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Physical Review Letters

Observation of Spontaneous Spin Polarization in an Optically Pumped Cesium Vapor

W. M. Klipstein, S. K. Lamoreaux, E. Norval Fortson

Published Mar 25, 1996• 21 citations
Publisher

Abstract

We have observed spontaneous atomic spin polarization of a Cs vapor undergoing rapid spin exchange in a cell when illuminated with plane-polarized 894 nm ( $D$1) light. The spin polarization shows marked hysteresis in switching between two stable orientations, fully aligned parallel or antiparallel to an external magnetic field, as the circular polarization of the pump light is varied about zero. We show that our observations agree with theoretical expectations and point out possible applications of spontaneous polarization.

DOI
10.1103/physrevlett.76.2266

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We have observed spontaneous atomic spin polarization of a Cs vapor undergoing rapid spin exchange in a cell when illuminated with plane-polarized 894 nm ( DDD1) light. The spin polarization shows marked hysteresis in switching between two stable orientations, fully aligned parallel or antiparallel to an external magnetic field, as the circular polarization of the pump light is varied about zero. We show that our observations agree with theoretical expectations and point out possible applications of spontaneous polarization.
A vapor of paramagnetic atoms, under conditions in which electronic spin exchange takes place faster than collisional spin relaxation, should acquire a spontaneous spin polarization when illuminated by unpolarized light tuned to excite only certain ground-state atomic hyperfine levels. In magnetic fields above a certain size, the atomic polarization has two stable states, either parallel or antiparallel to the field. Experiments to demonstrate this effect and explore possible applications may be feasible by the use of alkali-metal vapors.
We report results of measurements and theoretical modeling of spontaneous spin polarization and associated hysteresis in an optically pumped Cs vapor. We observe hysteresis over the full range (20--450 torr) of N2{\mathrm{N}}_{2}N2​ buffer gas pressures tested, and on two hyperfine components of the D1D1D1 transition, so long as both electronic spin exchange and optical pumping of a resolved ground state hyperfine level take place faster than spin relaxation. On the $F=\stackrel{\ensuremath{\rightarrow}}{3}{F}_{e}=4$ transition, the spin angular momentum is gained from the linearly polarized pump light, while on the $\stackrel{\ensuremath{\rightarrow}}{3}3$ transition, it comes instead from the buffer gas during collisional quenching. The density-matrix theoretical model we have developed includes the effects of coherence in the excited state. It reproduces all observed features of spontaneous polarization, including hysteresis curves in quantitative agreement with the data across a broad range of experimental parameters.
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.
We present the results of an experimental study of the anomalous anisotropy of alignment signals in cesium vapors under strong spin-exchange conditions near zero magnetic field with linearly polarized optical pumping. We show that the anisotropy of the Hanle resonances in the plane perpendicular to the pump beam increases with concentration: in one direction the widths remain broadened by spin-exchange, whereas in the other they approach the spin-exchange relaxation free limit. With a further increase in concentration, additional nonlinear effects arise, such as signal amplification, bistability, hysteresis, and memory. To explain these effects we construct a illustrative theoretical model incorporating spontaneous polarization effects under strong spin exchange conditions. The model qualitatively shows that the ultra-narrow alignment resonances may originate from quadrupole anisotropy arising from the projection of spontaneous transverse orientation onto the detection axis. The unique properties of these resonances, such as their extremely small width and magnetic field-controlled bistability with a long-term memory effect, make them promising for use in quantum sensing and information.
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.
The effect of optically driven spin precession discovered by Bell and Bloom [W. E. Bell and A. L. Bloom, Phys. Rev. Lett. 6, 280 (1961)] is widely used nowadays as a basis for numerous experiments in fundamental physics and for diverse applications. In this paper we consider a much less popular version of the light-induced spin precession that does not imply coherent precession of the spin-system magnetization and is excited by linearly (rather than circularly) polarized light. Pump-probe measurements performed on the D2{D}_{2}D2​ line of cesium vapor show that the magnitude of the signal of the optically driven spin-alignment precession, in ``vacuum'' cells (with no buffer gas) is close to that of classical spin-orientation precession. In the presence of buffer gas, however, the signal of spin-alignment precession appears to be strongly suppressed. The discovered effect is ascribed to spin mixing of excited states of cesium atoms in the cycle of optical pumping.