Polarization rotation due to femtosecond filamentation in an atomic gas.

The linear-to-elliptical transformation of a 400 nm femtosecond-probe pulse in the birefringent filament in argon of an 800 nm linearly polarized femtosecond-pump pulse is studied numerically and experimentally. The rotation of the probe elliptical polarization is the largest in the high-intensity filament core. With propagation, the rotated radiation diffracts outward by the pump-produced plasma. The transmission of the analyzer crossing the probe's polarization is maximum at the pump-probe angle of 45 degrees and gives equal values for each pair of angles symmetrically situated at both sides of the maximum.