Nonlinear index of air at 1.053μm

We have measured absolutely the intensity-dependent change in the state of elliptical polarization of a well-characterized 1-ns laser beam propagating in 68 m of air, at intensities ranging from 0.5 to 1.5 GW/cm². From the data we deduce a value for the nonlinear susceptibility coefficient c₁₂₂₁(-{ω},{ω},{ω},-{ω}) for air at 20 ^∘{}C and 1.053 {μ}m of (20±{}5)×{}10^-19 esu, corresponding to a nonlinear index n₂=1.0×{}10^-16 cm³ erg^-1. This value is consistent with the value inferred from low-power Kerr and harmonic-generation measurements in O₂ and N₂, but it is approximately one half of a previously reported experimental value for air. We also replaced 58 m of the air path with argon gas at 1 atm pressure and verified that a substantial reduction in nonlinear beam refraction can thereby be achieved.