Acoustical phase conjugation experiments: The generation of a reversed wave through three-wave mixing in a layer of stabilized microbubbles

Abstract

A phase-conjugate mirror is one that reverses an incident wave front so that it propagates back toward the source. Recent experiments [Kustoy et al., Sov. Phys. Acoust. 32, 500–504 (1986)] indicate that wavefront reversal can be established through the interaction of a pump wave of frequency f1 with a probe wave of frequency.f2 <f1 that diverges from a point source. Three-wave (nonlinear) mixing occurred in a layer of freely rising gas bubbles in water so as to produce a reversed wave having a frequency f3 = f1 − f2. The present research yields evidence of reversed wave generation resulting from three-wave mixing in a layer of stabilized microbubbles. One method of stabilization is to use the gas-filled micropores of a Nuclepore ® polycarbonate membrane. Our experiments were carried out in a water tank of diameter 164 cm and with frequencies f1 and f2 > 300 kHz so as to avoid spurious boundary reflections. The previously predicted [P. L. Marston, J. Acoust. Soc. Am. Suppl. 1 82, 12–13 (1987)] longitudinal and transverse focal point shifts for the reversed wave are investigated. The signal for the reversed wave at frequency f3 is enhanced through the use of a background subtraction technique. [Work supported by ONR.]