Guided-Acoustic Stimulated Brillouin Scattering in Silicon Nitride Photonic Circuits

TL;DR

This paper demonstrates enhanced stimulated Brillouin scattering in silicon nitride photonic circuits by optimizing multilayer structures, enabling high-gain SBS and applications like microwave filtering without complex fabrication.

Contribution

It introduces a novel multilayer silicon nitride waveguide design that achieves gigahertz acoustic guiding and high SBS gain in a standard platform, enabling practical integrated SBS devices.

Findings

Achieved up to 15 times higher SBS gain in silicon nitride waveguides.

Demonstrated on-demand SBS inhibition by controlling acoustic guiding.

Implemented a high-rejection microwave photonic notch filter using enhanced SBS.

Abstract

Coherent optomechanical interaction between acoustic and optical waves known as stimulated Brillouin scattering (SBS) can enable ultra-high resolution signal processing and narrow linewidth lasers important for next generation wireless communications, precision sensing, and quantum information processing. While SBS has recently been studied extensively in integrated waveguides, many implementations rely on complicated fabrication schemes, using suspended waveguides, or non-standard materials such as As$_2$S$_3$. The absence of SBS in standard and mature fabrication platforms prevents large-scale circuit integration and severely limits the potential of this technology. Notably, SBS in standard silicon nitride integration platform is currently rendered out of reach due to the lack of acoustic guiding and the infinitesimal photo-elastic response of the material. In this paper, we experimentally demonstrate advanced control of backward SBS in multilayer silicon nitride waveguides. By optimizing the separation between two silicon nitride layers, we unlock gigahertz acoustic waveguiding in this platform for the first time, leading up to 15 $\times$ higher SBS gain coefficient than previously possible in silicon nitride waveguides. Using the same principle, we experimentally demonstrate on-demand inhibition of SBS by preventing acoustic guiding in the waveguide platform. We utilize the enhanced SBS gain to demonstrate a microwave photonic notch filter with high rejection (30 dB). We accomplish this in a low-loss, standard, and versatile silicon nitride integration platform without the need of suspending the SBS-active waveguide or hybrid integration with other materials.