Interference of co-propagating Rayleigh and Sezawa waves observed with micro-focussed Brillouin Light Scattering Spectroscopy

We use micro-focussed Brillouin light scattering spectroscopy ($\mu$BLS) to investigate surface acoustic waves (SAWs) in a GaN layer on a Si substrate at GHz frequencies. Furthermore, we discuss the concept of $\mu$BLS for SAWs and show that the crucial parameters of SAW excitation and propagation can be measured. We investigate a broad range of excitation parameters and observe that Rayleigh and Sezawa waves are excited simultaneously at the same frequency. Spatially resolved measurements of these co-propagating waves show a periodic pattern, which proves their coherent interference. From the periodicity of the spatial phonon patterns, the wavevector difference between the two waves has been identified and compared to the dispersion relation. This concept of co-propagating phonons might be used to produce acoustic or magneto-elastic fields with a time-independent spatial variation similar to the situations realized using counter-propagating waves. However, co-propagating SAW have a well defined direction of the wave vector and thus, posses a finite phonon angular momentum which offers new opportunities, e.g. for angular momentum conversion experiments.