Guided-wave acousto-optic diffraction in Al_xGa_1-xN epitaxial layers

The acousto-optic (AO) diffraction of guided optical waves from surface acoustic waves in Al_xGa_1-xN layers grown on sapphire substrates by Migration Enhanced Metalorganic Chemical Vapor Deposition (MEMGCVD™) technique was studied at the optical wavelengths of 442 and 633 nm and acoustic wavelength of 16 μm. In the near-to-Bragg diffraction regime, the diffraction efficiency from 90% to 95% was attained at SAW powers of 0.28 and 0.72 W for the blue and red light, respectively. The simulation using photo-elastic and electro-optic constants reported in literature revealed the prevailing contribution of the photo-elastic effect to the AO diffraction. The calculated SAW power required to attain the diffraction maximum was about seven times larger than the measured values. This discrepancy implies that the photo-elastic constants of AlN and GaN available from literature are underestimated. The increase in the diffraction efficiency with the decreasing optical wavelength is in a good agreement with the theoretical prediction. This feature makes AlGaN very promising for AO applications in the deep UV region.