Non-Reciprocal Brillouin Scattering Induced Transparency
  Electromagnetically induced transparency (EIT) provides a powerful mechanism for controlling light propagation in a dielectric medium, and for producing slow and fast light. EIT traditionally arises from destructive interference induced by a nonradiative coherence in an atomic system. Stimulated Brillouin scattering (SBS) of light from propagating hypersonic acoustic waves has also been used successfully for the generation of slow and fast light. However, EIT-type processes based on SBS were considered infeasible because of the short coherence lifetime of hypersonic phonons. Here, we report a new Brillouin scattering induced transparency (BSIT) phenomenon generated by acousto-optic interaction of light with long-lived propagating phonons. We demonstrate that BSIT is uniquely non-reciprocal due to the propagating acoustic phonon wave and accompanying momentum conservation requirement. Using a silica microresonator having naturally occurring forward-SBS phase-matched modal configuration, we show that BSIT enables compact and ultralow-power slow-light generation with delay-bandwidth product comparable to state-of-the-art SBS systems. 
JunHwan Kim, Mark C. Kuzyk, Kewen Han, Hailin Wang, and Gaurav Bahl
physics.optics