Resonance-Induced Sign Reversal of Optical Gradient Forces and Three-Dimensional Singularity Trapping

Abstract

In atomic physics, tuning the light frequency across a resonance reverses the trapping force between bright and dark field regions, yet a unified analytical description of this principle applicable to photonic resonators in general has not been established. Here we show that sweeping the incident wavelength through a resonance in the optical response of the particle or device induces a $\pi$ phase shift, reversing the gradient force from attractive to repulsive. A generalized Fano line-shape model captures this quantitatively across physically distinct resonant systems, from plasmonic nanoparticles to high-Q metasurfaces, with full-wave simulations confirming the predictions in every case. Building on this framework, three-dimensional singularity trapping of silicon nanoparticles is demonstrated using counter-propagating vector beams and metasurfaces, with trapping potential depths competitive with conventional bright-field traps. These results establish a platform-independent design principle for controlling optical forces in resonant systems, with broad implications for optical manipulation, quantum optomechanics, and precision metrology.