Acousto-optical light diffraction in thin films

The diffraction of optical guided waves in thin films through an acousto-optic effect is treated. The diffraction of the bounded optical waves in thin films is a rather complicated phenomenon as compared with that of the free waves in bulk media. An expression for the diffraction efficiency has been derived using coupled-wave equations. Acousto-optical figure of merit and the mode coupling factor applicable to the diffraction in thin films are defined. The TE0 and TM0 waves in an As2S3 film deposited onto a LiNbO3 substrate were diffracted by Rayleigh waves with 93 and 72% efficiency at electrical input powers of 150 and 50 mW, respectively. The present experiment revealed that approximately 5% of the total acoustic power penetrated into the film and took part in the diffraction.