Photon trapping with amorphous-Ge gratings for high-efficiency eSWIR AlInAsSb avalanche photodiodes

The extended short-wavelength infrared (eSWIR) band is central to a wide range of scientific, industrial, and defense applications, driving continued interest in photodetectors that combine high quantum efficiency with low noise. However, using thin AlInAsSb absorbers as a means to reduce dark current, results in decreased absorption. Here, we introduce an all-dielectric amorphous-germanium (a-Ge) photon-trapping approach engineered to reshape the optical boundary conditions and enable efficient photon trapping in AlInAsSb SACM photodiodes. By tailoring the metasurface geometry to excite lateral resonant modes and achieve near-impedance-matched coupling, the structure concentrates electromagnetic energy within the absorber while simultaneously reducing surface reflectance by more than an order of magnitude. Devices incorporating the optimized a-Ge gratings show broadband increases in external quantum efficiency to ∼73%, more than a two-fold increase near 2 μm, and sustain improvement out to longer wavelengths. The resonance mechanism is polarization-robust, fabrication-tolerant, and fully compatible with high-field avalanche operation due to the lossless, electrically inert nature of a-Ge. These results establish dielectric Ge metasurfaces as a powerful and manufacturable approach to enhancing light–matter interaction in AlInAsSb SACM photodiodes, thereby enabling a scalable platform for next-generation eSWIR imaging and sensing technologies.