Optical Bessel beam illumination of a subwavelength prolate gold (Au) spheroid coated by a layer of plasmonic material: radiation force, spin and orbital torques

Abstract

The optical radiation force, spin and orbital torques exerted on a subwavelength prolate gold spheroid coated by a layer of plasmonic material with negative permittivity and illuminated by either a zeroth-order (non-vortex) or a first-order vector Bessel (vortex) beam are computed in the framework of the electric dipole approximation method. Calculations for the Cartesian components of the optical radiation force on a subwavelength spheroid with arbitrary orientation in space are performed, with emphasis on the order (or topological charge), half-cone angle of the beam, and the plasmonic layer thickness on- and off-resonance. A repulsive (pushing) force is predicted for the layered subwavelength prolate spheroid, on- and off-resonance along the direction of wave propagation. Moreover, the Cartesian components of the spin radiation torque are computed where a negative longitudinal spin torque component can arise, suggesting a rotational twist of the spheroid around its center of mass in either the counter-clockwise or the clockwise (negative) direction of spinning. In addition, the longitudinal component of the orbital radiation torque exhibits sign reversal, indicating a revolution around the beam axis in either the counter-clockwise or the clockwise directions. The results show that the plasmonic resonance strongly alters the force, spin and orbital torque components, causing major amplitude enhancements, signs twists, and complex distributions in the transverse plane.