  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.
