  Similar to electron waves, the phonon states in semiconductors can undergo
changes induced by external boundaries. Modification of acoustic phonon
spectrum in structures with periodically modulated elastic constant or mass
density - referred to as phononic crystals - has been proven experimentally and
utilized in practical applications. A possibility of modifying acoustic phonon
spectrum in individual nanostructures via spatial confinement would bring
tremendous benefits for controlling phonon-electron interaction and thermal
conduction at nanoscale. However, despite strong scientific and practical
importance, conclusive experimental evidence of acoustic phonon confinement in
individual free-standing nanostructures, e.g. nanowires, is still missing. The
length scale, at which phonon dispersion undergoes changes and a possibility of
the phonon group velocity reduction, are debated. Here, we utilize specially
designed high-quality GaAs nanowires (NWs) with different diameters, D, and
large inter-nanowire distances to directly demonstrate acoustic phonon
confinement. The measurements conducted with Brillouin - Mandelstam
spectroscopy reveal confined phonon polarization branches with frequencies from
4 GHz to 40 GHz in NWs with D as large as ~128 nm, i.e. at length scale, which
exceeds the "grey" phonon mean-free path in GaAs by an almost an order of
magnitude. The phonon dispersion modification and phonon energy scaling with D
in individual nanowires are in excellent agreement with theory. The obtained
results can lead to more efficient nanoscale control of acoustic phonons, with
benefits for nanoelectronics, thermoelectric energy conversion, thermal
management, and novel spintronic technologies.
