An ultra-stable three-dimensional photophoretic trap in air facilitated by a single multimode fiber

TL;DR

This paper demonstrates that a single multimode fiber creates a more stable and robust three-dimensional photophoretic trap in air for absorbing microparticles, outperforming single-mode Gaussian beam traps, with potential applications in optical trapping and spectroscopy.

Contribution

The study introduces a novel use of a single multimode fiber to achieve highly stable photophoretic trapping, highlighting the importance of speckle pattern size in trap stability.

Findings

Maximum trapping force occurs when speckle size matches particle diameter.

Multimode beams induce large oscillations along the axial direction.

Stable trapping conditions are confirmed through force balance simulations.

Abstract

Photophoretic forces - which are of thermal origin - have defined an alternative route of optical trapping of absorbing microparticles in air. Here, we show that a single multi-mode fiber facilitates significantly more robust optical traps compared to a pure Gaussian beam emanating from a single mode fiber for the trapping and manipulation of absorbing particles using photophoretic forces. We carefully study the dependency of trapping on speckle patterns generated from different modes from a multimode fiber, and experimentally observe that maximum trapping force can be obtained when the mean speckle size is comparable to the diameter of a trapped particle. We explain this observation by numerical simulations carried out to calculate the photophoretic force, and also determine stable trapping conditions from force balance equations. Interestingly, we also observe large oscillations of the trapped particle along the z-direction for multimode beams, which may be demonstrative of an effective restoring force for photophoretic trapping even in the axial direction. Our work may presage a new route for exciting applications on optical trapping and spectroscopy with photophoretic forces due to the inherent ease-of-use, portability, and flexibility of single muti-mode fiber based optical traps.