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

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.