Freestanding GaN grating couplers at visible wavelengths

Abstract

A freestanding GaN grating coupler is proposed for planar photonic applications within the visible-wavelength spectrum. This freestanding device was produced by double-sided fabrication, combining GaN front patterning with Si substrate back releasing and GaN slab back thinning. Transverse-electric (TE) and transverse-magnetic (TM) light-wave conversion into and out of the membrane through grating coupling was determined by optical measurement. The maximum coupling efficiency is up to 69% for TE waves and 66% for TM waves at each particular wavelength according to the finite element method (FEM) simulation results. The experimental results were also supported by reflective simulation based on rigorous coupled-wave analysis (RCWA). This work opens the way for freestanding GaN planar photonic devices operating within the visible-wavelength range. It also provides the possibility of monolithic integration of planar photonic and light sources on III-nitride active platforms.