Possible sorting mechanism for microparticles in an evanescent field

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.

18 pages, 7 figures

References in corpus (4)

• Mie scattering and optical forces from evanescent fields: A complex-angle approach
• Scattering of inhomogeneous circularly polarized optical field and mechanical manifestation of the internal energy flows
• Photophoretic trampoline - Interaction of single airborne absorbing droplets with light
• On the Possibility of Measuring the Abraham Force using Whispering Gallery Modes

Cited by in corpus (4)

• Spectrally resolved resonant propulsion of dielectric microspheres
• Negative azimuthal force of a nanofiber-guided light on a particle
• Transverse multipolar light-matter couplings in evanescent waves
• Engineering optical forces through Maxwell stress tensor inverse design