A thin-film-on-silicon acoustooptical modulator with multi-mode behavior

A thin film approach of an acoustooptical Bragg modulator on silicon substrates has been investigated. We used a piezoelectric ZnO layer for the SAW excitation and a light guiding oxynitride film separated by a lower refractive SiO₂-layer from the high index silicon substrate and zinc oxide film. The optical system was designed for a wavelength of 632.8 nm. Calculations for the optical field distribution yielded a 15 μm light guiding structure. Multi-mode acoustooptical interaction was concluded from numerical evaluation of SAW excitation and depth distribution of strain profiles for this 15 μm optical waveguide on (001) [110] silicon covered by an 8 μm ZnO top layer. By experiment, the Bragg reflection of the optical wave has been observed really in four different frequency regions due to the existence of three Sezawa modes additionally to the Rayleigh mode