Experimental and theoretical study of spontaneous spin polarization and hysteresis in cesium vapor

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=3⃗F_e=4 transition, the spin angular momentum is gained from the linearly polarized pump light, while on the 3⃗3 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.