Reflection of Microwave Pulses From Acoustic Waves: Summary of Experimental and Computational Studies

Abstract

Abstract : In [1, 4], the authors proposed and analyzed an interrogation (inverse problem) methodology based on use of an acoustic wave as a reflecting virtual interface for propagating impulses. It is by now well accepted (e.g., see [2, 7, 11, 14]) that acoustic pressure waves will interact with electromagnetic signals in ways that often mimic interfacial partial reflection/partial transmission for the electromagnetic waves. The response of atomic electrons to an applied electrical field in a material medium results in a material polarization with a concomitant index of refraction that is a function of the local density in the material. Thus, material density fluctuations produced by a sound wave induce perturbations in the index of refraction. Previous computational work in [1, 3] suggested that it might be possible to detect reflections of microwave frequency EM waves from a slowly (relative to the speed of the EM wave) moving acoustic wave front. These efforts are focused on reflections in a Debye medium. The authors made an argument for a simple pressure dependent dielectric model in which the Debye parameters exhibit a linear acoustic pressure dependence. In [1], finite-element simulations for a simple 1D geometry demonstrated computationally that EM reflections from the acoustic pulse are possible. These findings were confirmed with 2D computations in [3]. The results of [1, 3] consisting of a theoretical framework as well as computational validation of such an approach provide ample motivation for significant proof-of-concept experimental investigations of the proposed methodology. These prompted our preliminary experiment on which we report here to look for microwave frequency EM reflections from an acoustic pulse.