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We directly measure the nonlinear optical response in argon and nitrogen in athin gas target to laser intensities near the ionization threshold. Noinstantaneous negative nonlinear refractive index is observed, nor issaturation, in contrast with a previous measurement [Loriot et al., Opt.Express v. 17, 13429 (2009)] and calculations [Br\'ee et al., Phys. Rev. Lett.v. 106, 183902 (2011)]. In addition, we are able to cleanly separate theinstantaneous and rotational components of the nonlinear response in nitrogen.In both Ar and N$_2$, we observe the peak instantaneous index response scalelinearly with the laser intensity until the point of ionization, whereupon itturns abruptly negative and ~constant, consistent with plasma generation.

Optical nonlinearity in Ar and N$_2$ near the ionization threshold