Filament-induced ultrafast birefringence in gases

Femtosecond laser filaments generated by ultrashort laser pulses induce a remarkable birefringence over the whole filament length. Any linearly polarized femtosecond laser pulse probing this filament will be decomposed into two orthogonal polarization components propagating at different speeds resulting in an elliptical polarization. The latter in turn reflects the birefringence inside the filament zone. The filament-induced birefringence could be due to instantaneous electronic or delayed molecular response depending on different types of gases. In this paper, we review recent progress on filament-induced ultrafast birefringence in gases.