Optical waveguiding properties into porous gallium nitride structures investigated by prism coupling technique

In order to modulate the refractive index and the birefringence of Gallium Nitride (GaN), we have developed a chemical etching method to perform porous structures. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores density. GaN films are prepared on sapphire by metal organic chemical vapor deposition and the microstructure is characterized by transmission electron microscopy, and scanning electron microscope analysis. Optical waveguide experiment is demonstrated here to determine the key properties as the ordinary (n₀) and extraordinary (n_e) refractive indices of etched structures. We report here the dispersion of refractive index for porous GaN and compare it to the bulk material. We observe that the refractive index decreases when the porous density p is increased: results obtained at 0.975μm have shown that the ordinary index n₀ is 2.293 for a bulk layer and n₀ is 2.285 for a pores density of 20%. This value corresponds to GaN layer with a pore size of 30nm and inter-distance of 100nm. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices.