Fig. 1: Mechanical effects of light on small particles.
a For plane wave illumination, the optical force Frad is provided only by radiation pressure which acts in the propagation direction, and the optical spin dictates the torque Tspin. b Under structured light, gradient and curl forces may arise due to the field intensity gradient and spin-curl, it being also possible to realize the concepts of negative (NOF) and lateral (LOF) optical force, characterized by force directions opposite and orthogonal to the propagation, respectively. c The present work puts forward the rotational analogues of the forces shown in (b). We discover the gradient and curl torques, stemming from the inhomogeneity of the reactive helicity and electromagnetic (i.e., Poynting) momentum, respectively; showing how to harness them to create the negative (NOT) and the lateral (LOT) optical torque.
