Optical Torques on Dielectric Spheres in a Spin-Gradient Light Field

Abstract

Light carries spin angular momentum, which can be imparted to the particle, generating an optical torque (OT). For decades, it has been widely understood that this spin-induced OT only occurs on anisotropic (e.g., dumbbell and rod) particles or absorbing (e.g., gold) spheres. In contrast to previous interpretations, herein a nontrivial mechanism is presented for generating and measuring spin-determined OTs on non-absorbing spheres. This counterintuitive OT emerges from the gradient of the spin angular momentum, which imposes non-uniformly distributed force/energy vectors on the particle. The opposite force vectors on both sides of the beam induce extraordinary spin-gradient OTs whose sign depends on the particle's permittivity rather than its position. This negative spin-gradient OT is more readily observable on dielectric particles due to the elimination of the predominant positive absorbing OT. Experimentally, the spin-gradient OT is exquisitely measured using the polystyrene sphere delicately labeled with a tiny tail, showcasing its strong correlation with light polarization. The study enriches the fundamental understanding of OT, offering marvelous applications that circumvent heating effects in physical and biomedical sciences.