Enhanced Optomechanical Coupling between an Optically Levitated Particle and an Ultrahigh-<i>Q</i> Optical Microcavity

Exploring the dynamics of an optically levitated dielectric micro- and nanoparticle is an exciting new subject in quantum science. Recent years have witnessed rapid advancements in attaining quantum-limited optical detection and control of a nanoscale particle by coupling its motion to a high-finesse optical cavity in the resolved-sideband regime. In order to control the particle deeper in the quantum regime, it is necessary to significantly enhance the coupling between the particle and the cavity. Here, we present a novel platform that can allow for achieving this. Our system consists of a conventional optical tweezer and a toroidal optical microcavity with an ultrahigh quality (Q) factor. The optomechanical coupling between the particle and the cavity is established by placing the particle in the near field of the cavity. The significantly reduced mode volume allows us to achieve a 50-fold increase in the single photon optomechanical coupling compared to a conventional Fabry-Pérot cavity with macroscopic mirrors, while ultralow loss of the cavity brings the system close to the resolved-sideband regime. Our approach paves the way for enabling quantum experiments on levitated mesoscopic particles with high quantum cooperativity near the resolved-sideband regime.