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/cm²in 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.

This material is based upon work supported by the AFOSR under Award Number FA9550-16-1-0139 DEF, the ONR MURI (4-442521-JC-22891) and by the U.S. D.O.E. Grant DE-SC001006.