  The century-old study of photon-phonon coupling has seen a remarkable revival
in the past decade. Driven by early observations of dynamical back-action, the
field progressed to ground-state cooling and the counting of individual
phonons. A recent branch investigates the potential of traveling-wave,
optically broadband photon-phonon interaction in silicon circuits. Here, we
report continuous-wave Brillouin gain exceeding the optical losses in a series
of suspended silicon beams, a step towards selective on-chip amplifiers. We
obtain efficiencies up to $10^{4} \, \text{W}^{-1}\text{m}^{-1}$, the highest
to date in the phononic gigahertz range. We also find indications that
geometric disorder poses a significant challenge towards nanoscale phonon-based
technologies.
