Controllable Lateral Optical Forces on Janus Particles in Fluid Media

Optical forces−studied since the earliest days of laser physics−continue to reveal rich dynamics and enable powerful tools for the manipulation of objects on micro‐ and nanoscales, and even individual atoms. Lateral optical forces (LOFs), 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 forces 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 structural asymmetry of the particles, and can be tuned by adjusting not only the polarization angle of a linearly polarized beam, but also particle parameters, including 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.