  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.
