In this paper, we present a hybrid bistable optical device based on acoustooptic TE–TM mode conversion in an optical waveguide made of sputtered Ta2O5 on a glass substrate. The essential nonlinear transmission is provided by acoustic resonances. Bistability and optical limiting have been observed with less than 5 mW of acoustic power. A minimum value of 50 for the acoustooptic figure of merit (M2, relative to fused silica) of Ta2O5 is estimated from the results.