Nitrogen fluorescence emission induced by femtosecond vortex beams in air

Abstract

Abstract Fluorescence emission is a typical nonlinear phenomenon occurring during the femtosecond laser filamentation in air. The characteristics of nitrogen fluorescence emission induced by femtosecond vortex beams in air are experimentally studied. It is found that the topological charge has a great influence on the nitrogen fluorescence emission induced by a femtosecond filament: the fluorescence signals from nitrogen molecules (e.g., 337, 357 and 380 nm spectral lines) and molecular nitrogen ions (e.g., 391 and 428 nm spectral lines) differ much in the intensity distribution along the propagation direction. The discrepancy is explained by revisiting the generation mechanisms for nitrogen fluorescence, and it is found that the intersystem crossing scheme can account for it, indicating that the intersystem crossing scheme plays the main role in the formation of N 2 ( C 3 ∏ u + ) . This work is helpful to the understanding of the physical mechanism of fluorescence emission induced by the filamentation of femtosecond vortex beams in air.