Harnessing Photon Recoil for Enhanced Torque on Light-Driven Metarotors.

Contact-free rotation of microscopic objects in aqueous environments based on optical forces is a powerful concept in the development of light-driven microrobots, micromachines, torque transducers, and rheological sensors. Here, we demonstrate freely movable quasi-two-dimensional metasurface rotors with lateral dimensions up to 100 μm while still exhibiting controllable and steady rotation when submerged in water. The metarotors utilize photon recoil to produce strong optical torque by deflecting low-intensity laser light toward high angles via long lever arms, which amplify the creation of orbital angular momentum. We find that the torque generated by a single metarotor can be used to rotate hundreds of passive microparticles present in solution, suggesting potential applications as particle mixers in microfluidics and microbiology. Further development might involve utilizing metarotors as components in future microrobots for biomedicine and beyond.