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 buffer gas pressures tested, and on two hyperfine components of the
D1 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.
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.
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.