Transmission diffuse EUV scattering by nanoscale Lamb waves in thin membranes

Abstract

We report time-resolved diffuse extreme ultraviolet (EUV) scattering measurements of optically excited acoustic waves in thin Ti/SiN bilayers in transmission geometry. Following femtosecond optical excitation, the EUV diffuse scattering signal yields circular fringe patterns evolving as a function of the time delay between the optical pump and EUV probe pulses. We demonstrate that these patterns originate from multiple guided acoustic modes (Lamb waves) with wavelengths in the range 60–400 nm. By comparing the experimental frequency–wavevector maps with calculated dispersion curves, we show that diffuse scattering signal from Lamb waves is enhanced at discrete frequencies corresponding to longitudinal thickness resonances of the membrane. This observation indicates that Lamb waves with high in-plane wavevectors originate from the scattering of longitudinal thickness resonances by surface roughness. Our findings establish time-resolved diffuse EUV scattering as an efficient tool for probing nanoscale Lamb waves, applicable to the characterization of elastic properties of thin membranes.