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European Frequency and Time Forum

Optical beam size effects in spin polarized pumping

Xi Zeng, Christine Y. Wang, D. L. Boïko

Published Jun 1, 2014• 2 citations
Publisher

Abstract

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.

DOI
10.1109/eftf.2014.7331476

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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.
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.
Abstract In an optically pumped alkali vapor cell with a high density of atoms, the attenuation of the pump light generates a spatially non-uniform distribution of the electronic spin polarization of alkali atoms, which is detrimental to biomagnetism applications of magnetometers as well as the hyperpolarization of noble gas atoms. Therefore, in this study, we propose a new scheme to generate a nearly uniform, unsaturated spin polarization region based on counter-propagating pump beams and atomic diffusion. A finite element method-based simulation is used to demonstrate the three-dimensional distribution of the spin polarization in a spherical cell. The effects of cell temperature and pump light power on the homogeneity of the spin polarization are studied. The distribution of spin polarization near the center of the cell is experimentally measured and a 1 cm uniform spin polarization region is achieved in the center of the cell. The uniformity of spin polarization in the center region of the cell increased by 50% compared with single beam pumping. The advantage of our proposed scheme is that it can generate an unsaturated uniform region of spin polarization in the center of a cell using a single species of alkali atoms.
The standard model of Spin-Exchange Optical Pumping of 3 He has come under close scrutiny as researchers try to maximize the quantity and polarization of 3 He. This paper will review four important effects that have come to light: 1) pumping with frequency narrowed diode lasers significantly improves the polarization and production rate as compared to off-the-shelf broadband sources 2) Rb-K mixtures again improve the production rate due to reduced collisional loss in K-He collisions, 3) a unexplained 3 He wall relaxation mechanism that scales closely with alkali vapor pressure limits the attainable 3 He polarization to about 80%, 4) a small collisionally-induced breakdown in the selection rules for optical pumping explains why narrowband lasers are so advantageous, and may explain some observed limitations for Rb-K pumping as well.
The distribution of population in the ground-state sublevels of an optically pumped alkali-metal vapor has been found to be strongly dependent on the mechanism assumed responsible for relaxation. Two modes of optical pumping zero mixing and complete mixing in the excited state, and three modes of alkali relaxation, uniform, Zeeman, and electron randomization, are considered. Calculations of the sublevel relative populations are made in terms of experimentally measurable parameters for all six combinations of pumping and relaxation modes. Optical-pumping transient signals are then used to demonstrate that alkali relaxation due to wall collisions is of the low-frequency "Zeeman" variety, while relaxation due to alkali-rare-gas collisions takes place through randomization of the spin of the alkali valence electron. Equations for the rate of change of the electronic spin polarization are derived for all cases, and are found to be nonexponential, in contrast with the case of an alkali of zero nuclear spin.
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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.