Loss of Polarization in Collapsing Beams

TL;DR

This paper investigates the phenomenon of 'loss of polarization' in collapsing elliptically-polarized laser beams, showing both theoretical and experimental evidence that small power fluctuations lead to randomization of polarization after propagation.

Contribution

It introduces and demonstrates the 'loss of polarization' phenomenon, highlighting its dependence on input power fluctuations and dispersion regime, with experimental validation across different media.

Findings

Polarization angle becomes uniformly distributed after sufficient propagation.

Loss of polarization is more prominent in the anomalous GVD regime.

Significant polarization fluctuations occur beyond the critical power for self-focusing.

Abstract

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, 2$\pi$] from shot-to-shot. We term this novel 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.