Intracavity Optical Trapping

TL;DR

This paper introduces intracavity nonlinear feedback forces in optical trapping, enabling significantly higher confinement with lower laser intensities by placing particles inside an optical cavity, demonstrated experimentally with microscopic particles.

Contribution

The study demonstrates a novel intracavity trapping scheme that enhances confinement and reduces laser exposure by leveraging nonlinear feedback within an optical cavity.

Findings

Achieved orders-of-magnitude higher confinement per unit laser intensity.

Reduced laser intensity exposure by two orders of magnitude compared to standard tweezers.

Successfully trapped microscopic particles inside a fiber laser cavity.

Abstract

Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is \emph{outside} the laser cavity. Here, we demonstrate that \emph{intracavity nonlinear feedback forces} emerge when the particle is placed \emph{inside} the optical cavity, resulting in orders-of-magnitude higher confinement along the three axes per unit laser intensity on the sample. We present a toy model that intuitively explains how the microparticle position and the laser power become nonlinearly coupled: The loss of the laser cavity depends on the particle position due to scattering, so the laser intensity grows whenever the particle tries to escape. This scheme allows trapping at very low numerical apertures and reduces the laser intensity to which the particle is exposed by two orders of magnitude compared to a standard 3D optical tweezers. We experimentally realize this concept by optically trapping microscopic polystyrene and silica particles inside the ring cavity of a fiber laser. These results are highly relevant for many applications requiring manipulation of samples that are subject to photodamage, such as in biological systems and nanosciences.