In-Plane Optical Anisotropy of GaN Refractive Index in Visible Light Region

The optical properties of metal-organic chemical vapor deposition gallium nitride layers were measured with the use of a grating-assisted optical coupler. The application of a waveguide-based grating structure for in-coupling of radiated modes was the basis of the work presented here. A set of six grating couplers, providing different propagation angles Phi with respect to the a-axis of GaN, was fabricated in a multimode GaN planar waveguide layer grown directly on c-axis (0001) sapphire. Measurements of the refractive index were carried out for laser wavelength lambda = 632.8 nm and both transverse-electric and transverse-magnetic polarized light. It was found that the refractive indices were dependent not only on the polarization state but also on the propagation direction of the excited optical mode in the a-b plane. The 60^deg periodic in-plane anisotropy of GaN optical properties in the visible spectrum was clearly observed with maximum refractive index difference Deltan = 0.018 for both polarizations.