Optical Torque Reversal via Electric-Magnetic Spin-Channel Selection

Light carrying spin angular momentum (SAM) can exert optical torque and drive particles into rotational motion. Although optical torque reversal has been achieved by tailoring particle properties or incident field parameters, the respective roles of the electric and magnetic components of transverse SAM in governing the torque on an isotropic particle remain unresolved. Here, reversible control of the out of plane optical torque on an absorbing isotropic Mie sphere is demonstrated in interference fields formed by two or three plane waves. Two orthogonal linear polarization states selectively activate torque pathways mediated by the electric and magnetic components of the transverse SAM, while intermediate polarization states allow both pathways to coexist. Notably, polarization controls the optical torque by tuning the relative weights of electric and magnetic multipolar contributions. Once higher order Mie channels become appreciable, competition among these contributions can reverse the total torque and switch the particle rotation between clockwise and counterclockwise directions. These results reveal distinct mechanical roles of the electric and magnetic components of transverse SAM and establish spin channel selection as a mechanism for polarization controlled optical torque reversal.