Measurements of surface-wave harmonic generation in nonpiezoelectric materials

Abstract

Methods for exciting surface waves suitable for harmonic generation experiments in nonpiezoelectric materials have been investigated. Such methods are needed for improved characterization of engineered surfaces. Unlike conventional ultrasonic techniques, nonlinear experiments require the ability to produce finite-amplitude, spectrally pure surface waves. Comb structures were selected from the methods evaluated, because they appear to offer the greatest potential for harmonic generation experiments on arbitrary substrates. The out-of-plane component of the surface waves was measured using a Michelson interferometer. The apparatus enabled direct measurement of absolute displacements with a spatial resolution of approximately 50 μm. Using this combination of excitation and detection techniques, the evolution of narrowband surface waves in the 1–10 MHz range was studied. Displacement amplitudes as large as 35 nm at a fundamental frequency of 10 MHz were observed. The spatial behavior of the fundamental and second-harmonic displacements indicated that, for correct interpretation, it was necessary to account for diffraction and other geometrical effects. Quantitative results will be presented for a variety of experimental configurations, including different substrates and comb shapes. Experimental results will also be compared to predictions of available theories for nonlinear surface wave propagation.