Abstract
In order to pull objects towards the light source a single tractor beam inevitably needs to be strongly nonparaxial. This stringent requirement makes such a tractor beam somewhat hypothetical. Here we reveal that the cylindrical shape of dielectric particles can effectively mitigate the nonparaxiality requirements, reducing the incidence angle of the partial plane waves of the light beam down to 45° and even to 30° for respectively dipole and dipole-quadrupole objects. The optical pulling force attributed to the interaction of magnetic dipole and magnetic quadrupole moments of dielectric cylinders occurs due to the TE rather than TM polarization. Therefore, the polarization state of the incident beam can be utilized as an external control for switching between the pushing and pulling forces. The results have application values towards optical micromanipulation, transportation and sorting of targeted particles.
Highlights
Tractor beams have recently drawn great attention of researchers owing to intriguing and sometimes counterintuitive physics behind them[1,2,3,4,5,6,7,8,9,10,11]
When a light beam is absorbed by a lossy object, both the energy and momentum are transferred to the object, exerting naturally the pushing force due to the momentum conservation[28]
Bigger spherical beads in the dipolar approximation have both electric and magnetic dipole moments and require α ≥ 60°, where α is the angle between the optical axis x and wavevectors k of the partial plane waves of the light beam
Summary
Tractor beams have recently drawn great attention of researchers owing to intriguing and sometimes counterintuitive physics behind them[1,2,3,4,5,6,7,8,9,10,11]. Backward or pulling forces imposed by a continuous propagation-invariant tractor beam (single tractor beam, e.g., Bessel beam) are nonconservative forces. They emerge as a result of interaction of particle’s multipole moments, the most contribution being usually introduced by the dipole moments. When a light beam is absorbed by a lossy object, both the energy and momentum are transferred to the object, exerting naturally the pushing force due to the momentum conservation[28] In this connection, when loss is ‘reversed’, an object with optical gain may generate an optical pulling force[29], since it can greatly amplify the number of forward-scatterred photons. They can be employed in the Rayleigh approximation for attracting even tiny particles
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