Stimulated Forward Brillouin Scattering in Subwavelength Grating Silicon Membranes

ABSTRACT Brillouin scattering enables efficient and coherent conversion between optical photons and gigahertz‐frequency phonons. Implementing this interaction in silicon photonics provides a pathway toward scalable and cost‐effective devices, leveraging mature fabrication processes. However, achieving strong overlap and tight confinement of optical and mechanical modes in silicon nanophotonic waveguides remains a significant challenge. Here, we propose and demonstrate a novel strategy that enables independent control of optical and mechanical modes in periodically segmented silicon waveguides. Our approach combines two distinct periodic lattices: one with a period shorter than half of the optical wavelength, providing light guiding by metamaterial‐induced index contrast, and another that creates a complete phononic bandgap confining acoustic modes. This dual‐lattice strategy opens new degrees of freedom to optimize optomechanical confinement and coupling simultaneously. Based on this approach, we experimentally demonstrate remarkably high Brillouin gain of , resulting in a three‐tone Stokes gain of 3 and anti‐Stokes loss of 4 with 6.4 MHz mechanical linewidth. These results illustrate the potential of subwavelength silicon metamaterials for engineering on‐chip optomechanical interactions.