Self-Swerving Optical Force by Chiral Inhomogeneity

Light exerts optical forces on objects, finding numerous applications in physical and biomedical sciences. It is commonly known that the direction of the optical force on a single sphere is dictated by the light field. Thus, actively switching light properties such as polarization is an efficient strategy to control the locomotion of particles. Here, by exploiting the reversible optical force induced by chiral inhomogeneity, we observe self-swerving behavior in a chiral sphere embedded in water via a linearly polarized light wave. This counterintuitive optical force, arising from Poynting momentum conservation in chiral light-matter interaction, reverses direction with variations in the chirality gradient, inducing a self-swerving effect as the particle rotates. Experimentally, we observe that chiral particles, ranging from nanometers to micrometers in size, exhibit self-swerving behavior under a fixed linear polarization. Our study delves into a novel realm of optical forces in the presence of ubiquitous imperfect or inhomogeneous materials. It enriches the understanding of chiral-light interactions, and facilitates diverse applications in chiral detection, advanced optical manipulation, and micro-robots.