  The phonon analog of an optical laser has long been a subject of interest. We
demonstrate a compound microcavity system, coupled to a radio-frequency
mechanical mode, that operates in close analogy to a two-level laser system. An
inversion produces gain, causing phonon laser action above a pump power
threshold of around 50 $\mu$W. The device features a continuously tunable, gain
spectrum to selectively amplify mechanical modes from radio frequency to
microwave rates. Viewed as a Brillouin process, the system accesses a regime in
which the phonon plays what has traditionally been the role of the Stokes wave.
For this reason, it should also be possible to controllably switch between
phonon and photon laser regimes. Cooling of the mechanical mode is also
possible.
