  Brillouin laser oscillators offer powerful and flexible dynamics as the basis
for mode-locked lasers, microwave oscillators, and optical gyroscopes in a
variety of optical systems. However, Brillouin interactions are exceedingly
weak in conventional silicon photonic waveguides, stifling progress towards
silicon-based Brillouin lasers. The recent advent of hybrid photonic-phononic
waveguides has revealed Brillouin interactions to be one of the strongest and
most tailorable nonlinearities in silicon. Here, we harness these engineered
nonlinearities to demonstrate Brillouin lasing in silicon. Moreover, we show
that this silicon-based Brillouin laser enters an intriguing regime of
dynamics, in which optical self-oscillation produces phonon linewidth
narrowing. Our results provide a platform to develop a range of applications
for monolithic integration within silicon photonic circuits.
