Optical bistability from three-level atoms with the use of a coherent nonlinear mechanism

We report on studies on optical bistability from coherently driven $\Lambda$-type three-level atoms in a Fabry-Perot resonator. The nonlinear mechanism relies on transverse optical pumping and is due to population trapping in a coherent superposition of the ground-state sublevels. If, under appropriate conditions, a zero-field level-crossing resonance (Hanle resonance) occurs in the atomic system, the optical device is shown to display dispersive or absorptive bistability. Our theoretical studies are based on numerical solutions of the corresponding three-level Bloch equations in the presence of optical feedback. Experiments are performed on the $D_1$ line in a sodium-filled Fabry-Perot resonator using transverse optical Zeeman pumping of the ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ ground state. The measurements are found to be in fair agreement with the theoretical predictions from our simple model.