Negative nonlinear susceptibility of cesium vapor around 1.06 μm

We outline a complete theory of the nonlinear susceptibility of cesium around 1.06 \ensuremath{\mu}m, 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 $\ensuremath{-}(1.4\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$ esu, where $N$ is the atomic density. This is in reasonable agreement with our calculated value of $\ensuremath{-}2.62\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$. 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 $\ensuremath{-}(0.26\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$ and $\ensuremath{-}0.525\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$, respectively. In our experiments where the 35-psec pulses were shorter than the $6s\ensuremath{-}6p$ 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.

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