Polarization and incident angle insensitive germanium nano-pyramid array for perfect absorption in visible regime

An extraordinary light harvesting property of a periodic nano-pyramid array is reported. It is found that the nano-pyramid array can absorb light efficiently with an average absorptivity of 0.998 over the whole visible waveband. The ultra-high absorption is independent of the incoming light on polarization state and insensitive within a broad angle range. We attribute the efficient light harvesting property of the nano-pyramid array to Fabry-Perot resonance physically. This perfect absorber has advantages of easy fabrication, cost effective, and stable energy harvesting.