Bidirectional Chiral-Achiral Coupling for Tunable Lateral Optical Forces in Nanoparticle Systems

Abstract Light-induced optical forces provide a powerful route for manipulating nanoparticles, and chiral optical forces are particularly important for enantioselective trapping, separation, and sensing. Existing studies have mainly focused on the force response of chiral nanoparticles themselves, while the mutual photomechanical coupling between chiral and achiral nanoparticles remains insufficiently understood. Here, we reveal a bidirectional optical-force coupling mechanism in chiral-achiral nanoparticle systems. Under linearly polarized plane-wave illumination, an achiral nanoparticle can experience a tunable lateral optical force when coupled to a neighboring chiral nanoparticle. This counterintuitive force originates from chirality-mediated mirror-symmetry breaking in the coupled scattering process, which generates an asymmetric transverse momentum flow in the scattered field. The force direction reverses when the handedness of the chiral particle is inverted, and it can also be switched by rotating the incident polarization. Conversely, adjacent achiral Mie resonators can reshape and enhance the local electromagnetic field around a small chiral particle, leading to a substantially amplified chirality-dependent lateral force. Furthermore, by extending the system from spherical chiral particles to structurally chiral helical nanostructures, we show that chirality transfer remains robust and can further reverse the direction of the lateral force on adjacent achiral particles. These results provide a mechanistic understanding of chirality transfer and near-field-enhanced optical forces in hybrid chiral-achiral nanoparticle systems and suggest new opportunities for enantioselective optical manipulation, force-based chiral sensing, and chiral nanophotonic device design.