Superheterodyne techniques in laser ultrasonics.

Abstract

Laser-based ultrasonics is a powerful tool for the mechanical characterization of thin films and nanostructures. In this paper, superheterodyne approaches to single-point and full-field detection of laser generated ultrasonic signals will be discussed. In these techniques, an amplitude modulated laser source is used to excite a narrow-bandwidth signal. The detection laser, incorporated into an interferometric detection scheme, is also amplitude or phase modulated at frequency that is offset from the generation laser modulation frequency by a fixed amount, serving as the local oscillator for superheterodyne detection. Introduction of the local oscillator allows for the optical down-conversion of the high-frequency intensity modulation associated with sample motion to a low and fixed intermediate frequency given by the difference between excitation and detection laser modulation frequencies. The primary benefit of using this approach is that the upper frequency bound is not dictated by the speed of the detection photodiode and electronic circuitry or, in the case of full-field detection, the frame rate of the CCD. Results are presented demonstrating single-point superheterodyne detection of gigahertz frequency bulk and surface acoustic waves using a low-frequency photodiode and full-field superheterodyne detection of megahertz vibrations of nanostructure arrays using a low frame rate camera.