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

TL;DR

This study measures the nonlinear optical response of argon and nitrogen near ionization threshold, revealing plasma effects and separating different nonlinear components, contrasting previous findings of negative nonlinear index or saturation.

Contribution

The paper provides direct measurements of nonlinear optical responses in gases near ionization threshold, clarifying the behavior of the index and separating instantaneous and rotational effects.

Findings

No negative nonlinear index observed near ionization threshold.

Instantaneous index response scales linearly with intensity until ionization.

Index response becomes negative and constant after ionization, indicating plasma formation.

Abstract

We directly measure the nonlinear optical response in argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. No instantaneous negative nonlinear refractive index is observed, nor is saturation, 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 the instantaneous and rotational components of the nonlinear response in nitrogen. In both Ar and N$_2$, we observe the peak instantaneous index response scale linearly with the laser intensity until the point of ionization, whereupon it turns abruptly negative and ~constant, consistent with plasma generation.