# Negative optical torque on a microsphere in optical tweezers

## TL;DR

This paper demonstrates that optical tweezers with elliptically polarized beams can exert a negative optical torque on microspheres, with the effect observable across various particle sizes and configurations, enabling new characterization methods.

## Contribution

It introduces a method to observe and analyze negative optical torque in optical tweezers, accounting for astigmatism, and proposes using microsphere rotation measurements for system characterization.

## Key findings

- Confirmed negative optical torque across multiple particle sizes.
- Achieved agreement with theoretical predictions without fitting.
- Proposed a new method for characterizing trapping parameters.

## Abstract

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle.