Transverse spinning of a sphere in a plasmonic field

We evaluate optical forces and torques induced by a surface plasmon to a sphere of arbitrary size, i.e. beyond the point-like dipolar limit. Through a multipolar decomposition of the plasmonic field, we demonstrate that the induced torque is purely transverse to the plasmon propagation direction. Our approach removes the inherent ambiguities of the dipolar regime with respect to rotations and emphasizes the crucial role played by dissipation in the onset of the plasmonic torque. We also give realistic estimates of such plasmon-induced spinning of gold spheres immersed in water or air.