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

TL;DR

This paper develops a theoretical framework for optical tweezing of dielectric microspheres in chiral media, revealing enhanced trapping stability and optical torque effects that enable nanoscale chiral sensing.

Contribution

It introduces a novel Mie-based theory for optical forces and torques on particles in chiral media, enabling new applications in chiral characterization and manipulation.

Findings

Enhanced axial trap stability with matching polarization and medium handedness.

Significant optical torque and rotation angle dependence on particle size and chirality.

Potential for all-optical nanoscale chiral sensing using measurable rotation angles.

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.