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We report for the first time the theory of optical tweezers of sphericaldielectric particles embedded in a chiral medium. We develop a partial-wave(Mie) expansion to calculate the optical force acting on a dielectricmicrosphere illuminated by a circularly-polarized, highly focused laser beam.When choosing a polarization with the same handedness of the medium, the axialtrap stability is improved, thus allowing for tweezing of high-refractive-indexparticles. When the particle is displaced off-axis by an external force, itsequilibrium position is rotated around the optical axis by the mechanicaleffect of an optical torque. Both the optical torque and the angle of rotationare greatly enhanced in the presence of a chiral host medium when consideringradii a few times larger than the wavelength. In this range, the angle ofrotation depends strongly on the microsphere radius and the chirality parameterof the host medium, opening the way for a quantitative characterization of bothparameters. Measurable angles are predicted even in the case of naturallyoccurring chiral solutes, allowing for a novel all-optical method to locallyprobe the chiral response at the nanoscale.

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