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Journal of Physics Conference Series

Numerical simulation of wall relaxation in Rb vapor optically pumped with 21Ne

D. A. Dolgopyatov, Навасардян Екатерина Сергеевна, И. А. Архаров

Published Jan 30, 2015• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/584/1/012027

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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.
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.
The applicability of commonly used 129Xe nuclear polarization (PXe) buildup models to large optical cell polarizers, which operate at lower gas pressures when compared to small cell polarizers for which spin-exchange optical pumping (SEOP) models were originally developed, has yet to be systematically evaluated. In addition, spatial heterogeneity of Rb vapor density ([Rb]) and its impact on SEOP in low pressure, large optical cell polarizers remains poorly understood. In this theoretical study, it was shown that 1D analytical and numerical PXe buildup models yield diverging PXe and critical flow rate values at low flow rates and high laser absorption, contributing to the discrepancy between theoretical and experimentally measured production rates. Additionally, it was found that [Rb] heterogeneity leads to lower PXe buildup and an undesirable increase in optimal cell temperature, while the optimal laser absorption remains relatively unchanged. The updated simulation framework presented here can be utilized to model other SEOP systems.
We have experimentally and theoretically investigate the effect of optical pump beam size on narrowband spin polarized optical pumping of alkali atom vapor. We developed a six-level model describing narrowband spin polarized optical pumping of alkali atoms and applied this model to an atomic vapor cell, accounting for atomic transit time through the optical beam and wall collision effects. Our expression for the wall collision effects is derived from the diffusion model. Overall, we obtained good agreement between theory and experimentally measured relaxation time constants.
We have investigated high-density Rb optical pumping in polarized 3He^{3}\mathrm{He}3He targets and present measurements of collisional relaxation rates and studies of diffusion-driven wall relaxation of the Rb polarization. We show that a boundary layer resides in the vicinity of the cell walls within which a polarization gradient is established by diffusion. Absorption of the incident laser radiation within this layer leads to a striking decrease in optical-pumping efficiency for resonant light but has little effect for off-resonant light. Our theory introduces polarization-dependent slowing of optical pumping rates due to the Rb nuclear spin. We have observed a strong frequency dependence of the optical-pumping efficiency, consistent with our theoretical predictions. We report the following rate constants for collisional relaxation of Rb polarization: due to Rb (${\mathit{k}}_{\mathrm{Rb}\mathrm{\ensuremath{-}}\mathrm{Rb}}$=8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm3{\mathrm{cm}}^{3}cm3/s), due to N2{\mathrm{N}}_{2}N2​ (${\mathit{k}}_{\mathrm{N}2\mathrm{\ensuremath{-}}}$Rb=8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ cm3{\mathrm{cm}}^{3}cm3/s), and due to 3He^{3}\mathrm{He}3He, an upper limit of (k3{\mathit{k}}^{3}k3He-Rb\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ cm3{\mathrm{cm}}^{3}cm3/s). The data and model presented are used to predict laser power and intensity requirements for high-density polarized 3He^{3}\mathrm{He}3He targets.
We present a detailed experimental analysis of Rb-polarization imaging in high-pressure gas cells. The Rb vapor in these cells is optically pumped by high-power diode-laser arrays. We present images for high (35 G) and low (4 G) magnetic fields and for different He and Xe buffer-gas mixtures. We demonstrate that high-field imaging provides an absolute measurement of the Rb-polarization distribution in the cell, based on the fact that a spin-temperature distribution of the hyperfine magnetic sublevels is established in high-pressure buffer gases. A survey of various mechanisms that broaden the Rb magnetic-resonance lines is presented. These broadening mechanisms determine the limits of the spatial resolution achievable for images of the Rb-polarization distribution.
We present detailed images of the Rb polarization in a high-pressure optical pumping cell. Images taken at high magnetic field strengths (≈37 G) yield precise information on the relative occupation probability of the hyperfine Zeeman sublevels. Low-field (⩽4 G) images yield very high resolution (less than 1 mm) maps of the spatial distribution of the Rb polarization. For optical pumping in the presence of high-pressure buffer gases, the measured populations of the hyperfine magnetic sublevels are shown to be well described by a spin temperature, even though the Rb–Rb spin exchange rate is much smaller than the rates of optical pumping or electron-randomizing Rb–Xe collisions in the buffer gas.