Resonant noninear refraction of 4-5 $\mu$m light in CO and CO$_2$ gas

TL;DR

This study demonstrates resonant nonlinear refraction in CO and CO$_2$ gases at mid-infrared wavelengths, revealing intensity-dependent sign reversals and rapid response times, explained by a density matrix model of a driven two-level system.

Contribution

It provides the first experimental observation of resonant nonlinear refraction in these gases at 4-5 μm and introduces a theoretical model explaining the phenomena.

Findings

Nonlinear refraction exhibits sign reversals at high intensities.

Response time of the nonlinearity is less than 4 ns.

Theoretical model qualitatively matches experimental results.

Abstract

The resonant nonlinear refraction of 4-5 $\mu$m light in CO and CO$_2$ gas at a peak intensity of 15 MW/cm$^2$ was demonstrated using time- and frequency-resolved measurements of self-focusing and self-defocusing. The nonlinearity of these molecular gases exhibits intensity-dependent sign reversals and a < 4 ns response time. A change from self-focusing to self-defocusing or vice-versa was observed to occur for Rabi frequencies that are comparable to the collisional linewidth. A density matrix model for the nonlinear susceptibility of a strongly driven two-level system provides a qualitative explanation for these results.