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arXiv:2004.07685

Spin selection rule for {\it S} level transitions in atomic rubidium under paraxial and nonparaxial two-photon excitation

Krishnapriya Subramonian Rajasree, Ratnesh Kumar Gupta, Vandna Gokhroo, Fam Le Kien, Thomas Nieddu, Tridib Ray, Síle Nic Chormaic, Georgiy V. Tkachenko

Published Apr 16, 2020• 1 citation
arXivHTMLPDF

Abstract

We report on an experimental test of the spin selection rule for two-photon transitions in atoms. In particular, we demonstrate that the $5S_{1/2}\to 6S_{1/2}$ transition rate in a rubidium gas follows a quadratic dependency on the helicity parameter linked to the polarization of the excitation light. For excitation via a single Gaussian beam or two counterpropagating beams in a hot vapor cell, the transition rate scales as the squared degree of linear polarization. The rate reaches zero when the light is circularly polarized. In contrast, when the excitation is realized via an evanescent field near an optical nanofiber, the two-photon transition cannot be completely extinguished (theoretically, not lower than 13\% of the maximum rate, under our experimental conditions) by only varying the polarization of the fiber-guided light. Our findings lead to a deeper understanding of the physics of multiphoton processes in atoms in strongly nonparaxial light.

DOI
10.48550/arxiv.2004.07685

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We report on an experimental test of the spin selection rule for two-photon transitions in atoms. In particular, we demonstrate that the 5S1/2→6S1/25S_{1/2}\to 6S_{1/2}5S1/2​→6S1/2​ transition rate in a rubidium gas follows a quadratic dependency on the helicity parameter linked to the polarization of the excitation light. For excitation via a single Gaussian beam or two counterpropagating beams in a hot vapor cell, the transition rate scales as the squared degree of linear polarization. The rate reaches zero when the light is circularly polarized. In contrast, when the excitation is realized via an evanescent field near an optical nanofiber, the two-photon transition cannot be completely extinguished (theoretically, not lower than 13% of the maximum rate, under our experimental conditions) by only varying the polarization of the fiber-guided light. Our findings lead to a deeper understanding of the physics of multiphoton processes in atoms in strongly nonparaxial light.
Abstract We present the first evidence of excitation of the 5 p 3 / 2 → 6 p 1 / 2 electric dipole-forbidden transition in atomic rubidium. The experiments were done in a rubidium vapor cell using Doppler-free optical–optical double resonance spectroscopy with counter-propagating beams, with a similar scheme to the excitation of the other fine transition 6 p 3/2 in the 6 p J manifold reported in Ponciano-Ojeda et al (2015 Phys. Rev. A 92 042511). The 5 p 3 / 2 → 6 p 1 / 2 dipole-forbidden excitation occurs at a wavelength λ ≈ 917.5 nm. The spectra showed the two main hyperfine states of the 6 p 1/2 manifold, and also resolved up to three satellites that appear due to effects of selection of velocities in thermal rubidium atoms. The polarization dependence of the relative intensities of the hyperfine lines of the decay fluorescence is also investigated. The data show that the individual line intensities and also the total cross-section have a strong dependence on the polarization direction. Experimental data for different polarization configurations of the light beams used in this two-photon spectroscopy are compared with the results of calculations that consider a strong atom-field coupling in the preparation step, followed by a weak electric quadrupole excitation and the blue fluorescence decay emission. Good agreement between experiment and this three-step model is found in the case of linear–linear polarizations, which confirms the results found for the 5 p 3 / 2 → 6 p 3 / 2 forbidden transition. The results presented here complete the information about the 5 p 3/2 → 6 p J electric dipole-forbidden transition in rubidium.
Abstract Doppler-free optical double-resonance spectroscopy is used to study the 5 s 1 / 2 → 5 p 3 / 2 → 6 p 3 / 2 excitation sequence in room-temperature rubidium atoms. This involves a 5 s 1 / 2 → 5 p 3 / 2 electric dipole preparation step followed by the 5 p 3 / 2 → 6 p 3 / 2 electric quadrupole excitation. A detailed experimental and theoretical study of the dependance on the excitation beams polarization from the 420 nm decay fluorescence ( 6 p 3 / 2 → 5 s 1 / 2 ) is presented. When a circularly polarized preparation beam is used, it produces a strongly oriented 5 p 3 / 2 intermediate state. In this case a linear quadrupole excitation beam transfers the oriented state to the 6 p 3 / 2 hyperfine states. For linearly polarized preparation and quadrupole excitation beams the spectra of the 6 p 3 / 2 hyperfine lines follow a cosine squared dependence on the angle betwe
The two-photon excitation of cold 7 Li atoms at the 2 S –120 S transition is studied with variable polarization of one of the two counter-propagating optical beams. The maximum efficiency of the two-photon transfer of atoms from the ground state 2 S to the Rydberg state 120 S can be reached when the circular polarizations of the beams are orthogonal. In this case the selection rules Δ L = 0 are satisfied. By using a rotating quarter-wave plate it is possible to vary the polarization of the selected optical beam and change the amplitude of the two-photon resonance. The recorded dependence of the two-photon resonance on the beam polarization is discussed. The obtained results can be used to compensate the unwanted variations of the beam polarization after reflections from mirrors.
Doppler-free optical double-resonance spectroscopy is used to study the 5S1/2 → 5P3/2 → 6Pj (j = 3/2,1/2) excitation sequence in room-temperature rubidium atoms. This involves a 5S1/2 → 5P3/2 electric dipole preparation step followed by the 5P3/2 → 6Pj electric quadrupole excitation. The electric dipole forbidden transitions occur at 911.0 nm (j = 3/2) and 917.5 nm (j = 1/2). Production of atoms in the 6Pj states is detected by observing their direct decay to the ground state through emission of blue photons (λ ≈ 420 nm). A detailed experimental and theoretical study of the dependence on the relative linear polarizations of excitation beams is made. It is shown that specific electric quadrupole selection rules over magnetic quantum numbers are directly related to the relative orientation of the linear polarization of the excitation beams.
Polarization rotation effects in atomic sodium vapor are studied both experimentally and theoretically. Two pulsed dye laser beams propagate collinearly through a cell containing sodium vapor. The direction of polarization of a linearly polarized dye laser beam of frequency ${\ensuremath{\omega}}_{1}$ is found to be rotated by a circularly polarized beam of frequency ${\ensuremath{\omega}}_{2}$. This rotation is due to the difference in refractive index at ${\ensuremath{\omega}}_{1}$ for right and left circularly polarized light which is produced by the circularly polarized beam. When ${\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$ is tuned near the $3S\ensuremath{-}5S$ two-photon transition of atomic sodium, this difference is primarily due to the dispersion associated with the two-photon transition. Other sources of polarization rotation which were identified include (i) real transfers of population to excited atomic levels due to collisionally induced transitions and (ii) optically induced atomic energy-level shifts. The theory includes both the effects of two-photon dispersion and optically induced energy shifts. The application of the two-photon polarization rotation effect as a fast, optically controlled shutter or modulator and as a sensitive detection technique for two-photon spectroscopy are briefly discussed.