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arXiv:1007.5486 · Physical Review A

Circular dichroism of RbHe and RbN 2 molecules

Brian Lancor, Earl Babcock, Robert Wyllie, Thad G Walker

Published Oct 27, 2010• 24 citations
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Abstract

We present measurements of the circular dichroism of optically pumped Rb vapor near the ${D}_{1}$ resonance line. Collisions with the buffer gases $^{3}\mathrm{He}$ and ${\mathrm{N}}_{2}$ reduce the transparency of the vapor, even when fully polarized. We use two methods to measure this effect, show that the He results can be understood from RbHe potential curves, and show how this effect conspires with the spectral profile of the optical pumping light to increase the laser power demands for optical pumping of very optically thick samples.

DOI
10.1103/physreva.82.043435

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We present measurements of the circular dichroism of optically pumped Rb vapor near the D1{D}_{1}D1​ resonance line. Collisions with the buffer gases 3He^{3}\mathrm{He}3He and N2{\mathrm{N}}_{2}N2​ reduce the transparency of the vapor, even when fully polarized. We use two methods to measure this effect, show that the He results can be understood from RbHe potential curves, and show how this effect conspires with the spectral profile of the optical pumping light to increase the laser power demands for optical pumping of very optically thick samples.
We have studied the optical pumping of mixtures of Rb vapor and N2{\mathrm{N}}_{2}N2​ buffer gas by laser light tuned to the D1{D}_{1}D1​ transition having a spectral width of  500~500 500 MHz. The Rb densities are of the order of 1013{10}^{13}1013 cm${}^{\ensuremath{-}3}$, while the buffer-gas pressures range from 0.1 to 10 torr. As the frequency of the right-hand circularly polarized laser is varied across the D1{D}_{1}D1​ absorption profile, the electron spin polarization of the Rb is found to take on negative values for small negative values of pump detuning from the absorption profile center. This occurs for N2{\mathrm{N}}_{2}N2​ pressures below  1~1 1 torr; at 10 torr the electron spins consistently point in the same direction as the angular momentum of the pump light. The spin-reversal effect can be understood in terms of populations of the F=2F=2F=2 (85Rb^{85}\mathrm{Rb}85Rb) and F=1F=1F=1 (87Rb^{87}\mathrm{Rb}87Rb) states caused by small unpolarized fractions in the pump beam and its elimination in terms of pressure broadening caused by the N2{\mathrm{N}}_{2}N2​ buffer gas. We speculate that this effect could be used for fast Rb spin modulation.
The use of high-power ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Alx{\mathrm{Al}}_{\mathit{x}}Alx​As diode laser arrays for optical pumping of Rb vapor has been examined both theoretically and experimentally. Our studies show that large atomic polarizations can be obtained using the broad spectral output of a diode laser array if the Rb resonance line is collisionally broadened by several atmospheres of a buffer gas. The objective was to study the effectiveness of diode laser arrays as light sources for optically pumped spin-exchange polarized 3He^{3}\mathrm{He}3He targets. The average Rb polarization was deduced at three different Rb vapor pressures using the polarization induced in the 3He^{3}\mathrm{He}3He buffer gas. The largest 3He^{3}\mathrm{He}3He polarization observed was 53% using a 20-W diode array with 1.9-nm full width at half maximum spectral width in 10.5 amagat of 3He^{3}\mathrm{He}3He. This result compares favorably with other light sources for this target. The measured average Rb polarizations are compared to theoretical calculations.
We present the transformation in the Hanle configuration of the transmission that results from coherent population trapping (CPT) into the level-crossing absorption (LCA) that results from the single-photon optical pumping in the 87Rb^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ line of a Rb vapor cell with a Ne buffer gas when the polarization of the laser field is changed from linear to circular. The LCA spectrum, with a narrow spectral width of 2.42.42.4 mG (1.71.71.7 kHz), was observed in the ${F}_{g}\ensuremath{\rightarrow}{F}_{e}\ensuremath{\leqslant}{F}_{g}$ transition with the circularly polarized laser. This may be because the LCA is both related to the transverse magnetic field and the atom-laser interaction time resulting from diffusive atomic motion in the cell with the buffer gas. The CPT and LCA spectra were calculated numerically using the full density matrix equations for the relevant magnetic sublevels of the hyperfine levels, considering the residual magnetic fields perpendicular to laser propagation and the collision effects resulting from the buffer gas. There was good qualitative agreement between theoretical and experimental results.
A cryogenic technique has been used to produce polarized 3He^{3}\mathrm{He}3He targets of up to relative density p=12 atm (\ensuremath{\approxeq}3\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 3He^{3}\mathrm{He}3He atoms/cm3{\mathrm{cm}}^{3}cm3; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets 3He^{3}\mathrm{He}3He nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790--800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\rightarrow}5P1/2{\mathit{P}}_{1/2}P1/2​ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus 3He^{3}\mathrm{He}3He pressure, which indicates the importance of Rb${\mathrm{\ensuremath{-}}}^{3}$He${\mathrm{\ensuremath{-}}}^{3}$He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, 3He^{3}\mathrm{He}3He pressure, light intensity, and cell geometry. The Rb${\mathrm{\ensuremath{-}}}^{3}$He spin-exchange cross section, 〈${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SE}}$v〉=6.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, was found to be independent of 3He^{3}\mathrm{He}3He pressure up to p=12.1 atm. Maximum 3He^{3}\mathrm{He}3He polarizations of 72--79% were observed with cells of 17 cm3{\mathrm{cm}}^{3}cm3 volume that contained 3He^{3}\mathrm{He}3He at p=6--9 atm.
A new method based on optical pumping is proposed for increasing the parameters of dark resonance excitation on D 2 line in 87 Rb vapor. The pumping is provided by linearly polarized two-frequency laser radiation propagating in the direction perpendicular to the probing field directed along the cell with atomic rubidium vapor. This method significantly improves the dark resonance parameters as compared to the case of pumping by a circularly polarized field. Qualitative considerations are confirmed by the results of numerical calculations.
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.