  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.
