  We report the observation of low-frequency modes in the Raman spectra of
thin-film superlattices of the high-temperature superconductor YBa$ _{2} $Cu$
_{3} $O$ _{7-\delta} $ and various manganite perovskites. Our study shows that
these modes are caused by the backfolding of acoustic phonons due to the
additional periodicity introduced by the superlattice. Such modes were
previously only observed for ultra-pure semiconductor superlattices. They can
be used to determine the bilayer thickness of the superlattice and its speed of
sound. Moreover, we use the spatial resolution of Raman microscopy to map the
film thickness inhomogeneity across a sample, making these modes a useful tool
to characterize thin-film superlattices.
