Negative Optical Torque

TL;DR

This paper reports the discovery of negative optical torque, where light with angular momentum causes an object to rotate in the opposite direction, demonstrated in simple planar structures with specific conditions.

Contribution

It introduces the phenomenon of negative optical torque and explains how it can be achieved in simple planar structures, expanding understanding of light-matter angular momentum interactions.

Findings

Negative optical torque can be realized in simple planar structures.

Field retardation is essential for negative torque.

Optimal conditions for negative torque are identified and explained.

Abstract

Maxwell noted that light carries angular momentum, and as such it can exert torques on material objects. This was subsequently proved by Beth in 1936. Applications of these opto-mechanical effects were limited initially due to their smallness in magnitude, but later enabled by the invention of laser. Novel and practical approaches for harvesting light for particle rotation have been demonstrated, where the structure is subjected to a positive optical torque along a certain axis21 if the incident angular momentum has a positive projection on the same axis. We report here a counter-intuitive phenomenon of negative optical torque, meaning that incoming photons carrying angular momentum rotate an object in the opposite sense. Surprisingly this can be realized quite straightforwardly in simple planar structures. Field retardation is a necessary condition. The optimal conditions are explored and explained.