High efficient photodetectors are of paramount importance in optical communications as the advent of the big data era. The bandwidth-efficiency trade-off of detectors has always been being a limiting problem. We demonstrate here that ultrahigh absorption more than 85% can be achieved at wavelength 1300 nm by only patterning an ultrathin germanium (Ge) slab with a periodic array of air holes, which is about 3 times comparing with that of a uniform Ge slab of the same thickness. The enhanced absorption is mainly attributed to the critical coupling of the guided resonance of the photonic crystal slab. This work paves a way for high-responsivity surface-illuminated photodetection with a patterned ultrathin Ge slab.
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