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Physical Review A

Transfer of spin angular momentum from Cs vapor to nearby Cs salts through laser-induced spin currents

Kiyoshi Ishikawa, Brian Patton, Ben A. Olsen, Yuan‐Yu Jau, W. Happer

Published Jun 14, 2011• 14 citations
Publisher

Abstract

Optical pumping of alkali-metal atoms in vapor cells causes spin currents to flow to the cell walls where excess angular momentum accumulates in the wall nuclei. Experiments reported here indicate that the substantial enhancement of the nuclear-spin polarization of salts at the cell walls is primarily due to the nuclear-spin current, with a lesser contribution from the electron-spin current of the vapor.

DOI
10.1103/physreva.83.063410

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Over the four year period covered by this grant, we have carried out extensive research involving polarized atoms and nuclei. Using laser optical pumping, high polarizations can be produced in alkali-metal vapors. From this starting point, angular momentum can be transferred to other atoms and nuclei through several different types of spin-exchange collisions. This powerful technique enables us to study a wide variety of spin interactions and spin related phenomena. Among the systems we work with are polarized nobel gases, which display extremely long spin-relaxation times that vary from several minutes to many weeks. The use of optical pumping and spin exchange has grown enormously over the past four years.

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Optical pumping and spectroscopy of Cs vapor at high magnetic field

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We have measured changes in the ground-state populations of Cs vapor induced by optical pumping at high magnetic field. The $2.7$-T field of our experiments is strong enough to decouple the nuclear and electronic spins, allowing us to independently measure each population. The spatial dependence of the Cs populations in small amounts of buffer gas obeys a simple coupled diffusion model and the relative populations reveal the details of relaxation within the vapor cell. Optical pumping can produce high nuclear polarization in the Cs vapor due to perturbations of the hyperfine interaction during collisions with buffer-gas particles and depending on the pumping transition, radiation trapping can strongly influence the electronic and nuclear polarizations in the vapor.

Physical Review A•Dec 7, 2011•79 citations
Optical pumping of alkali-metal atoms in vapor cells causes spin currents to flow to the cell walls where excess angular momentum accumulates in the wall nuclei. Experiments reported here indicate that the substantial enhancement of the nuclear-spin polarization of salts at the cell walls is primarily due to the nuclear-spin current, with a lesser contribution from the electron-spin current of the vapor.
The nuclear spin polarization of optically pumped Cs atoms flows to the surface of Cs hydride in a vapor cell. A fine glass wool lightly coated with the salt helps greatly increase the surface area in contact with the pumped atoms and enhance the spin polarization of the salt nuclei. Even though the glass wool randomly scatters the pump light, the atomic vapor can be polarized with unpolarized light in a magnetic field. The measured enhancement in the salt enables study of the polarizations of light and atomic nuclei very near the salt surface.
The spin angular momentum accumulates in the Cs nuclei of salt on contact with optically pumped Cs vapor. The spin polarization in stable chloride as well as dissociative hydride indicates that nuclear dipole interaction works in spin transferring with a lesser role of atom exchange. In the solid film, not only the spin buildup but also the decay of enhanced polarization is faster than the thermal recovery rate for the bulk salt. Eliminating the signal of thick salt, we find that the nuclear spin polarization in the chloride film reaches over 100 times the thermal equilibrium.
The theory of spin exchange between optically pumped alkali-metal atoms and noble-gas nuclei is presented. Spin exchange with heavy noble gases is dominated by interactions in long-lived van der Waals molecules. The main spin interactions are assumed to be the spin-rotation interactions $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the rotational angular momentum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}$ of the alkali-metal---noble-gas pair and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ of the alkali-metal atom, and the contact hyperfine interaction $\ensuremath{\alpha}\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the nuclear spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ of the noble-gas atom and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$. Arbitrary values for KKK and for the nuclear spin III of the alkali-metal atom are assumed. Precise formal expressions for spin transfer coefficients are given along with convenient approximations based on a perturbation expansion in powers of ${(\frac{\ensuremath{\alpha}}{\ensuremath{\gamma}N})}^{2}$, a quantity which has been shown to be small by experiment.
Over the four year period covered by this grant, we have carried out extensive research involving polarized atoms and nuclei. Using laser optical pumping, high polarizations can be produced in alkali-metal vapors. From this starting point, angular momentum can be transferred to other atoms and nuclei through several different types of spin-exchange collisions. This powerful technique enables us to study a wide variety of spin interactions and spin related phenomena. Among the systems we work with are polarized nobel gases, which display extremely long spin-relaxation times that vary from several minutes to many weeks. The use of optical pumping and spin exchange has grown enormously over the past four years.
We have measured changes in the ground-state populations of Cs vapor induced by optical pumping at high magnetic field. The 2.72.72.7-T field of our experiments is strong enough to decouple the nuclear and electronic spins, allowing us to independently measure each population. The spatial dependence of the Cs populations in small amounts of buffer gas obeys a simple coupled diffusion model and the relative populations reveal the details of relaxation within the vapor cell. Optical pumping can produce high nuclear polarization in the Cs vapor due to perturbations of the hyperfine interaction during collisions with buffer-gas particles and depending on the pumping transition, radiation trapping can strongly influence the electronic and nuclear polarizations in the vapor.