Origin of the Nonlinear Refractive Index of Liquid C<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cl</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We report here the first determination for a simple liquid (specifically liquid C${\mathrm{Cl}}_{4}$) of the fraction of its Kerr effect that arises from the (nearly instantaneous) nonlinear response of its electronic currents, and hence would exist even if the nuclei were frozen in position. To do this, we have remeasured the power dependence of the rotation of the polarization ellipse of a monochromatic beam in C${\mathrm{Cl}}_{4}$ with greatly improved accuracy (\ifmmode\pm\else\textpm\fi{} 10% absolute) using a single-Gaussian-mode ruby (giant pulse) laser. We then compare the results of this ellipse rotation measurement with existing Kerr data, and, using a general relation between the relative electronic contributions to both effects which we demonstrate, we show that (54 \ifmmode\pm\else\textpm\fi{} 16)% of the Kerr effect in C${\mathrm{Cl}}_{4}$ arises from nonlinear electronic response. The method should be useful for any isotropic material.

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