Controllable lateral optical force in a double-sphere system

Extensive studies have been conducted on the lateral optical force, which is perpendicular to the direction of light propagation. Current research is primarily focused on the lateral force experienced by a single sphere. In this study, we investigate the lateral optical force exerted on a double-sphere system aligned along the direction of evanescent-wave propagation, utilizing full-wave simulations and the multipole-expansion method. Despite no apparent lateral asymmetry, we find that each sphere can experience either positive or negative lateral optical force by tuning the incident wave's frequency and polarization. Unlike the lateral force experienced by a single sphere in an evanescent wave, arising solely from recoil force due to Belinfante's spin momentum in a circularly polarized evanescent wave or spin angular momentum in a linearly diagonally polarized evanescent wave, the lateral force exerted on each sphere in the double-sphere system can also stem from an interception force. This interception force occurs due to the field gradient caused by the spheres' asymmetric scattering. This asymmetry is rooted in the real or imaginary part of the complex Poynting vector of the incident wave. It is the interception force that enables continuous tuning of the lateral optical force of each sphere and their sign flip. This research advances the understanding of lateral optical force and offers more flexibility for optical manipulation in multiparticle systems, opening up new opportunities for lateral-force-based applications.