Electromagnetically induced transparency (EIT) [1,2] 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 [3] has also been used successfully for the generation of slow and fast light [4][5][6][7]. 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 [8,9]. 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 phasematched modal configuration [8,9], we show that BSIT enables compact and ultralow-power slow-light generation with delay-bandwidth product comparable to state-of-the-art SBS systems. [3,10,11] is a fundamental materiallevel nonlinearity occurring in all states of matter [12] in which two optical fields are coupled to a traveling acoustic wave through photoelastic scattering and electrostriction. The light fields scatter from the periodic photoelastic perturbation generated by the traveling acoustic wave, while simultaneously writing a spatiotemporally beating electrostriction force whose momentum and frequency matches the acoustic wave. Phase matching for SBS is thus defined by both 1 arXiv:1408.1739v2 [physics.optics] 11 Aug 2014 energy and momentum conservation, and is satisfied in back-scattering only by multi-GHz phonon modes in most solids. SBS is frequently used for optical gain [3,13], laser linewidth narrowing [14], optical phase conjugation [15], dynamic gratings [16], and even material characterization and microscopy [17][18][19][20][21][22]. The applications of SBS in superluminal and slow light experiments have been recognized as well [4][5][6][7]. However, unlike the case of electromagnetically induced transparency (EIT) [1,2], the generation of transparency with SBS has never been demonstrated. This is because in typical Brillouin scattering pump-probe systems the lifetimes of phonons at multi-GHz frequencies are much shorter than the photon lifetimes [3,23,24], effectively disabling the coherent interference of probe Stokes scattering and pump anti-Stokes scattering pathways. In a forward-scattering SBS system however, this lifetime relationship can be reversed by coupling the light fields through a low-frequency long-lived phonon mode as shown recently in the experimental demonstration of forward-SBS lasing [8] and Brillouin cooling [9].
Stimulated Brillouin scattering (SBS)Nonlinear optical processes such as EIT need to satisfy the energy-momentum conservation, leading to the phase-matching requirement. For...