Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

Rfaqat Ali,Paulo A Maia Neto,Felipe A Pinheiro,Rafael S Dutra,Felipe S S Rosa

doi:10.1038/s41598-020-73530-1

Rfaqat Ali, Paulo A Maia Neto + Show 3 more

Open Access

https://doi.org/10.1038/s41598-020-73530-1

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Abstract

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.

Highlights

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium
The constitutive relations for a chiral medium contain a direct coupling between the electric field E and the auxiliary field H proportional to the chirality parameter κ
We have shown that the optical force acting on a dielectric trapped microsphere embedded in a chiral medium strongly depends on the chirality parameter κ and on the handedness of the CP trapping beam

Summary

Introduction

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. Our results for the particle rotation open the way for the characterization of the local chiral response at the nanoscale, that nicely interconnects with already existing local probing techniques to determine viscoelastic properties[47,48] and micro-rheological properties of small p articles[49,50,51,52]

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