Enhancing the absorption of a thin germanium slab with periodical patterning

We propose a germanium (Ge) photonic crystal thin slab to achieve perfect absorption efficiency in telecom band. The maximum absorption is increased 14-fold compared to a planar cell with the same thickness at normal incidence. The absorption enhancement is attributed to the critical coupling with guided resonances by introducing an airhole photonic crystal on the thin slab. This novel design constitutes an important step towards high responsivity and high-speed photodetection with strong energy harvesting.