Nonreciprocity in Brillouin scattering

Nonreciprocal devices are essential in nearly all complex optical systems as they play a major role for laser protection and directional signal routing. While the common implementation of these devices is with magneto-optic materials, the approach is not yet suitable for on-chip integration at scale, or for applications that are sensitive to magnetic fields. Notably, the phase-matching relations intrinsic to Brillouin scattering offer an effective solution to producing these devices in nearly any optically transparent material system. In this chapter we discuss how Brillouin scattering-based nonreciprocity can be utilized in the spontaneous scattering, stimulated scattering, and induced transparency regimes, and we show how these nonreciprocal effects offer a path to producing idealized nonreciprocal behavior. We also perform a review of the wide range of nonreciprocal platforms that have been demonstrated to date, spanning dielectric-based fibers and resonators, to on-chip silicon and silicon nitride photonics, to piezoelectric photonic devices.