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 (Si 3 N 4 ) 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 Si 3 N 4 waveguides. By optimizing the separation between two Si 3 N 4 layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in Si 3 N 4 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 Si 3 N 4 platform.

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Photonic integration, advanced functionality, reconfigurability, and high radio frequency (RF) performance are key features in integrated microwave photonic systems that are still difficult to achieve simultaneously. In this work, we demonstrate an integrated microwave photonic circuit that can be reconfigured for two distinct RF functions, namely, a tunable notch filter and a phase shifter. We achieved >50 dB high-extinction notch filtering over 6–16 GHz and 2π continuously tunable phase shifting over 12–20 GHz frequencies. At the same time, we implemented an on-chip linearization technique to achieve a spurious-free dynamic range of more than 120 dB · Hz4/5 for both functions. Our work combines multi-functionality and linearization in one photonic integrated circuit and paves the way to reconfigurable RF photonic front-ends with very high performance.