Effects of photon scattering torque in off-axis levitated torsional cavity optomechanics

We consider theoretically a dielectric nanoparticle levitated in an optical ring trap inside a cavity and probed by an angular lattice, with all electromagnetic fields carrying orbital angular momentum. Analyzing the torsional motion of the particle about the cavity axis, we find that photon scattering from the trap beam plays an important role in the optomechanical system. First we show that the presence of the torque introduces an instability. Subsequently, we demonstrate that for bound motion near a stable equilibrium, varying the optical torque strength allows for tuning the linear optomechanical coupling. Finally, we indicate that the relative strengths of the linear and quadratic couplings can be detected directly by homodyning the cavity output. Our studies should be of interest to researchers exploring quantum mechanics using torsional optomechanics.