Spin dynamics and torque-mediated relaxation mechanisms of a nonbirefringent optomechanical microrotor in vacuum

The vacuum-levitated microrotor has emerged as a versatile strategy for nondestructive detection and precision measurement, which always relies on spin angular momentum transfer from circularly polarized light. Here, we present a deflection optical torque introduction scheme that is independent of beam polarization for a nonbirefringent microparticle. Without active feedback cooling, the stably trapped pressure level achieved is one order of magnitude lower than that attainable with the conventional levitated microrotor method, reaching down to 1.6 × 10^-4 mbar, and a maximum rotational frequency of 0.48 MHz is obtained. From rotation relaxation measurements, the input optical torque is inferred to be (1.384 ± 0.005) × 10^-17 N·m. This work establishes a clean, stable high-speed rotating platform under vacuum conditions, paving the way for ultrasensitive torque measurements and enabling potential applications in optically levitated gyroscopes.