Detection of evanescent acoustical waves on water surface by using defocusing optical method

The acoustical perturbation by targets smaller than the wavelength generates evanescent waves which decay with the distance of propagation. By putting such targets immediately under the free surface of water, the diffracted acoustical field can excite the surface membrane before evanescence, and produces a static ripple because of the radiation pressure. A collimated beam of light reflects at the perturbed surface and it becomes modulated in phase. In this work we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from an array-like target. The spatial periodicity of such an array is smaller than the acoustical wavelength. We also show experimental results where we achieve an optical image of the acoustical evanescent waves produced by the target. With this method we look for a spatial resolution smaller than acoustical wavelength (Rayleigh criterion). 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 mm of periodicity.