Nitrogen Fluorescence Emission Induced By Femtosecond Vortex Beams In Air

As the femtosecond laser pulse propagates in air,a self-induced plasma channel is generated due to the Kerr self-focusing effect and plasma defocusing effect.Femtosecond laser filaments in the air can radiate fluorescence.By analyzing and measuring the fluorescence spectrum induced by femtosecond laser filamentation,we can gain insight into the dynamics of atoms and molecules in the air.Vortex beams have a helical wave-front and a phase singularity at the pulse center,and each photon in vortex beams carries an OAM.The OAM carried by the beam will be transferred to the excited molecules and molecular ions generated during the filamentation in air.It will increase the complexity of the filamentation process,and inevitably affect the accompanied fluorescence emission.However,there are few studies on the fluorescence emission induced by the filamentation of femtosecond vortex beams.In this paper,we experimentally study the variation of characteristic nitrogen fluorescence signals induced by femtosecond vortex beams in air with the propagation distance.In theory,we firstly introduce the physical process of the femtosecond laser filamentation,including the mechanism of filament formation,nonlinear phenomena in filamention.Then the vortex beam is described from three aspects:beam type,generation and measurement.In experiments,we study the effect of topological charge on the nitrogen fluorescence emission induced by femtosecond vortex beams in air by controlling the spatial light modulator to change the topological charge,and the fluorescence signals of spectral lines with different wavelengths are recorded by the spectrometer along with the propagation distance.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.Based on the experimental results,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 in the formation ofN₂（C³?_u⁺）,the intersystem crossing scheme plays a leading role.This work is helpful to the understanding of the physical mechanism of fluorescence emission induced by the filamentation of femtosecond vortex beams in air.And it lays a foundation for the study of spin-orbit interaction between light and matter.