Fig. 1. Overview of SBS. Taking backward SBS, for example, the pump light ω_p and a counterpropagating Stokes light ω_s generate a beat pattern. This beat pattern generates a moving elastic wave Ω in the waveguide through electrostriction and radiation pressure, and the elastic wave perturbates the dielectric tensor of the material through the photoelastic effect, resulting in periodic refractive index changes in the waveguide. This periodic change in refractive index resembles a moving index grating, scattering the pump light to the Stokes light.

Fig. 2. Schematic of Brillouin acousto-optic interaction. (a) Stokes process; the pump photon is scattered into a redshifted photon and a phonon. (b) Anti-Stokes process; the pump photon absorbs a phonon and scatters into a blueshifted photon.

Fig. 3. Dynamics of three different SBS types in integrated waveguides. (a) BSBS; (b) intramodal FSBS; (c) intermodal FSBS.

Fig. 4. Typical on-chip SBS devices with different materials and structures. (a) As₂S₃ rib waveguide on silica^[16]; (b) As₂S₃ spiral ring resonator^[25]; (c) suspended LN waveguide^[91]; (d) SiO₂ microsphere^[20]; (e) SiO₂ microdisk^[23]; (f) SiGe alloy waveguide^[46]; (g) suspended Si racetrack microring^[53]; (h) suspended Si waveguide in Si₃N₄^[26]; (i) Si waveguide with phononic crystals^[3]; (j) Si waveguide with silica pillar^[61]; (k) leaky Si₃N₄ microring^[29]; (l) Si₃N₄ waveguide^[70]; (m) Si₃N₄ slot waveguide^[54]; (n) suspended AlN microring^[30]; (o) suspended AlN waveguide^[56]; (p) suspended AlN membrane^[90].

Fig. 5. Applications of on-chip SBS. (a) Microwave synthesizer based on SiO₂ microdisk^[93]; (b) MWP filter based on As₂S₃ rib waveguide^[102]; (c) reconfigurable MWP filter based on As₂S₃ rib waveguide^[103]; (d) MWP filter based on Si waveguide^[65]; (e) slow and fast light based on As₂S₃ rib waveguide^[109]; (f) light storage based on SiO₂ microsphere^[107]; (g) light storage based on As₂S₃ rib waveguide^[32]; (h) isolator based on Si waveguide^[55]; (i) isolator based on AlN microring with external acoustic pumping^[30]; (j) optical gyroscope based on SiO₂ microdisk^[39]; (k) optical gyroscope based on Si₃N₄ microring^[29]; (l) optical frequency comb based on As₂S₃ rib waveguide^[127]; (m) optical frequency comb based on As₂S₃ microring^[128]; (n) optical frequency comb based on SiO₂ microbubble^[129].

Fig. 6. Experimental setups and principles of on-chip SBS applications. (a) MWP filters^[103]; (b) light storage and time-delay components^[107]; (c) optical isolators and nonreciprocal components^[55]; (d) optical gyroscopes^[29].