Focusing membrane metamirrors for integrated cavity optomechanics

Membrane-based cavity optomechanical systems have been widely successful; however, their chip-scale integration remains a significant challenge. Here we present a solution based on metasurface design. Specifically, by non-periodic photonic crystal patterning of a Si₃N₄ membrane, we realize a suspended metamirror with a finite focal length, enabling formation of a stable optical cavity with a plane end-mirror. We present simulation, fabrication, and characterization of the metamirror using both free-space and cavity-based measurements, demonstrating reflectivities as high as 99% and cavity finesse as high as 600. The mirror radius of curvature (∼30 cm) is inferred from the cavity mode spectrum. In combination with phononic engineering, focusing membrane mirrors offer a route towards high-cooperativity, vertically integrated cavity optomechanical systems with applications ranging from precision force sensing to hybrid quantum transduction.