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arXiv:2110.09963 · Journal of Physics B Atomic Molecular and Optical Physics

Absorption of hybrid fibre modes by Cs atoms in quadrupole transitions

Smail B. Bougouffa, M. Babiker

Published Sep 26, 2023• 2 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/1361-6455/acfd71

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Journal of Optics•Sep 29, 2016•8 citations
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.
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 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 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.
We investigate the electric quadrupole interaction of an alkali-metal atom with guided light in the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the quadrupole oscillator strength, and their enhancement factors. In the example of a 87Rb^{87}\mathrm{Rb}87Rb atom, we study the dependencies of the quadrupole Rabi frequency on the quantum numbers of the transition, the mode type, the phase circulation direction, the propagation direction, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the root-mean-square (rms) quadrupole Rabi frequency reduces quickly but the quadrupole oscillator strength varies slowly with increasing radial distance. We show that the enhancement factors of the rms Rabi frequency and the oscillator strength do not depend on any characteristics of the internal atomic states except for the atomic transition frequency. The enhancement factor of the oscillator strength can be significant even when the atom is far away from the fiber. We show that, in the case where the atom is positioned on the fiber surface, the oscillator strength for the quasicircularly polarized fundamental mode HE11{\mathrm{HE}}_{11}HE11​ has a local minimum at the fiber radius $a\ensuremath{\simeq}107$ nm, and is larger than that for quasicircularly polarized higher-order hybrid modes, transverse electric modes, and transverse magnetic modes in the region a<498.2a<498.2a<498.2 nm.
Optical nanofibres are increasingly being used in cold atom experiments due to their versatility and the clear advantages they have when developing all-fibred systems for quantum technologies. They provide researchers with a method of overcoming the Rayleigh range for achieving high intensities in a focussed beam over a relatively long distance, and can act as a noninvasive tool for probing cold atoms. In this review article, we will briefly introduce the theory of mode propagation in an ultrathin optical fibre and highlight some of the more significant theoretical and experimental progresses to date, including the early work on atom probing, manipulation and trapping, the study of atom-dielectric surface interactions, and the more recent observation of nanofibre-mediated nonlinear optics phenomena in atomic media. The functionality of optical nanofibres in relation to the realisation of atom–photon hybrid quantum systems is also becoming more evident as some of the earlier technical challenges are surpassed and, recently, several schemes to implement optical memories have been proposed. We also discuss some possible directions where this research field may head, in particular, in relation to the use of optical nanofibres that can support higher-order modes with an associated orbital angular momentum.
Optical nanofibres are ultrathin optical fibres with a waist diameter typically less than the wavelength of light being guided through them. Cold atoms can couple to the evanescent field of the nanofibre-guided modes and such systems are emerging as promising technologies for the development of atom-photon hybrid quantum devices. Atoms within the evanescent field region of an optical nanofibre can be probed by sending near or on-resonant light through the fibre; however, the probe light can detrimentally affect the properties of the atoms. In this paper, we report on the modification of the local temperature of laser-cooled 87 Rb atoms in a magneto-optical trap centred around an optical nanofibre when near-resonant probe light propagates through it. A transient absorption technique has been used to measure the temperature of the affected atoms and temperature variations from 160 μ k to 850 μ k, for a probe power ranging from 0 to 50 nW, have been observed. This effect could have implications in relation to using optical nanofibres for probing and manipulating cold or ultracold atoms.