  Inspired by concepts developed for fermionic systems in the framework of
condensed matter physics, topology and topological states are recently being
explored also in bosonic systems. The possibility of engineering systems with
unidirectional wave propagation and protected against disorder is at the heart
of this growing interest. Topogical acoustic effects have been observed in a
variety of systems, most of them based on kHz-MHz sound waves, with typical
wavelength of the order of the centimeter. Recently, some of these concepts
have been successfully transferred to acoustic phonons in nanoscaled
multilayered systems. The reported demonstration of confined topological
phononic modes was based on Raman scattering spectroscopy, yet the resolution
did not suffice to determine lifetimes and to identify other acoustic modes in
the system. Here, we use time-resolved pump-probe measurements using an
asynchronous optical sampling (ASOPS) technique to overcome these resolution
limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors
(DBRs) as building blocks, we engineer high frequency ($\sim$ 200 GHz)
topological acoustic interface states. We are able to clearly distinguish
confined topological states from stationary band edge modes. The detection
scheme reflects the symmetry of the modes directly through the selection rules,
evidencing the topological nature of the measured confined state. These
experiments enable a new tool in the study of the more complex topology-driven
phonon dynamics such as phonon nonlinearities and optomechanical systems with
simultaneous confinement of light and sound.
