Negative optical torque

While it is now clear that optical scattering force can be negative (opposite to the propagating direction of the beam), less attention is being paid to the optical torque. Here, we show that the optical torque exerted on a structure by an electromagnetic wave can be in an opposite direction to the angular momentum of the incident beam. This negative torque is closely related to the symmetry of the structure, and by playing around with the rotational symmetry and density of the structure, one can control the magnitude of the negative as well as the positive torque.