Optical pulling forces via off-resonance near-field hybridization

In general, the realization of strong optical pulling force on certain particle in a multiple particle system depends on the electromagnetic resonances of the particle. However, the accompanying resonant heat absorption of particle impairs the stable light traction. Moreover, the pulling force acting on an entire dimer is related to the configuration of its two constituent particles, yet the roles of the near-field interactions in mediating this force remain unclear. Here, we address these challenges by considering a silicon-silver heterodimer under normal illumination of a plane wave. The analytical results demonstrate that the pulling forces on two particles arise not only from electric dipolar interaction but also from near-field interactions such as electric-magnetic dipolar coupling and hybridization. Notably, the two large pulling forces can be independently achieved under an off-resonance condition because of the hybridization. Moreover, the coupling and hybridization can even dominate the pulling forces, respectively. In particular, the pulling forces depend on the particle sizes, wavelength, and gap between particles. Importantly, the optical pulling forces can be regulated by the polarization angle of the incident light. The presented theoretical approach is general for a dielectric-metal heterodimer within the electromagnetic dipolar approximation and the results provide insights for stable optical traction, sorting, separation, and delivery of particles and cells.