F. G. Mitri
The interaction of Bessel pincers light-sheets (i.e., finite optical beam “slices” in 2D) with a light-absorptive subwavelength sphere coated by a plasmonic layer in vacuum is examined in the framework of the electric dipole approximation. The vector angular spectrum decomposition method in addition to the Lorenz gauge condition and Maxwell’s equations are used to determine the Cartesian components of the incident radiated electric field of the Bessel pincers light-sheets. Two main effects are thoroughly investigated from the standpoint of the optical radiation force and spin torque theories. The numerical results for the optical radiation force and spin torque components show that a plasmonic layer of optimal thickness coating a small sphere can enhance the longitudinal and transverse force components as well as the axial spin torque induced by Bessel pincers light-sheets at the plasmonic resonance. Moreover, sign reversal of the longitudinal force component is manifested off the plasmonic resonance, resulting in particle trapping in a confined region in space. An axial spin torque sign reversal also occurs, which induces counterclockwise or clockwise rotation around the center of mass of the coated absorptive sphere depending on its position in the cross-sectional transverse plane. Furthermore, based on Newton’s second law of motion, a particle dynamics numerical analysis is performed, which reveals the emergence of retrograde motions of the coated sphere off-resonance as well as complex non-rectilinear/wiggly trajectories depending on its position in the transverse cross-sectional plane. At the plasmonic resonance, the coated sphere is propelled away from the source in the transverse cross-sectional plane along quasi-rectilinear trajectories. The results find related applications using autofocusing Bessel pincers light-sheets in particle manipulation and optical sorting devices.
DOI
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