Comparative study of the optical nonlinearity of atomic and molecular gases driven by an intense 10 µm CO₂ laser

Measurements of the optical nonlinearity of atomic and molecular gases are important for understanding light-matter interactions at high intensities. While the wavelength scaling of the nonlinear refractive index of these gases at visible and near-IR wavelengths has been studied extensively, there is a lack of measurements for the long-wave infrared(LWIR) wavelengths that are far from electronic resonance. In this range, rovibrational Raman transitions may introduce a resonant contribution to the optical nonlinearity of molecules. Further, LWIR radiation at intensities up to 10¹² W/cm² may modify the electronic nonlinearity of these gases while still remaining below the tunnel or multi-photon ionization threshold. We have experimentally studied the four-wave mixing of picosecond, 10 µm, CO₂ laser pulses at an intensity up to 10¹⁰ W/cm² in a gas-filled cell. Here we report the first measurements of the effective nonlinear refractive index of the diatomic molecules N₂ and O₂ and the noble gases Kr and Xe at 10 µm. We have observed that the molecular nonlinearity dominates the effective nonlinear refractive index.