High efficiency resonant-metasurface germanium photodetector with ultra-thin intrinsic layer

Photodetectors at telecommunications-band with high efficiency and high speed are becoming increasingly important as the booming of big data, 5G, internet of things, cloud computing, artificial intelligence and relevent applications. Silicon-based Germanium photodetectors exhibit great potential in reducing the cost and power dissipation, due to its compatibility of monolithic integration with signal-processing electronics. We report the first demonstration of normal incident resonant-metasurface germanium photodetector, to address the trade-off between quantum efficiency, bandwidth and wavelength coverage for free-space detectors. With an ultra-thin intrinsic layer thickness of 350 nm, a high external quantum efficiency of more than 60% and clearly open eyes at the speed of 20 Gbps are achieved, for a 30 {\mu}m-diameter device. The photodetector employs multiple trapped-mode resonances to enhance the localized electromagnetic field, which not only enhances the external quantum efficiency by more than 300% at 1550 nm, but also extends to the whole C band. Further simulation and measurement with small incident spot size show the feasibility to achieve >50 GHz 3 dB bandwidth by simply reducing the mesa size, with minor sacrifice of the enhanced absorption. This work paves the way for the future development of low-cost, high efficiency normal incident germanium photodetectors operating at data rates of 50 Gbps or higher.