Nonlinear index of air at 1.053 microm.

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/cm/sup 2/. From the data we deduce a value for the nonlinear susceptibility coefficient c/sub 1221/(-..omega..,..omega..,..omega..,-..omega..) for air at 20 /sup 0/C and 1.053 ..mu..m of (20 +- 5) x 10/sup -19/ esu, corresponding to a nonlinear index n/sub 2/ = 1.0 x 10/sup -16/ cm/sup 3/ erg/sup -1/. This value is consistent with the value inferred from low-power Kerr and harmonic-generation measurements in O/sub 2/ and N/sub 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.