Optical Generation and Characterization of Acoustic Waves in Thin Films: Fundamentals and Applications

This is an invited contribution to Annual Review of Materials Science. It summarizes our work in the area of picosecond pulsed laser excitation and detection of high frequency acoustic waves in thin films and other microstructures. The mechanical and acoustic properties of thin films and multilayer assemblies are important both for technological applications of these materials and for basic scientific studies of their physical behavior. Techniques that use short pulsed lasers to stimulate acoustic waves in thin films, and probe lasers to monitor these motions, are useful for accurately and nondestructively characterizing the high frequency acoustic physics of these systems. This review briefly summarizes some of these techniques, and focuses on a method known as impulsive stimulated thermal scattering, or transient grating photoacoustics. It describes the most advanced experimental techniques for performing this measurement, and outlines its application to the study of acoustic waveguide modes in a variety of thin films. These measurements, coupled with models for the physics of the modes, can be used to determine intrinsic mechanical properties of materials and structures that occur, for example, in microelectronics and high frequency acoustic filters. This article summarizes a selected set of existing applications, and concludes with an overview of future directions that include studies of the acoustics of complex microstructures such as microfluidic networks and synthetic phonic crystals.