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Quadrupole absorption of hybrid optical fibre modes by Cs atoms

Smail B. Bougouffa, M. Babiker

Published Oct 19, 2021• 0 citations
OpenAlex

Abstract

The rate of the absorption of light endowed with orbital angular momentum by the process of upward quadrupole transitions in Cs atoms is evaluated when the Cs atoms are localised in the vicinity of the outer surface of an optical fibre. The coupling is between the quadrupole tensor components and the gradients of the vector components of the electric field of a hybrid fibre mode. Values of the absorption rates are obtainable for quadrupole transitions assuming a moderate laser power and experimentally accessible parameters, we find that the processes of absorption of the hybrid modes by the Cs atoms proceed at rates of the order $0.1 \gamma$ where $\gamma^{-1}$ is the lifetime of the upper state involved in the quadrupole transition.

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The rate of the absorption of light endowed with orbital angular momentum by the process of upward quadrupole transitions in Cs atoms is evaluated when the Cs atoms are localised in the vicinity of the outer surface of an optical fibre. The coupling is between the quadrupole tensor components and the gradients of the vector components of the electric field of a hybrid fibre mode. Values of the absorption rates are obtainable for quadrupole transitions assuming a moderate laser power and experimentally accessible parameters, we find that the processes of absorption of the hybrid modes by the Cs atoms proceed at rates of the order 0.1γ0.1 \gamma0.1γ where γ−1\gamma^{-1}γ−1 is the lifetime of the upper state involved in the quadrupole transition.
Abstract We evaluate the rate of the absorption of an optical nanofibre mode by a Cs atom in an electric quadrupole transition. With the Cs atom localised near the outer surface of the optical nano-fibre, an interaction occurs between the atomic quadrupole tensor components and the gradients of the vector components of the electric field of a hybrid fibre mode. The absorption rate is evaluated as a function of the radial position of the atom from the fibre axis, assuming a specific value of the laser power and we use experimentally accessible parameters. We find that the absorption of the hybrid modes by the Cs atom decreases as the atom recedes away from the fibre axis and it formally vanishes at sufficiently large radial distances. Close to the fibre, however, the absorption rate for the input power chosen can be two orders of magnitude larger than the quadrupole de-excitation rate despite the moderate power used.
Recent experiments involving the interaction of optical vortices with atoms in quadrupole transitions have shown it to be accompanied by the exchange of orbital angular momentum (OAM) between the electronic states of the atom and the optical vortex field. Earlier work, both theoretical and experimental, had ruled out the transfer of a vortex OAM to the electronic degrees of freedom in an electric dipole atomic transition and it has been confirmed that the lowest multipolar order involving an OAM transfer to the electronic motion is indeed the electric quadrupole. Hitherto, the quadrupole transition involving optical vortices has not been quantified and we thus set out to evaluate the absorption rate accompanied by an OAM transfer with reference to the $6^{2}S_{1/2}\ensuremath{\rightarrow}5^{2}D_{5/2}$ in Cs when cesium atoms are subject to the field of a linearly polarized optical vortex. Our results assuming typical experimentally accessible parameters indicate that the absorption rate for moderate light intensities is lower than the quadrupole spontaneous emission rate but should still be within the measurement capabilities of modern spectroscopic techniques.
Twisted light beams, or optical vortices, have been used to drive the circular motion of microscopic particles in optical tweezers and have been shown to generate vortices in quantum gases. Recent studies have established that electric quadrupole interactions can mediate an orbital angular momentum exchange between twisted light and the electronic degrees of freedom of atoms. Here we consider a quadrupole atomic transition mediated by a circularly-polarized optical vortex. We evaluate the transfer rate of the optical angular momentum to a Ca+ ion involving the 42S1∕2→32D5∕2 quadrupole transition and explain how the polarization state and the topological charge of the vortex beam determine the selection rules.
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.
Abstract Light guided by an optical nanofibre has a very steep evanescent field gradient extending from the fibre surface. This gradient can be exploited to drive electric quadrupole transitions in nearby quantum emitters. In this paper, we report on the observation of the 5S1/2 → 4D3/2 electric quadrupole transition at 516.6 nm (in vacuum) in laser-cooled 87Rb atoms using only a few μW of laser power propagating through an optical nanofibre embedded in the atom cloud. This work extends the range of applications for optical nanofibres in atomic physics to include more fundamental tests such as high-precision measurements of parity non-conservation.
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.