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Physical Review A

Effect of radiation trapping on the polarization of an optically pumped alkali-metal vapor in a weak magnetic field

D. Tupa, L. W. Anderson

Published Sep 1, 1987• 51 citations
Publisher

Abstract

Calculations are presented on the effect of radiation trapping on the electron-spin polarization that can be produced by optical pumping of an alkali-metal vapor in a weak magnetic field. Radiation trapping severely limits the polarization that can be obtained in a weak magnetic field.
DOI
10.1103/physreva.36.2142

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Calculations are presented of the limitations imposed by radiation trapping on the electron spin polarization produced in an alkali-metal vapor by optical pumping in a large magnetic field. It is found that electron spin polarizations of 90% are possible with Na densities up to 1019{10}^{19}1019 atoms/m3{\mathrm{m}}^{3}m3 and ground-level relaxation times of 150 \ensuremath{\mu}s in a large magnetic field using a cylindrical geometry of radius 7.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ m.
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.
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.
A vapor of paramagnetic atoms, under conditions in which electronic spin exchange takes place faster than collisional spin relaxation, should acquire a spontaneous spin polarization when illuminated by unpolarized light tuned to excite only certain ground-state atomic hyperfine levels. In magnetic fields above a certain size, the atomic polarization has two stable states, either parallel or antiparallel to the field. Experiments to demonstrate this effect and explore possible applications may be feasible by the use of alkali-metal vapors.
We report an unusual phenomenon, peculiar sign reversals of the ground-state magnetic resonances and of the zero-dip resonance (Zeeman resonance at zero field) of optically pumped, alkali-metal vapors. These anomalies occur when a weak circularly polarized D1D1D1 laser light is tuned to pump atoms predominantly from the lower ground-state hyperfine multiplet. One can understand the signal reversals in a simple, semiquantitative way with reference to the spin-temperature distribution. Quantitative computer simulations are in excellent agreement with observations.