High Efficient Ge Surface Absorption Through Hybridized Cavity and Horizontal Surface Plasmon Modes

A new Au-SiO₂-Si stack grating structure to efficiently couple normally incident light to Au-Ge surface plasmon modes for enhancing Ge surface absorption is presented. Through an optimized design on the grating dimension, the surface plasmon modes are hybridized with a cavity mode for both suppressing light reflection and transmission to the Ge substrate. About 70% Ge absorption occurs in a 0.15-μm shallow layer near the Ge surface at the wavelength of 1.38 μm. Such a short absorption length is desirable for implementing a low-noised Ge avalanche photodiode.