Quantitative assessment of non-conservative radiation forces in an optical trap

TL;DR

This paper introduces a method to quantify non-conservative forces in optical trapping, confirming their negligible impact under typical conditions and providing a standard calibration extension.

Contribution

It presents a novel technique to measure non-conservative forces in optical tweezers, extending standard calibration methods.

Findings

Non-conservative forces are negligible in typical trapping conditions.

The technique can verify the conservative nature of optical forces.

Non-conservative effects only matter at very low trapping powers.

Abstract

The forces acting on an optically trapped particle are usually assumed to be conservative. However, the presence of a non-conservative component has recently been demonstrated. Here we propose a technique that permits one to quantify the contribution of such non-conservative component. This is an extension of a standard optical tweezers calibration technique and, therefore, can easily become a standard test to verify the conservative optical force assumption. Using this technique we have analyzed different-sized optically trapped particles under different trapping conditions. We conclude that the non-conservative effects are effectively negligible and do not affect the standard calibration procedure, unless for extremely low-power trapping, far away from the trapping regimes usually used in experiments.