Guided-acoustic stimulated Brillouin scattering in silicon nitride photonic circuits.

Coherent optomechanical interaction known as stimulated Brillouin scattering (SBS) can enable ultrahigh resolution signal processing and narrow-linewidth lasers. SBS has recently been studied extensively in integrated waveguides; however, many implementations rely on complicated fabrication schemes. The absence of SBS in standard and mature fabrication platforms prevents its large-scale circuit integration. Notably, SBS in the emerging silicon nitride (SiN) photonic integration platform is currently out of reach because of the lack of acoustic guidance. Here, we demonstrate advanced control of backward SBS in multilayer SiN waveguides. By optimizing the separation between two SiN layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in SiN waveguides. We use the enhanced SBS gain to demonstrate a high-rejection microwave photonic notch filter. This demonstration opens a path to achieving Brillouin-based photonic circuits in a standard, low-loss SiN platform.