Loss of polarization of elliptically polarized collapsing beams

We show theoretically and demonstrate experimentally that collapsing elliptically polarized laser beams experience a nonlinear ellipse rotation that is highly sensitive to small fluctuations in the input power. For arbitrarily small fluctuations in the input power and after a sufficiently large propagation distance, the polarization angle becomes uniformly distributed in [0,2π] from shot to shot. We term this phenomenon loss of polarization. We perform experiments in fused-silica glass, nitrogen gas, and water and observe a significant increase in the fluctuations of the output polarization angle for elliptically polarized femtosecond pulses as the power is increased beyond the critical power for self-focusing. We also show numerically and confirm experimentally that this effect is more prominent in the anomalous group-velocity dispersion (GVD) regime compared to the normal-GVD regime due to the extended lengths of the filaments for the former. Such effects could play an important role in intense-field light-matter interactions in which elliptically polarized pulses are utilized.