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Optical tweezers use light from a tightly focused laser beam to manipulate the motion of tiny particles. When the laser light is strongly focused, but still paraxial, its e/m field is characterized by a longitudinal component which is of magnitude comparable to the transverse ones and which has been ignored in the theoretical analysis of the tweezing forces. In our work we revise the calculations of the various components of the radiation pressure force, within the limits of Rayleigh regime or dipole approximation, in the case where a tiny particle interacts, in free space, with a circularly polarized optical vortex beam, by taking into account this ignored field term. We show that this term is responsible for considerable modifications in the magnitude of the various components, moreover and also for the appearance of terms involving the coupling of the spin angular momentum (SAM) and the orbital angular momentum (OAM) of the photons of the vortex beam. We compare our findings with the ones taken ignoring the longitudinal field component.

The classical dynamics of a cold atom trapped inside a vertical rotating helical optical tube (HOT) is investigated by taking also into account the gravitational field. The resulting equations of motion are solved numerically. The rotation induces a vertical motion for an atom initially at rest. The motion is a result of the action of two inertial forces, namely the centrifugal force and the Coriolis force. Both inertial forces force the atom to rotate in a direction opposite to that of the angular velocity of the HOT. The frequency and the turning points of the atom's global oscillation can be controlled by the value and the direction of the angular velocity of the HOT. However, at large values of the angular velocity of the HOT the atom can escape from the global oscillation and be transported along the axis of the HOT. In this case, the rotating HOT operates as an Optical Archimedes' Screw (OAS) for atoms.

We study the directional properties of a radiation field emitted by a geometrically small system composed of two identical two-level emitters located at short distances and driven by an optical vortex beam, a Laguerre-Gaussian beam which possesses a structured phase and amplitude. We find that the system may operate as a nanoantenna for controlled and tunable directional emission. Polar diagrams of the radiation intensity are presented showing that a constant phase or amplitude difference at the positions of the emitters plays an essential role in the directivity of the emission. We find that the radiation patterns may differ dramatically for different phase and amplitude differences at the positions of the emitters. As a result the system may operate as a two- or one-sided nanoantenna. In particular, a two-sided highly focused directional emission can be achieved when the emitters experience the same amplitude and a constant phase difference of the driving field. We find a general directional property of the emitted field that when the phase differences at the positions of the emitters equal an even multiple of \pi/4, the system behaves as a two-sided antenna. When the phase difference equals an odd multiple of \pi/4, the system behaves as an one-sided antenna. The case when the emitters experience the same phase but different amplitudes of the driving field is also considered and it is found that the effect of different amplitudes is to cause the system to behave as a uni-directional antenna radiating along the interatomic axis., Comment: published version

We determine how a system composed of two nonidentical two-level atoms with different resonance frequencies and different damping rates could work as a nano-antenna for controlled mode switching and light routing. We calculate the angular distribution of the emitted field detected in a far-field zone of the system including the direct interatomic interactions and arbitrary linear dimensions of the system. The calculation is carried out in terms of the symmetric and antisymmetric modes of the two atom system. We find that as long as the atoms are identical, the emission cannot be switched between the symmetric and antisymmetric modes. The switching may occur when the atoms are non-identical and the emission can then be routed to different modes by changing the relative ratio of the atomic frequencies, or damping rates or by a proper tuning of the laser frequency to the atomic resonance frequencies. It is shown that in the case of atoms of different resonance frequencies but equal damping rates, the light routing is independent of the frequency of the driving laser field. It depends only on the sign of the detuning between the atomic resonance frequencies. In the case of atoms of different damping rates, the emission can be switched between different modes by changing the laser frequency from the blue to red detuned from the atomic resonance. The effect of the interatomic interactions is also considered and it is found that in the case of unequal resonance frequencies of the atoms, the interactions slightly modify the visibility of the intensity pattern. The case of unequal damping rates of the atoms is affected rather more drastically, the light routing becoming asymmetric under the dipole-dipole interaction with the enhanced intensities of the modes turned towards the atom of smaller damping rate., Comment: 14 pages, 14 figures