Measurement of fast dynamic strain generated by focusing of surface acoustic waves

We have measured the spatial and temporal dependence of high frequency (88 MHz) strain waves in a focused surface acoustic wave (FSAW). Increasing the strain generated by a SAW to levels that are comparable with epitaxial strains in thin films ($\sim$ 1{\%}) necessitate annular inter-digital transducers (AIDT) that follow the constant velocity curve of the piezoelectric surface to focus the strain wave over a small focal area. A detailed analysis of the strain around the focal center of an AIDT patterned on 128 Y-cut LiNbO$_{3}$ shows shifts in the (104) x-ray diffraction peak. The AIDT spacing was chosen to produce a resonance at the ring frequency of the Advanced Photon Source, where x -ray diffraction measurements were carried out with a focus of 7 $\mu$m. The spatial dependence of the strain is identical to that measured using light reflectivity. From x-ray diffraction measurements, we obtained a quantitative measure of the strain amplitude, 0.5{\%} at an AIDT excitation power of 24 dBm. The temporal dependence showed a sinusoidally varying strain that cycles between compressive and tensile at the frequency of the AIDT excitation. Diffraction measurements of a Pt thin film grown on the LiNbO$_{3}$ show significant strain transfer. These data lay the groundwork for future experiments that involve tuning the physical properties of strain sensitive thin film materials at high frequencies.