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The electromechanical coupling coefficient K2 for surface acoustic waves propagating on c and a surfaces of bulk AlN single crystals has been measured using the S11-parameter method in the frequency range of 160–360 MHz. The extracted values of K2 are 0.11% and 0.47% for the c and a surfaces, respectively. By fitting our experimental data to our numerical simulation results, we have estimated piezoelectric constants, which are in a reasonable agreement with literature data. Our results are consistent with the negative sign of the e15 constant.

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The acousto-optic (AO) diffraction of guided optical waves from surface acoustic waves in Al x Ga 1−x N 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 m. 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.

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