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/${\mathrm{cm}}^2$. From the data we deduce a value for the nonlinear susceptibility coefficient $c_{1221}$(-ω,ω,ω,-ω) for air at 20 °C and 1.053 μm of (20±5)×${10}^{\mathrm{-}19}$ esu, corresponding to a nonlinear index $n_2$=1.0×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^3$ ${\mathrm{erg}}^{\mathrm{-}1}$. This value is consistent with the value inferred from low-power Kerr and harmonic-generation measurements in ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$, 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.