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IEEE Journal of Quantum Electronics

An Investigation on Radiation Trapping and Quenching in Optically Pumped Atomic Vapor Cell

Sheng Li, Jingbiao Chen, Hong Mei Guo

Published May 29, 2019• 1 citation
Publisher

Abstract

A model is presented to investigate the effects of quenching gas on atomic polarization in optically pumped atomic vapor cell. For Rb atomic vapor cell, the formula obtained by the present model can well describe the relation between the atomic polarization and the pressure of quenching gas nitrogen or hydrogen, and shows that if we keep the pumping rate and the quenching gas pressure unchanged, the atomic polarization tends to be one definite value as the atomic density increases. In addition, the fitting values of the pumping rate show certain dependance on radiation trapping and a model is presented to investigate it.

DOI
10.1109/jqe.2019.2919793

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A model is presented to investigate the effects of quenching gas on atomic polarization in optically pumped atomic vapor cell. For Rb atomic vapor cell, the formula obtained by the present model can well describe the relation between the atomic polarization and the pressure of quenching gas nitrogen or hydrogen, and shows that if we keep the pumping rate and the quenching gas pressure unchanged, the atomic polarization tends to be one definite value as the atomic density increases. In addition, the fitting values of the pumping rate show certain dependance on radiation trapping and a model is presented to investigate it.
We have made a systematic study of rubidium optical pumping in a simple cylindrical cell geometry with a high-power 10W10\phantom{\rule{0.3em}{0ex}}\mathrm{W}10W diode laser array, low magnetic fields, and buffer-gas pressures of less than 50torr50\phantom{\rule{0.3em}{0ex}}\text{torr}50torr. We have determined rubidium polarizations experimentally for H2{\mathrm{H}}_{2}H2​, N2{\mathrm{N}}_{2}N2​, He, and Ar buffer gases, with Rb number densities from ${10}^{12}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Comparison to a relatively simple optical pumping model allows us to extract useful information about radiation trapping and quenching effects.
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.
We simulated the optical pumping rate of 87Rb atoms with Xe and high-pressure buffer gas in a cubic cell. We calculated absorption cross-section as a function of pumping laser frequency by considering a pressure and Doppler broadening as gas temperature varies from 364 K to 414 K. In a steady state the spin polarization and the atomic population of 87Rb atoms were obtained by optical pumping in each ground state. We calculated the optimum beam waist, linewidth and detuning of pumping beam to achieve higher population of polarized atoms at cell parameters such as temperature, size. We simulated that the optimum beam waist is 0.3 times of cubic cell length at 250 Torr of di-nitrogen and 50 Torr of Xe gas pressure. The optimum laser beam linewidth parabolically increases with increasing the temperature in range of few ten kelvin and it exponentially decrease with increasing the cubic cell length. We also observed that few ten GHz detuned laser frequency increases the optical pumping rate. These numerical results show appropriate experimental parameter as a function of cell temperature and size to get maximum population of spin polarized atoms in given temperature range.
We report investigations of spin exchange between 21Ne^{21}\mathrm{Ne}21Ne nuclei and optically pumped Rb vapor. Polarization of 30% (〈Kz〉=0.46〈{K}_{z}〉=0.46〈Kz​〉=0.46) of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of N2{\mathrm{N}}_{2}N2​. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm3{\mathrm{cm}}^{3}cm3-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to 3He^{3}\mathrm{He}3He has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.
Spatial distribution of the polarization in Rb vapor optically pumped with 21 Ne has been modeled numerically with 40 mW laser beam power, 180°C cell temperature, 1 bar cell pressure, and 6 mm × 15 mm pancake-cell geometry. An inhomogeneity of polarization throughout the cell results from a wall relaxation of polarized media and a decay of the pumping beam. For small cells characteristic of experimental conditions, a contribution of wall relaxation to the total alkali relaxation rate can become significant, demanding more accurate evaluation than afforded by analytical model based on average parameters.
Calculations of the limitations imposed by radiation trapping on the polarization of an alkali-metal atomic beam by optical pumping are presented. In an atomic beam radiation trapping limits the density of alkali-metal atoms that can be highly polarized by optical pumping. This density is about 1/3 the density of an alkali-metal vapor that can be highly polarized by optical pumping. In a low magnetic field the maximum intensity of an atomic beam that can be optically pumped is a few particle milliamperes.