Direct measurement of the nonconservative force field generated by optical tweezers

TL;DR

This paper introduces a novel method to measure the three-dimensional force field of optical tweezers, revealing that it is nonconservative and influenced by scattering forces, with implications for energy flux in trapping experiments.

Contribution

The study provides the first direct measurement of the nonconservative force component in optical tweezers with high spatial and force resolution.

Findings

Force field is nonconservative, increasing radially from the axis.

Nonconservative forces cause complex energy flux depending on conditions.

Measurement aligns with Gaussian beam model predictions.

Abstract

The force field of optical tweezers is commonly assumed to be conservative, neglecting the complex action of the scattering force. Using a novel method that extracts local forces from trajectories of an optically trapped particle, we measure the three dimensional force field experienced by a Rayleigh particle with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is nonconservative with the nonconservative component increasing radially away from the optical axis, in agreement with the Gaussian beam model of the optical trap. Together with thermal position fluctuations of the trapped particle, the presence of the nonconservative force can cause a complex flux of energy into the optical trap depending on the experimental conditions.