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Physical Review A

Glass-wool study of laser-induced spin currents en route to hyperpolarized Cs salt

Kiyoshi Ishikawa

Published Jul 7, 2011• 5 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.84.013403

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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 interaction of spin polarized cesium atoms on coated surfaces is examined. Coated surfaces find extensive applications in atomic devices-hydrogen masers and advanced rubidium frequency standards utilize wall coatings to obtain very narrow hyperfine resonances. A Cs atomic beam is hyperfine or spin polarized by optical pumping techniques using a single mode AlGaAs diode laser. The ground state distribution is probed before and after scattering from coated surfaces using laser induced fluorescence spectroscopy. The coatings under investigations are paraffin and organosilanes and these are known to be very weakly relaxing surfaces for polarized alkali atoms. The spin relaxation properties of paraffin and organosilane coatings have been studied in glass cell-vapor environment. In our experiments using polarized atomic beams, our goal is to examine in detail the hyperfine and spin relaxation properties of these coatings in a carefully controlled environment which is not possible in the gas cell environment. We measure the spin relaxation probability per wall collision as a function of temperature of the surface. Results of our studies will be presented.
The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. 115 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.
The polarizations of sodium atoms optically pumped by a single-mode cw ring dye laser were measured as a function of the pumping power, sodium-vapor thickness, and external magnetic field for Pyrex glass and copper walls with and without dry-film coating. Observed quantities were analyzed by a simple optical pumping model in which the effect of the radiation trapping was taken into account. Assuming that the depolarization of the sodium atom was mainly induced by the local magnetic field at the sodium atom during its adsorption on the wall surface, the calculations could successfully reproduce the observed polarizations.
A new optical pumping scheme utilizing selective reflection is proposed and studied for the first time. This method enables us to observe the spin-polarization of atoms even at the high density, at which the pumping beam cannot be transmitted through a sample cell. Theoretical analysis shows that the light reflectivity at the glass-vapor interface is strongly dependent on the direction of the macroscopic magnetic moment of optical pumped atoms. Two types of demonstrating experiments are carried out with Cs and Rb vapors at the temperature ∼ 200°C. In the transverse pumping experiment with a modulated laser beam, asymmetry of the magnetic resonance signal with respect to the inversion of the magnetic field direction is observed as predicted by the theory. In the longitudinal pumping experiment, the transfer of a large amount of the magnetic moment by spin-exchange collisions is directly observed.