Nonconservative forcing and diffusion in refractive optical traps

TL;DR

This paper investigates how nonconservative optical forces near equilibrium points affect particle diffusion, energy distribution, and heating in refractive optical traps, revealing mechanisms for potential instabilities.

Contribution

It introduces a model of nonconservative forces in optical traps and analyzes their effects on particle diffusion and energy equipartition.

Findings

Nonconservative forces reduce to circular forcing in a plane near equilibrium.

Equipartition of potential energy persists despite nonconservative flow.

Dissipated power is proportional to temperature, indicating possible heating instability.

Abstract

Refractive optical trapping forces can be nonconservative in the vicinity of a stable equilibrium point even in the absence of radiation pressure. We discuss how nonconservative 3D force fields, in the vicinity of an equilibrium point, reduce to circular forcing in a plane; a simple model of such forcing is the refractive trapping of a sphere by a four rays. We discuss in general the diffusion of an anisotropically trapped, circularly forced particle and obtain its spectrum of motion. Equipartition of potential energy holds even though the nonconservative flow does not follow equipotentials of the trap. We find that the dissipated nonconservative power is proportional to temperature, providing a mechanism for a runaway heating instability in traps.