Mode conversion in GaN based laser structures on sapphire due to the birefringence of the nitrides

In this paper, we present our investigations of the waveguiding properties of various GaInN/GaN-DH and laser structures; particularly with regard to the influence of the substrate. Due to the small refractive index of sapphire and the neccessity of a thick GaN buffer, several higher order modes (up to 30) are guided in a complete laser structure on sapphire. This and the fact, that the nitrides are birefringent, lead to mode coupling phenomena. To support our experimental results we performed calculations with a 4 x 4 transfer-matrix method, taking into account the birefringence of the material. The results are in good qualitative agreement with our experimental data.