Optical multipolar torque in structured electromagnetic fields: on `helicity gradient' torque, quadrupolar torque and the spin of field gradient

Abstract

Structured light mechanically interacts with matter via optical forces and torques. The optical torque is traditionally calculated via the flux of total angular momentum (AM) into a volume enclosing an object. In [Phys. Rev. A 92, 043843 (2015)] a powerful method was suggested to calculate optical torque separately from the flux of the spin and the orbital parts of optical AM, providing useful physical insight. However, the method predicted a new type of dipolar torque dependent on the gradient of the helicity density of the optical beam, inconsistent with prior torque calculations. In this work we intend to clarify this discrepancy and clear up the confusion. We re-derive, from first principles and with detailed derivations, both the traditional dipolar total torque using total AM flux, and the spin and orbital torque components based on the corresponding AM contributions, ensuring that their sum agrees with the total torque. We also test our derived analytical expressions against numerical integration, with exact agreement. We find that `helicity gradient' torque terms indeed exist in the spin and orbital components separately, but we present corrected prefactors, such that upon adding them, they cancel out, and the `helicity gradient' term vanishes from the total dipolar torque, reconciling literature results. We also derive the analytical expression of the quadrupolar torque, showing that it is proportional to the spin of the EM field gradient, rather than the local EM field spin, as sometimes wrongly assumed in the literature. We provide examples of counter-intuitive situations where the spin of the EM field gradient behaves very differently to the local EM spin. Naively using the local EM field spin leads to wrong predictions of the torque on large particles with strong contributions of quadrupole and higher-order multipoles, especially in a structured incident field.