Negative nonlinear susceptibility of cesium vapor around 1.06 μm

We outline a complete theory of the nonlinear susceptibility of cesium around 1.06 {μ}m, and present the first measurements of the negative nonlinear refractive index n₂ primarily responsible for the self-defocusing that is observed. For linearly polarized light, our measured value of n₂ is -(1.4±0.2)×10^-30N esu, where N is the atomic density. This is in reasonable agreement with our calculated value of -2.62×10^-30N. The main portion of n₂ comes from a two-photon resonance between the $6s$ and $7s$ levels, and an additional negative term arises from induced population changes between $6s$ and $6p$. For circular polarization, n₂ arises mainly from the induced population changes, giving the measured and calculated values of -(0.26±0.03)×10^-30N and -0.525×10^-30N, respectively. In our experiments where the 35-psec pulses were shorter than the 6s-6p inverse linewidth, the nonlinear susceptibility depends mainly on the instantaneous intensity; however, for longer pulses, one would obtain additional contributions proportional to time integrals over the intensity. Since the useful output power from large Nd laser systems is limited by self-focusing due to the laser glass, our results suggest the possibility of increasing this power by using Cs vapor for compensation.