Optically levitated systems have emerged as a significant platform for physics and bioscience research due to their non-contact and non-invasive nature, pioneered by Arthur Ashkin. In vacuum, such systems offer exceptional isolation from environmental noise and allow controlled decoherence from background gas, making them a focus in fundamental physics and precision measurement. Notable progress includes ground-state cooling of the center-of-mass motion. Furthermore, these systems exhibit richer physical phenomena compared to cantilever-based optomechanical systems, such as observed libration and GHz rotation of anisotropic levitated particles.
However, the polarization characteristics of the scattered field from an optically levitated anisotropic particle---critical for motion manipulation, detection, and cooling. Here, we present a theoretical study of these characteristics for an intrinsically isotropic ellipsoidal particle driven by linearly or circularly polarized laser light. We first calculate the far-field distributions of the scattered field and then numerically evaluate the signal amplitudes for libration and rotation from the interference field, as detected by a collection lens displaced perpendicularly to the trapping laser axis.
Our analysis reveals several counterintuitive results. The libration signal amplitude is minimal when the collection lens is centered on the beam axis and increases as the lens is displaced off-axis. Moreover, The signal can be increased in the detection when the half wave plate is rotated by a certain angle. Meanwhile, the β rotation signal is detectable. By measuring a specific polarization component of the interference field, we find that the signal amplitudes for the α and β rotation are not maximal on-axis but are instead located in the four quadrants of the transverse plane.
This work provides an important foundation for the manipulation, detection, and cooling of optically levitated anisotropic particles in vacuum.