(color online) (a) FDTD simulation absorptions versus wavelength for the bare Si NW arrays (black solid line) and Si NW arrays with 80-nm SiO₂ shell (red solid line). (b) Optical generation rates for uncoated Si NWA and 60-nm-thick SiO₂-layer coated Si NWA at three typical wavelengths: 400, 600, and 800 nm. (c) Average electric field ⟨ |E|²⟩ as a function of axial position (Z, where the bottom of Si NW is set to be the zero point) of the NW (⟨|E|²⟩ is normalized to the maximum value in the Si/SiO₂ (60 nm) structure).