Boundary-dominated optomechanics in silicon metamaterial membranes

TL;DR

This paper demonstrates boundary-dominated optomechanical interactions in silicon membranes with record-high Brillouin gain at 12 GHz, enabling scalable on-chip microwave photonics.

Contribution

It introduces a novel silicon membrane design with metamaterial claddings that enhances boundary effects for strong Brillouin interactions.

Findings

Achieved 12 GHz forward Brillouin interactions with high gain and quality factor.

Demonstrated net Brillouin amplification with low pump power in millimeter-scale waveguides.

Established a scalable platform for high-frequency opto-acoustic signal processing.

Abstract

Stimulated Brillouin scattering in integrated photonic waveguides enables coherent coupling between optical photons and gigahertz acoustic phonons, providing a powerful mechanism for on-chip microwave photonics and opto-acoustic signal processing. Despite theoretical predictions of ultra-strong Brillouin interactions arising from enhanced light-sound coupling at device boundaries, most state-of-the-art integrated demonstrations remain governed by bulk photoelastic effects. This limitation stems from trade-offs between optical loss, interaction with waveguide boundaries and accessible phonon frequencies associated with the use of transverse-electric optical modes coupled to horizontally breathing mechanical modes. Here we demonstrate a new approach based on transverse-magnetic optical modes coupled to vertically breathing mechanical modes in suspended silicon membranes engineered with subwavelength metamaterial claddings. In this geometry, the interaction is dominated by the moving-boundary effect occurring at smooth top and bottom interfaces, while the phonon frequency is set primarily by the membrane thickness rather than its width. We observe forward Brillouin interactions at a record frequency of 12 GHz with a gain of 7200 W$^{-1}$ m$^{-1}$ and a mechanical quality factor of 620, yielding the highest Brillouin gain-to-quality-factor ratio reported in silicon waveguides. The devices exhibit net Brillouin amplification in millimeter-scale waveguides with pump powers below 15 mW, establishing a scalable platform for high-frequency integrated opto-acoustic signal processing.