Possible sorting mechanism for microparticles in an evanescent field

TL;DR

This paper explores a potential microparticle sorting method using evanescent wave radiation forces, showing how adjusting laser parameters can selectively expel particles based on slight refractive index differences.

Contribution

It demonstrates a novel sorting mechanism leveraging oscillatory radiation forces influenced by wave number tuning, with practical feasibility for particles with specific refractive index differences.

Findings

Vertical force can be used to separate microspheres by refractive index.

Sorting is feasible at wave numbers around 18-20 or higher.

Incidence angle variations affect the radiation force and sorting efficiency.

Abstract

Mie scattering theory is used to calculate radiation forces on a dielectric microsphere illuminated by evanescent waves, produced by laser light transmitted obliquely through a flat horizontal dielectric surface. The incident field is identified with the evanescent field, and both $p$ and $s$ polarizations are considered. Our investigation consists of three parts. First, after highlighting the basic formalism, we report results for the radiation force published in an earlier paper [J. Opt. Soc. Am. B {\bf 12}, 2429 (1995)], correcting a few trivial calculational errors. Second - the main objective of our paper - is to show how the vertical (lifting) force on microspheres, typically via a proper adjustment of the laser frequency, can be used to separate spheres differing by a slight amount in their refractive index. This is caused by an oscillatory behavior in the force with respect to the nondimensional wave number $\alpha$ in the surrounding medium. Fine-tuning the wave number $\alpha$, relative to the given refractive indices in the system, may lead to particle expulsion. The sorting mechanism turns out to be feasible when $\alpha$ is about 18-20 or larger, which actually is in the region of practical interest. Finally, we investigate how variations in the angle of incidence $\theta_1$ for the laser beam influences the resulting radiation force.