Stimulated Brillouin interactions mediate nonlinear coupling between photonsand acoustic phonons through an optomechanical three-wave interaction. Thoughthese nonlinearities were previously very weak in silicon photonic systems, therecent emergence of new optomechanical waveguide structures have transformedBrillouin processes into one of the strongest and most tailorable on-chipnonlinear interactions. New technologies based on Brillouin couplings haveformed a basis for amplification, filtering, and nonreciprocal signalprocessing techniques. In this paper, we demonstrate strong guided-waveBrillouin scattering between light fields guided in distinct spatial modes of asilicon waveguide for the first time. This inter-modal coupling createsdispersive symmetry breaking between Stokes and anti-Stokes processes,permitting single-sideband amplification and wave dynamics that permitnear-unity power conversion. Combining these physics with integratedmode-multiplexers enables novel device topologies and eliminates the need foroptical circulators or narrowband spectral filtering to separate pump andsignal waves as in traditional Brillouin processes. We demonstrate 3.5 dB ofoptical gain, over 2.3 dB of net amplification, and 50% single-sideband energytransfer between two optical modes in a pure silicon waveguide, expanding thedesign space for flexible on-chip light sources, amplifiers, nonreciprocaldevices, and signal processing technologies.
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