# Nonequilibrium dynamics induced by scattering forces for optically trapped nanoparticles in strongly inertial regimes

## TL;DR

This paper investigates how nonconservative scattering forces influence the nonlinear dynamics of optically trapped nanoparticles in underdamped regimes, revealing new behaviors like Brownian vortices and noise excess.

## Contribution

It provides the first experimental and theoretical analysis of nonconservative forces in underdamped optical trapping, expanding understanding beyond overdamped environments.

## Key findings

- Significant low-frequency noise excess observed along the optical axis.
- Emergence of toroidal Brownian vortices in trapped particles.
- Numerical simulations and models support experimental results.

## Abstract

As a first approximation, the forces acting on optically trapped particles are commonly assumed to be conservative. The influence of the nonconservative force has been shown to be negligible in overdamped liquid environments. However, its impact in the underdamped regime remains unexplored. Here, we experimentally study the combined effects of gradient and scattering forces on the nonlinear dynamics of trapped particles at various air pressures, leading to significant low-frequency noise excess along the optical axis and the emergence of toroidal Brownian vortices. Our results supported, by numerical simulations and a theoretical model, provide fundamental insights into the functioning of optical tweezers and a means for investigating nonequilibrium steady states in the presence of nonconservative forces.