On-chip Inter-modal Brillouin Scattering
  Stimulated Brillouin interactions mediate nonlinear coupling between photons and acoustic phonons through an optomechanical three-wave interaction. Though these nonlinearities were previously very weak in silicon photonic systems, the recent emergence of new optomechanical waveguide structures have transformed Brillouin processes into one of the strongest and most tailorable on-chip nonlinear interactions. New technologies based on Brillouin couplings have formed a basis for amplification, filtering, and nonreciprocal signal processing techniques. In this paper, we demonstrate strong guided-wave Brillouin scattering between light fields guided in distinct spatial modes of a silicon waveguide for the first time. This inter-modal coupling creates dispersive symmetry breaking between Stokes and anti-Stokes processes, permitting single-sideband amplification and wave dynamics that permit near-unity power conversion. Combining these physics with integrated mode-multiplexers enables novel device topologies and eliminates the need for optical circulators or narrowband spectral filtering to separate pump and signal waves as in traditional Brillouin processes. We demonstrate 3.5 dB of optical gain, over 2.3 dB of net amplification, and 50% single-sideband energy transfer between two optical modes in a pure silicon waveguide, expanding the design space for flexible on-chip light sources, amplifiers, nonreciprocal devices, and signal processing technologies. 
Eric A. Kittlaus, Nils T. Otterstrom, Peter T. Rakich
physics.optics