Visualization of Dense Spurious Modes in a 5 GHz Laterally Excited Bulk Acoustic Resonator Using Pulsed Laser Interferometry

This paper presents a breakthrough in high-frequency acoustic resonator characterization by employing scanning pulsed laser interferometry to directly visualize dense spurious modes in a 5 GHz laterally excited bulk acoustic resonator (XBAR). Through spatial Fourier transform analysis of the interferometric measurements, we systematically resolve superimposed vibration patterns and track their dynamic evolution across both fundamental and spurious resonant modes. A novel spectral filtering reconstruction methodology is developed to effectively isolate individual mode components from complex multimodal vibrations. Complementary dispersion diagram simulations reveal the physical origin of observed spurious modes as higher-order Lamb wave propagations. This work establishes the first experimental demonstration of direct spatial mapping for spurious modes beyond 5 GHz, benefiting design optimization in the future.