Time Reversed Ultrasonic Echoes as an Experimental Tool

Abstract

Ultrasonic parametric backward wave interactions were first observed by Damon and Van de Vaart1 who utilized the “parallel pumping” magnetoelastic interaction in YIG. In this method the backward propagating wave, at frequency ω, is generated through the parametric interaction of a forward propagating wave at ω, with a non-propagating pump field at 2ω. Subsequently, a similar interaction, second order piezoelectricity, was investigated by Thompson and Quate2 in LiNbO3. This effect exists in all piezoelectric crystals, and has been extensively utilized in nonlinear surface wave devices. More recently, two other ultrasonic backward wave interactions, ultrasonic spin echoes3, and polarization echoes4, have been discovered. For both these phenomena the pump frequency is the same as that of the forward and backward waves, ω5. The source of the nonlinearity in the case of polarization echoes occurs through electric field induced ionization of trapped electrons6, resulting in a space and time modulated free electron distribution, and/or a space modulated trapped charge distribution. We define two types of polarization echoes, parametric and holographic, according as the backward wave is generated by interaction of the forward wave with either the free or trapped distributions, respectively.