Negative nonlinear susceptibility of cesium vapor around 1.06 μm

We outline a complete theory of the nonlinear susceptibility of cesium around 1.06 μm, and present the first measurements of the negative nonlinear refractive index $n_2$ primarily responsible for the self-defocusing that is observed. For linearly polarized light, our measured value of $n_2$ is $-(1.4\mathrm{}\pm 0.2)\times {10}^{-30\mathrm{}}N$ esu, where $N$ is the atomic density. This is in reasonable agreement with our calculated value of $-2.62\mathrm{}\times {10}^{-30\mathrm{}}N$. The main portion of $n_2$ 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, $n_2$ arises mainly from the induced population changes, giving the measured and calculated values of $-(0.26\mathrm{}\pm 0.03)\times {10}^{-30\mathrm{}}N$ and $-0.525\mathrm{}\times {10}^{-30\mathrm{}}N$, 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.