Anomalous optical gradient force induced by polarization-tuned antisymmetry in energy density gradient

Abstract

The spatial inhomogeneity of electromagnetic energy density in an optical field typically gives rise to conservative gradient forces, which serve as the fundamental mechanism for trapping nanoparticles in optical tweezers. Surprisingly, however, we demonstrate that even in the absence of an energy density gradient, optical gradient forces can still act on isotropic, achiral particles when the incident field consists of counter-propagating plane waves engineered to exhibit polarization-controlled antisymmetry between the electric and magnetic energy density gradients. Through both numerical simulations and analytical derivations based on multipole expansion theory, we show that this anomalous gradient force arises from the electromagnetic symmetry breaking induced by the particle itself, irrespective of its size. Notably, this electromagnetic symmetry breaking-induced gradient force reaches its maximum under elliptical polarization at the specific position, rather than linear or circular polarization, underscoring the critical role of polarization configuration in modulating energy density gradients. These findings reveal a previously unrecognized mechanism for optical gradient force generation and deepen our understanding of the role of hidden antisymmetry in structured light fields.