Transverse optical gradient force in untethered rotating metaspinners

TL;DR

This paper introduces untethered microscopic metaspinners made from dielectric metasurfaces that can be efficiently rotated by light, revealing novel transverse optical forces and potential applications in active matter and micromachinery.

Contribution

It demonstrates the design and operation of optical metasurface-based metaspinners capable of light-induced rotation and reveals an anomalous transverse optical gradient force affecting their collective motion.

Findings

Metaspinners generate torque via photon recoil and orbital angular momentum.

A transverse optical gradient force causes collective orbiting opposite to spinning direction.

Potential applications include active matter systems and light-driven micromachines.

Abstract

Nanostructured dielectric metasurfaces offer unprecedented opportunities to control light-matter momentum exchange, and thereby the forces and torques that light can exert on matter. Here we introduce optical metasurfaces as components of ultracompact untethered microscopic metaspinners capable of efficient light-induced rotation in a liquid environment. Illuminated by weakly focused light, a metaspinner generates torque via photon recoil through the metasurfaces ability to bend light towards high angles despite their sub-wavelength thickness, thereby creating orbital angular momentum. We find that a metaspinner is subject to an anomalous transverse lateral optical gradient force that acts in concert with the classical gradient force. Consequently, when two or more metaspinners are trapped together in a laser beam, they collectively orbit the optical axis in the opposite direction to their spinning motion, in stark contrast to rotors coupled through hydrodynamic or mechanical interactions. The metaspinners delineated herein not only serve to illustrate the vast possibilities of utilizing optical metasurfaces for fundamental exploration of optical torques, but they also represent potential building-blocks of artificial active matter systems, light-driven micromachinery, and general-purpose optomechanical devices.