Abstract
An interaction analogous to optical four-wave mixing in photorefractive materials is demonstrated for ultrasonic waves in a suspension of spheres in water. Counterpropagating 800-kHz acoustical waves establish a standing wave field in ∼5% saline solution in which the suspended ∼20-μm-diam Latex spheres are neutrally buoyant. When these pump waves are absent, the spheres are distributed uniformly. In response to the imposed acoustical radiation pressure from the pump waves, the spheres are attracted to pressure nodes and form bands. An ultrasonic probe wave is incident on these bands in the form of a higher frequency tone burst. The banded suspension acts as a diffraction grating for the probe wave giving rise to coherent scattering from the suspension when the Bragg condition is satisfied. The Bragg scattering amplitude was studied as a function of the pump amplitude and compared to simple models. The scattered wave has the same frequency as the probe wave. The temporal evolution of the Bragg scattering amplitude in response to sudden changes in the pump amplitude was also studied. [Work supported by ONR.]
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