  The ability to amplify light within silicon waveguides is central to the
development of high-performance silicon photonic device technologies. To this
end, the large optical nonlinearities made possible through stimulated
Brillouin scattering offer a promising avenue for power-efficient all-silicon
amplifiers, with recent demonstrations producing several dB of net
amplification. However, scaling the degree of amplification to technologically
compelling levels (>10 dB), necessary for everything from filtering to small
signal detection, remains an important goal. Here, we significantly enhance the
Brillouin amplification process by harnessing an inter-modal Brillouin
interaction within a multi-spatial-mode silicon racetrack resonator. Using this
approach, we demonstrate more than 20 dB of net Brillouin amplification in
silicon, advancing state-of-the-art performance by a factor of 30. This degree
of amplification is achieved with modest (~15 mW) continuous-wave pump powers
and produces low out-of-band noise. Moreover, we show that this same system
behaves as a unidirectional amplifier, providing more than 28 dB of optical
nonreciprocity without insertion loss in an all-silicon platform. Building on
these results, this device concept opens the door to new types of all-silicon
injection-locked Brillouin lasers, high-performance photonic filters, and
waveguide-compatible distributed optomechanical phenomena.
