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Journal of Taibah University for Science

Generate localized optical vortices by quadrupole transition and twisted light

S. Al‐Awfi

Published Jan 1, 2021• 1 citation
Publisher

Abstract

We explain how quadrupole-active atomic interactions be able to be enhanced once the neutral atom interacts with an evanescent winding light modes. The optical system presented is a semi-bounded space based on neutral atoms situated close to the evanescent Bessel-Gaussian modes. The quadrupole Rabi frequency (and hence the optical quadrupole potential) generated by such evanescent modes is large enough to be exploited in atoms manipulation. In particular, the atomic states in which the dipole transitions are prohibited but the quadrupole transitions are permissible create the optical quadrupole potential of accessible value to play an important role. It is also shown how such optical system acts to create of evanescent modes with ℓ>0 having an orbital angular momentum ℓℏ in which the intensity distribution is restricted in the vicinity of the dielectric surface.

DOI
10.1080/16583655.2021.2007634

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We explain how quadrupole-active atomic interactions be able to be enhanced once the neutral atom interacts with an evanescent winding light modes. The optical system presented is a semi-bounded space based on neutral atoms situated close to the evanescent Bessel-Gaussian modes. The quadrupole Rabi frequency (and hence the optical quadrupole potential) generated by such evanescent modes is large enough to be exploited in atoms manipulation. In particular, the atomic states in which the dipole transitions are prohibited but the quadrupole transitions are permissible create the optical quadrupole potential of accessible value to play an important role. It is also shown how such optical system acts to create of evanescent modes with ℓ>0 having an orbital angular momentum ℓℏ in which the intensity distribution is restricted in the vicinity of the dielectric surface.
Recent studies have confirmed the coupling of optical vortices, such as Laguerre-Gaussian and Bessel-Gaussian modes, to quadrupole-active atomic transitions. This interaction has been shown to be enhanced considerably in the case of Laguerre-Gaussian beams due to the gradient coupling, particularly, in the case of a relatively large winding number. Here, we consider the trapping and the dynamics of atoms in the optical quadrupole potential generated by two coaxial counterpropagating optical vortex beams. We focus on the atomic transition $6{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}5{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{5/2}$ in Cs which is a dipole-forbidden but a quadrupole-allowed transition. We show how this atomic transition engages with the optical vortex fields at near resonance, leading to atom trapping in the optical quadrupole potential well accompanied by translational motion. We show how the optical forces generate the motion of the atoms trapped within the quadrupole potential, illustrating the results using typical experimentally accessible parameters.
The total internal reflection of a light with phase singularity, such as Laguerre–Gaussian (LG) light, can generate an evanescent wave. The distribution of this type of light is firmly localized in the vicinity above the dielectric surface and displays a rotational character. Any atom approaching the surface from the vacuum region interacts with evanescent wave and produces interesting rotational effects. Recent studies have established the coupling of LG beams to both electric atomic transitions; dipole and quadrupole according to the selection rules of each, permitted and prohibited. Due to the small magnitude of the quadrupole interaction compared to the dipole interaction, we chose the two counter-propagating beams configuration to generate the doubling Rabi frequencies with more stable surface optical forces. We describe the essential rotational features and discuss how they can be used to influence atoms. Atom confinement and dynamics in the interaction of surface optical vortices with the two different atomic transitions are illustrated using typical experimentally accessible parameters for the case of the Cs atom interacts with the LG10 mode.
In this paper, we explain how a weak optical quadrupole interaction in atoms can be utilized. Our proposed physical model is based on the atom, which interacts at near resonance with laser light prepared as a Bessel mode. This type of beam is a class of light, which possesses a well-defined orbital angular momentum that is distinct from spin angular momentum. This property, in particular, causes optical forces to ensue from such interactions, giving distinctive features. Although the magnitude of the quadrupole interaction in standard situation is small, it plays an important role in some special cases, such as those allowing processes involving dipole-forbidden transitions or in the presence of external fields. We describe the essential features of the optical-force effects and discuss how they can, in principle, be used to influence atoms.
Recently, some physical research has recognized the coupling of optical phase singularity light with quadrupole-active atomic transitions. We show that a Laguerre-Gaussian beam, when completely internally reflected at the dielectric/vacuum interface, can generate a well-defined surface spiral beam with a two-dimensional intensity distribution confined to a sub-wavelength region near the interface outside of the dielectric. The quadrupole interaction has been shown to be significantly enhanced in the case of coating the planar surface of a dielectric prism using multilayer dielectric thin films of different thicknesses. The mechanical action on the atom due to the optical forces characterized by optical quadrupole transitions could greatly facilitate the revolution of optical manipulation of neutral atoms along the dielectric/vacuum interface. The typical optical quadrupole potential, evaluated for the atomic transition, corresponds to λ=675.0nm with ΓQ=7.8×105s-1, in Cesium atoms, which is a dipole-forbidden but quadrupole-allowed transition. The parameters used to compute the optical quadrupole potential are similar to those in recent experiments. The factors controlling the general enhancement are pointed out and discussed.
Modern studies have confirmed the possibility of making atomic mirrors using quadrupole-active atomic transition evanescent waves. The reflection process has been shown to be enhanced to approach specular behavior in several ways. In this study, we considered the atomic mirror due to the optical quadrupole potential generated by two counter-propagating optical Gaussian beams. This arrangement provides good enhancement for the evanescent field in the vacuum above the surface, which can be used as an atomic mirror. A high-quality atomic mirror should reflect the atoms specularly while maintaining the coherence of the de Broglie wave function. We focused on a particular atomic transition in Cs, which is a dipole-forbidden but quadrupole-allowed transition. We illustrate the optical quadrupole potential and suggest that quadrupolar transitions generally ignored in atom optics exist for exploitation. This study investigated the conditions facilitating atomic mirrors using typical experimentally accessible parameters.
We show that the normally weak optical quadrupole interaction in atoms is enhanced significantly when the atom interacts at near resonance with an optical vortex. In particular, the forces and torque acting on the atom are shown here to scale up with the square of the winding number l of the vortex. Because the integer l can be arranged to be large, this property allows for processes involving dipole-forbidden, but quadrupole-allowed, transitions in atoms, such as cesium and oxygen, to come into play. We show that the mechanical effects of vortex light on atoms involving translational and rotational motion as well as trapping should be significantly enhanced for quadrupole transitions and present novel features with useful implications for the emerging field of atomtronics.