Investigating and Controlling the Libration and Rotation Dynamics of Nanoparticles in an Optomechanical System

Abstract In optomechanical systems, the libration and rotation of nanoparticles provide profound insights for ultrasensitive torque measurements and macroscopic quantum superpositions. The achievements include transitioning the libration to the rotation up to 6 GHz and cooling the libration to millikelvin temperatures. Libration and rotation are driven by restoring and constant optical torques, respectively. However, the transition mechanisms between these two states warrant further exploration. From this perspective, in this study, monitoring lateral-scattered light enables real-time observation of the libration/rotation transitions and associated hysteresis as the ellipticities of trapping laser fields are varied. By calculating optical torques and solving the Langevin equation, the transitions are linked to the balance between anisotropic-polarization-induced sinusoidal optical torques and constant torques, and absorption is identified as the main contributor to constant torques. These findings enable direct weak torque sensing and precise nanoparticle control at rotational degrees, facilitating the study of quantum effects such as nonadiabatic phase shifts and macroscopic quantum superpositions, and thereby enriching quantum optomechanics research.