection efficiency ηC increased from 0.499 to 0.680, corresponding to a 36.2% enhancement, and the internally collected carrier-rate proxy increased by 76.1% relative to the planar reference. Under ideal fully-passivated side-wall conditions, the internally collected carrier-rate proxy increased by 76.1% relative to the planar reference. This study provides theoretical guidance for designing and optimizing high-performance micro/nanostructured InGaAs photocathodes.