Controllable Lateral Optical Forces on Janus Particles in Fluid Media

TL;DR

This paper demonstrates a novel method to generate and control lateral optical forces on Janus particles in fluid media, enabling reversible propulsion through polarization adjustments, with potential applications in biophotonics and microfluidics.

Contribution

It introduces a tunable lateral optical force mechanism within fluid environments using Janus particles, expanding optical manipulation capabilities beyond interface geometries.

Findings

Lateral optical forces can be induced and tuned in fluid media.

Reversible propulsion of Janus particles achieved by polarization rotation.

Experimental results agree with theoretical predictions.

Abstract

Optical forces - studied since the earliest days of laser physics - continue to reveal rich dynamics and enable powerful tools for manipulation of objects on micro- and nanoscales, and even individual atoms. Lateral optical forces, which act perpendicular to the direction of beam propagation, are particularly intriguing but have largely been restricted to interface geometries such as air - water boundaries. Here, we realize tunable lateral optical force entirely within a fluid environment by using Janus particles: dielectric microspheres half-coated with gold. We show that the lateral optical force arises from scattering asymmetry induced by the asymmetric structure of the particles; it can be tuned by adjusting the polarization angle of a linearly polarized beam, but also particle parameters including their size and orientation. Experimentally, we directly observe fully reversible lateral propulsion of Janus particles in water merely by rotating the polarization direction, in excellent agreement with theoretical predictions. These results establish a new mechanism for programmable, polarization-controlled optical manipulation, with promising implications for biophotonics, microfluidics, and active soft-matter systems.