Friday, May 19, 2017

Optical Bessel tractor beam on active dielectric Rayleigh prolate and oblate spheroids

F. G. Mitri

Optical Bessel tractor beams, designed to produce a negative pulling force on a particle, are gaining increased attention for applications in noncontact remote sampling, particle manipulation, and handling, to name some examples. In the long-wavelength (Rayleigh) limit, known also as the electric dipole approximation, earlier investigations demonstrated that a zeroth-order Bessel beam incident upon a passive dielectric sphere (i.e., no radiating sources in its core) always acts as a repulsor beam, which causes the particle to be pushed away from the source in the forward direction of the linear momentum. In contrast to what has already been established, this work shows that the incident wave field can act as a tractor beam (where a small spheroid is pulled backwards towards the source due to a negative attractive force) in the dipole approximation (Rayleigh) limit, provided that the particle is made of an active material, i.e., a dielectric spheroid acting as an oscillating source for which the extinction energy efficiency is negative. Numerical computations for the Cartesian components of the optical radiation force on active prolate and oblate spheroids with arbitrary orientation are performed. Emphasis is placed on the emergence of the tractor beam behavior and its dependence upon the half-cone angle, the polarization type of the incident beam, the spheroid aspect ratio, as well as its orientation in space. The analysis is extended to calculate the Cartesian components of the spin radiation torque, which causes a rotation of the spheroid around its center of mass in either the counterclockwise or the clockwise (negative) direction of spinning. Unlike the case of a sphere, the optical spin torque arises for a nonabsorptive oblate or prolate spheroid with arbitrary orientation in the field of a zeroth-order Bessel beam. Potential applications in optically engineered metamaterials, optical tractor beams, tweezers, particle manipulation, rotation, and handling would benefit from the results of this study.

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