Optical trapping of a perfect electromagnetic conductor (PEMC) sphere exhibiting rotary polarization using nonparaxial focused Gaussian single-beam tweezers

Abstract

A perfect electromagnetic conductor (PEMC) sphere illuminated by linearly-polarized plane progressive waves is known to experience a positive/repulsive force regardless of the sphere size. The present analysis shows that the situation changes when a Rayleigh (small) PEMC sphere is located along the axis of wave propagation of a linearly-polarized nonparaxial focused Gaussian beam. A negative longitudinal force arises as the sphere is translated beyond the center of the focus in the diverging region of the beam along the axis of wave propagation. Mathematical series related to the co-polarized and cross-polarized force components are derived. An exact expression for the on-axis beam-shape coefficients (BSCs) representing the incident nonparaxial focused Gaussian beam is obtained stemming from the complex source-point (CSP) method. The BSCs are compared to the standard solution obtained using the principle of localization. The case of plane progressive waves is recovered for a highly collimated nonparaxial focused Gaussian beam. In addition, the scattering asymmetry parameter is derived. Computations for the longitudinal radiation force efficiency, the dimensionless scattering asymmetry parameter, and the co-polarized and cross-polarized components are presented and discussed. In the Rayleigh regime, the results for the PEMC sphere show that the co-polarized component of the radiation force efficiency is negative beyond the center of the focal spot, while it is positive in the converging region of the beam. Trapping occurs when the longitudinal force vanishes. The cross-polarized radiation force efficiency is always positive, and counteracts the pulling force. Moreover, the scattering asymmetry parameter and its cross-polarized component are always negative while its co-polarized component is positive. Computations for the force efficiency for a radially-polarized nonparaxial focused Gaussian beam are also performed and compared to those obtained using a linearly-polarized focused field. Although the linearly-polarized focused beam induces larger pulling longitudinal force, improved spatial confinement with radial polarization is achieved.