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Physical Review A

Effect of radiation trapping on the polarization of an optically pumped alkali-metal atomic beam

D. Peterson, L.W. Anderson

Published May 1, 1991• 13 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.43.4883

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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.
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.
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 variation of the usual method of atomic orientation of alkali metal vapors by optical pumping is proposed. This variation consists of employing a single circularly polarized D1{D}_{1}D1​ line as pumping radiation, instead of a mixture of the two DDD lines. Under these conditions, one magnetic substate of the ground state is transparent to the pumping radiation. It is shown that for a sufficiently intense light source and relatively weak relaxation, the entire population of the vapor will tend to be pumped into this single state, resulting in nearly one-hundred percent atomic orientation. The pumping process is analyzed in detail for sodium vapor illuminated with $3{P}_{\frac{1}{2}}\ensuremath{-}3{S}_{\frac{1}{2}}$ radiation, and the time dependence of the populations of the eight magnetic sublevels of the ground state is computed. It is shown that in the steady state, the light absorption by the vapor is linear rather than exponential with distance. A simple method of measuring relaxation times based on this observation is suggested. The conditions under which the imprisonment of resonance radiation limits the size of the oriented sample are discussed, and it is suggested that the proposed technique will allow the alignment of larger samples than is possible by the usual method of optical pumping.
In atomic vapor cells, atoms collide with the inner surface, causing their spin to randomize on the walls. This wall-depolarizing effect can be diffusive and it becomes more pronounced in smaller vapor cells under high temperatures. In this work we investigate the polarization of optically pumped alkali-metal atoms in a millimeter-sized cell heated to $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$. We consider two extreme boundary conditions, fully depolarizing and nondepolarizing boundaries, and we provide an analytical estimation of the polarization difference between them. In the nondepolarizing case, the pump beam's absorption is proportional to the average atomic polarization. However, for fully depolarizing walls, the absorption peak may correspond to a polarization minimum. To mitigate the wall effect, we propose reducing the pump beam's diameter while maintaining the pump power to prevent illumination of the cell wall and increase the pump intensity in the central area. This is crucial for compact vapor-cell devices where the laser frequency cannot be detuned since it is locked to the absorption peaks. Additionally, we analyze the wall-depolarizing effect on the performance of an alkali-metal atomic magnetometer operating in the spin-exchange relaxation-free regime. We show that the signal strength is highly limited by wall collisions and we provide an upper bound for it.