Micromotors Driven by Spin-Orbit Interaction of Light:Mimicking Planetary Motion at the Microscale

We introduce a new class of optical micromotors driven by synergistic action between the spin-orbit interaction (SOI) of light and spin-driven fluid flows leads to simultaneous rotation and revolution of the micromotors. The micromotors are essentially birefringent liquid crystal (LC) particles, that can efficiently convert the angular momentum of light into high-frequency rotational motion. By tightly focusing circularly polarized Gaussian beams through a high numerical aperture (NA) objective into a refractive index (RI) stratified medium, we create a spherically aberrated intensity profile where the spinning motion of a micromotor optically trapped at the centre of the profile induces fluid flows that causes orbiting motion of the off-axially trapped surrounding particles (secondary micromotors). In addition, the interaction between the input helicity of light and the anisotropic properties of the LC medium leads to intriguing SOI effects that drive the conversion of right- to left-circular polarization and vice versa. This opposite helicity, or spin-to-spin conversion, induces spin in the orbiting secondary micromotors depending on their birefringence properties, so that the entire system of spinning primary micromotor and revolving and spinning secondary micromotors is reminiscent of planetary motion at mesoscopic scales. Our findings, supported by both theoretical modelling and experimental validation, not only advance the understanding of light-matter interactions at the microscale but also open new avenues for the design and control of next-generation micromotors and spin-orbit optomechanics, offering a versatile platform for exploring novel optical manipulation techniques in combination with complex fluid dynamics.