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Birefringence of GaN/AlGaN optical waveguides

We have experimentally studied the birefringence of wurtzite GaN grown on a sapphire substrate. The measurements were done with single-mode GaN/AlGaN planar optical waveguides on c-plane grown heterostructure films. The refractive indices were found to be different for signal optical field perpendicular or parallel to the crystal c axis (n⊥≠n∥). More importantly, we found an approximately 10% change in index difference Δn=n∥−n⊥ with variation of the waveguide orientation in the a–b plane, and a 60° periodicity was clearly observed. This is attributed to the hexagonal structure of nitride materials.