Optical forces on cylinders near subwavelength slits illuminated by a photonic nanojet

TL;DR

This paper investigates how optical forces on dielectric and metallic cylinders near subwavelength slits are affected by photonic nanojet illumination, revealing force enhancements and resonance effects that influence particle manipulation.

Contribution

It demonstrates the impact of nanojet illumination and particle resonances on optical forces near subwavelength slits, including force enhancement and force direction changes.

Findings

Nanojet illumination increases optical forces by 3 to 10 times.

Localized plasmon excitation enhances force strength.

Resonance effects can switch forces from attractive to repulsive.

Abstract

We discuss optical forces exerted on particles, either dielectric or metallic, near a subwavelength slit illuminated by a photonic nanojet. We compare those cases in which the Mie resonances are or are not excited. The configurations on study are 2D, hence those particles are infinite cylinders and, in order to obtain extraordinary transmission, the illuminating beam is p-polarized. We show the different effects of these particle resonances on the optical forces: while whispering gallery modes under those illumination conditions weaken the force strength, this latter is enhanced by localized plasmon excitation. Also, illuminating the slit with a nanojet enhances the optical forces on the particle at the exit of the aperture by a factor between 3 and 10 compared with illumination of the slit with a Gausian beam. In addition, the pulling force that such a small resonant metallic particle suffers on direct illumination by a nanojet, can change by the presence of the slit, so that it may become repulsive at certain lateral positions of the particle.