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Journal of the Optical Society of America B

Coherent population trapping in optically thin 133Cs atomic vapor in a finite-size cell

К. А. Баранцев, Sergey V. Bozhokin, A. S. Kuraptsev, A. N. Litvinov, I. M. Sokolov

Published Mar 23, 2021• 13 citations

Abstract

We study the peculiarities of the coherent population trapping (CPT) dark resonance in atomic vapor confined in a finite-size cell. Taking into account the scattering of atoms on anti-relaxation coating of the walls, we analyze the line shape of the CPT resonance in 133 C s depending on various parameters, such as the width of laser radiation spectrum, the type of pumping, and the magnitude of static magnetic field for both lin || lin and l i n ⊥ l i n polarization schemes. We show that for different parameters, the dependence of the line shape on the size of a cell is different, i.e., different factors affect the CPT resonance nonadditively. Thus, in the case when CPT resonance is excited by a narrowband laser, the shape of the resonance weakly depends on the size of a cell due to the laser-induced narrowing effect. We also show that the distortion of the line shape of the CPT resonance caused by nonresonant hyperfine sublevels of atomic excited states is modified with the size of a cell. Our calculation reveals the important role of accounting for the hyperfine and Zeeman structure of atomic levels. We have shown that the contrast of the CPT resonance differs about 5 times from the prediction of the simple three-level Λ model. The obtained results can be used for the optimization of the quantum frequency standards based on the CPT effect. In the final part of the paper, on the basis of the shot noise approximation we estimate the short-term stability of such standards.

DOI: 10.1364/josab.412513

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We study the peculiarities of the coherent population trapping (CPT) dark resonance in atomic vapor confined in a finite-size cell. Taking into account the scattering of atoms on anti-relaxation coating of the walls, we analyze the line shape of the CPT resonance in 133 C s depending on various parameters, such as the width of laser radiation spectrum, the type of pumping, and the magnitude of static magnetic field for both lin || lin and l i n ⊥ l i n polarization schemes. We show that for different parameters, the dependence of the line shape on the size of a cell is different, i.e., different factors affect the CPT resonance nonadditively. Thus, in the case when CPT resonance is excited by a narrowband laser, the shape of the resonance weakly depends on the size of a cell due to the laser-induced narrowing effect. We also show that the distortion of the line shape of the CPT resonance caused by nonresonant hyperfine sublevels of atomic excited states is modified with the size of a cell. Our calculation reveals the important role of accounting for the hyperfine and Zeeman structure of atomic levels. We have shown that the contrast of the CPT resonance differs about 5 times from the prediction of the simple three-level Λ model. The obtained results can be used for the optimization of the quantum frequency standards based on the CPT effect. In the final part of the paper, on the basis of the shot noise approximation we estimate the short-term stability of such standards.

We examine experimentally and theoretically the Ramsey effects on the coherent population trapping (CPT) resonance in a paraffin-coated Rb vapor cell. The observed CPT spectrum in the Hanle configuration of the 5S 1/2(F = 2)−5P 1/2(F′ = 1) transition of 87Rb atoms consists of a narrow structure with a spectral width of 0.10 mG due to the wall-induced Ramsey effect and a broad structure with a spectral width of 23 mG due to the transit effect. Considering the velocity distribution of atoms and the number of wall-atom collisions, we numerically calculated the transmittance of the laser beam as a function of the longitudinal magnetic field. The experimental results of the CPT Ramsey interference were in good agreement with the theoretical results, and the dependences of the CPT Ramsey effects on the atom-laser interaction time were investigated in detail.

We present a detailed study of coherent population trapping (CPT) in the Hanle configuration in sodium atoms confined inside a glass cell and irradiated with a broadband multimode dye laser light resonant with the

{D}_{1}

line. The presence of several modes spaced by tens of MHz is effective in inhibiting hyperfine optical pumping, thus reducing losses and allowing for efficient trapping. A narrow resonance, whose width is of the order of 1 mG, is observed superimposed on the electromagnetically induced transparency (EIT) resonance. Such a structure is more than two orders of magnitude narrower than the broader one, with significantly reduced power broadening. In order to describe this phenomenon, we developed a theoretical model, based on numerical solutions of density matrix equations, which takes into account the peculiarity of CPT induced by broadband multimode laser light. Results are shown to be in very good agreement with the experimental data. In this framework, the small width and the small amount of power broadening exhibited by this narrow spectral structure are related to the time spent by the atoms to re-enter the laser beam volume after collisions that preserve the orientation. This study offers insight into the mechanisms of atoms depolarization caused by collisions with the cell walls and represents a starting point for the development of techniques for the diagnostics of cell-coating parameters and new applications in metrology and magnetometry.

The fast development of CPT applications and the need of good magnetooptical sensors result in an increased interest in the Coherent Population Trapping (CPT) resonances and the processes that determine their shape. In this work the shape and width of the CPT resonances are investigated in two different paraffin-coated Rb vapor cells from point of view of understanding the processes influencing the shape of the resonances and building of miniature and sensitive detector. The dependence of the shape of the resonances on the laser power is measured. Narrow resonances on three hyperfine transitions of the D1 87Rb line are registered. For explanation of the bright structure in the resonance shapes at low laser powers analysis of the influence of different processes is made.

According to the Λ -type three-level system consisting of cesium 6S1/2 (F=3 and F=4) long-lived ground states and 6P3/2 (F'=4) excited state, we experimentally investigate and theoretically analyze the parameters of coherent population trapping (CPT) spectra in a cesium atomic vapor cell with or without neon as buffer gas. The CPT of the full width at half maximum (FWHM), the signal amplitude as a function of optical intensity of the two phased-locked laser beams with a frequency difference of 9.19263177 GHz (the hyperfine splitting in the cesium ground state 6S1/2), relative intensity ratio, beam spot size, residual magnetic field of the magnetic shielding, and the influence of with and without the cesium vapor cell containing neon as buffer gas are investigated experimentally. With the optimized parameters, we obtain the CPT signal FWHM to be as narrow as ～ 340 Hz.