OPTICAL-DETECTION OF EVANESCENT ULTRASOUND WAVES IN WATER

Acoustical perturbation by targets smaller than wavelength can generat e evanescent waves, which decay with the distance of propagation. By p utting such targets immediately under the free surface of water, the d iffracted acoustical field can excite the surface membrane before evan esce, and it produces a static ripple because of the radiation pressur e. A collimated beam of light reflects at the perturbed surface, and i t becomes modulated in phase. In this work we show experimental result s where we achieve an optical image of the acoustical evanescent waves produced by an array-like target. Arising from the theory, we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from the target in order to verify the experimental r esults. With this method, we look for a spatial resolution smaller tha n acoustical wavelength, for normal incidence and plane waves. In our experimental device, we use a sound wavelength in water greater than 1 .5 mm, generated by a PZT transducer. We can resolve an array of 1.0 m m of periodicity.