  Femtosecond pump-probe spectroscopy has been carried out on suspended gold
nanostructures with a rectangular cross section lithographed on a silicon
substrate. With a thickness fixed to 110 nm and a width ranging from 200 nm to
800 nm, size dependent measurements are used to distinguish which confined
acoustic modes are detected. Furthermore, in order to avoid any ambiguity due
to the measurement uncertainties on both the frequency and size, pump and probe
beams are also spatially shifted to detect guided acoustic phonons. This leads
us to the observation of backward propagating acoustic phonons in the gigahertz
range ($\sim$3 GHz) in such nanostructures. While backward wave propagation in
elastic waveguides has been predicted and already observed at the macroscale,
very few studies have been done at the nanoscale. Here, we show that these
backward waves can be used as the unique signature of the width dilatational
acoustic mode.
