Simulated Picosecond-laser-generated Surface Acoustic Waves in Nanostructures
Luke Thatcher ’22 and Professor Brian Daly (Physics)
We have run computer simulations of an ultrafast pump-probe laser experiment that generates and detects surface acoustic waves (SAW) in periodic structures at the nanoscale. In these experiments, a ‘pump’ beam of light causes strain and generates surface waves in a structure by heating the surface and causing thermal expansion, and a ‘probe’ beam of light is used to measure the change in reflectivity of the structure, which can be used to identify surface waves with frequencies up to 50 GHz (the highest SAW frequencies that can be detected). Our simulations consisted of two parts: a mechanical simulation that uses the elasticities and densities of the structure to calculate and show strains in the structure, and an electromagnetic simulation that uses the strains to simulate changes in the optical reflectivity of the structure. We focused on how different polarizations of the probe beam affect the changes in reflectivity at both 400 nm and 800 nm wavelengths of light. Our simulations predicted significant differences in reflectivity between polarizations for some structures, and no significant differences in other structures. We compared our simulations to published experiments and found surface waves at similar frequencies as were found in experimental data, but polarization and wavelength dependence do not consistently agree with experimental data.