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Optical Multipolar Torque in Structured Electromagnetic Fields: on `Helicity Gradient' Torque, Quadrupolar Torque and the Spin of Field Gradient by Wei, Lei; Fortuño, Francisco J. Rodríguez is a scholarly article available to read on EtoBox.

What is Optical Multipolar Torque in Structured Electromagnetic Fields: on `Helicity Gradient' Torque, Quadrupolar Torque and the Spin of Field Gradient about?

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

Author

Wei, Lei; Fortuño, Francisco J. Rodríguez

Published

2021

Language

EN