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arXiv:2305.00761

Effect of depolarizing and quenching collisions on contrast of the coherent population trapping resonance

Kirill M. Sabakar, Maria I. Vaskovskaya, Dmitry S. Chuchelov, Eugene A. Tsygankov, Vitali V. Vassiliev, Sergei A. Zibrov, Vladimir L Velichansky

Published May 1, 2023• 0 citations
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Abstract

We investigate the effect of buffer gases on the coherent population trapping resonance induced by a $σ$-polarized optical field in $^{87}$Rb atoms. Our experimental results show that inert gases, which depolarize the excited state of the alkali-metal atoms, provide higher contrast than nitrogen that effectively quenches their fluorescence. We also demonstrate that elimination of the spontaneous radiation does not significantly decrease the width at moderate temperatures of an atomic medium. Therefore, a mixture of inert gases can be preferable over a mixture with nitrogen for atomic clocks.

DOI
10.48550/arxiv.2305.00761

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We investigate the effect of buffer gases on the coherent population trapping resonance induced by a σσσ-polarized optical field in 87^{87}87Rb atoms. Our experimental results show that inert gases, which depolarize the excited state of the alkali-metal atoms, provide higher contrast than nitrogen that effectively quenches their fluorescence. We also demonstrate that elimination of the spontaneous radiation does not significantly decrease the width at moderate temperatures of an atomic medium. Therefore, a mixture of inert gases can be preferable over a mixture with nitrogen for atomic clocks.
We investigate the effect of buffer gases on the coherent population trapping resonance induced by a $\ensuremath{\sigma}$-polarized optical field in 87Rb{}^{87}\mathrm{Rb}87Rb atoms. Our experimental results show that inert gases, which depolarize the excited state of alkali-metal atoms, provide higher contrast than nitrogen that effectively quenches their fluorescence. We also demonstrate that elimination of spontaneous radiation does not significantly decrease the width at moderate temperatures of an atomic medium. Therefore, a mixture of inert gases can be preferable over a mixture with nitrogen for atomic clocks.
Molecular nitrogen is often used as a buffer gas in cells with alkali metals due to its known ability to quench resonant fluorescence. It is widely believed that the suppression of spontaneous emission decreases the width of coherent population trapping resonance. However, our recent results have not confirmed this positive action of molecular nitrogen within the typical range of 87Rb concentrations and buffer gas pressures. On the opposite, we have observed the negative influence of quenching, the decrease in contrast of the coherent population trapping resonance in σ+{{\sigma }^{ + }}σ+ – σ+{{\sigma }^{ + }}σ+ configuration. In this work, we further confirm these results implementing the Ramsey spectroscopy, and compare the characteristics of the central fringe in nitrogen and neon, and show that the latter provides a significantly better contrast-to-width ratio.
We present the results of an experimental study of Coherent Population Trapping (CPT) in potassium, obtained by means of modulation of laser light amplitude with kHz frequency. The radiation from an external cavity diode laser, matching the D1 line of K, is modulated by an acousto-optical modulator. In the cell containing buffer gas, the CPT resonance width is reduced more than three orders of magnitude as compared to the cell containing pure potassium vapor. In K this resonance narrowing occurs with high resonance contrast; such behavior is not observed in buffered cells containing Rb or Cs, where the optical pumping to the non-interacting with the light ground level is very effective and depletes the population of the working ground Zeeman sublevels. The narrow CPT resonance of reduced fluorescence transforms to the one of enhanced fluorescence with the cell temperature rising. The transformed resonance exhibits higher contrast and lower width than those of the reduced fluorescence resonance. Hence, beside its scientific importance the resonance sign reversal can be used for the improvement of the CPT resonance parameters.
We report an experimental study of an all-optical three-photon-absorption resonance (known as an ``NNN resonance'') and discuss its potential application as an alternative to atomic clocks based on coherent population trapping. We present measurements of the NNN-resonance contrast, width and light shift for the D1{D}_{1}D1​ line of 87Rb^{87}\mathrm{Rb}87Rb with varying buffer gases, and find good agreement with an analytical model of this resonance. The results suggest that NNN resonances are promising for atomic clock applications.
An upper limit of ${10}^{\ensuremath{-}2}$ A˚2{\mathrm{\AA{}}}^{2}A˚2 is obtained for the quenching cross section of sodium resonance fluorescence when the excited sodium atoms are in collision with inert-gas atoms in their ground state. Excitation of sodium vapor at a pressure of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ Torr is followed by observation of the decay of the fluorescence using the method of delayed coincidences. The decay time of the fluorescence is unaffected by the presence of the inert gases He, Ne, A, Kr, and Xe at pressures up to 800 Torr, and at temperatures of 130 and 210\ifmmode^\circ\else\textdegree\fi{}C.
Optical pumping of an optically thick atomic vapor typically requires a quenching buffer gas, such as N2, to prevent radiation trapping of unpolarized photons which would depolarize the atoms. We show that optical pumping of a trace contamination of Rb present in K metal results in a 4.5 times higher polarization of K than direct optical pumping of K in the absence of N2. Such spin-exchange polarization transfer from optically thin species is useful in a variety of areas, including spin-polarized nuclear scattering targets and electron beams, quantum-nondemolition spin measurements, and ultrasensitive magnetometry.