Excitation of elastic surface pulses by fiber optics and near-field optical devices

Abstract

Elastic surface pulses or surface acoustic wave (SAW) pulses were launched at the surface by confining pulsed laser radiation with single-mode fibers and tapered fiber tips, produced by the focused ion beam technique, in near-field configuration. The propagation of the elastic surface pulses was studied in a point-source point-probe experiment. The transient surface displacements of the propagating SAW pulses were detected by an actively stabilized Michelson interferometer with sub-angstrom resolution. For tip aperture sizes in the micrometer range confinement effects could be observed, extending the frequency spectrum of the launched SAW pulses to approximately 800 MHz. The actual frequency spectrum was limited by the applied detection method. An inverse square root dependence of the SAW amplitude with propagation distance was confirmed experimentally, as expected for the excitation geometry employed. Results are presented for layered systems with dispersion of the ultrasonic surface pulses. The relevance of the frequency spectrum of the SAW pulse for the determination of elastic and mechanical film properties in dispersive media is discussed.