Single-shot capable surface acoustic wave dispersion measurement of a layered plate
Established techniques for characterizing a layer on a substrate system via surface acoustic wave (SAW) dispersion measurement are often slow due to the need for scanning excitation or detection positions. We present a method for determining discrete points on the SAW mode at equidistant wavenumbers that requires only a single measurement, overcoming these speed limitations. A pulsed laser, shaped into an array of equidistant lines, generates elastic waves on the sample surface. A vibrometer detects the resulting surface displacement next to the line array. The periodic excitation arrangement results in constructive interference of the SAW at wavelengths corresponding to integer fractions of the line spacing. This leads to distinct peaks in the response spectrum, whose higher orders we term spatial harmonics of the SAW. The known line spacing determines the wavelengths, allowing the peak frequencies to be mapped to discrete points on the SAW dispersion curve, effectively sampling the SAW at specific equidistant wavenumbers. By solving an inverse problem, a model can be fit to the experimental data to obtain properties of the layered system. We demonstrate this technique on copper-clad epoxy laminate and roll-cladded aluminum alloy plates and compare our results with dispersion relation scans and micrography. Additionally, we analyze the method's sensitivity to elastic parameters and layer thickness. This approach offers a viable alternative to established SAW scanning methods, particularly in scenarios where the trade-off of lower wavenumber sampling density is acceptable to achieve exceptionally fast measurements while avoiding moving parts.