  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.
