Polarization Effect On Critical Power And Luminescence In Ultrashort Laser Filamentation In Air

Laser-induced filamentation is a unique nonlinear phenomenon that has attracted a lot of interests since the first experimental observation in 1995.Femtosecond laser filamentation can be attributed to a dynamic interplay between Kerr self-focusing and defocusing of the plasma generated by multiphoton/tunneling ionization during the propagation of laser beam in optical media.Due to the unique properties such as ultrahigh clamping intensity,femtosecond filament has shown many potential applications including remote sensing,combustion diagnosis,THz radiation generation and so forth.However,this nonlinear process is strongly dependent on the external experimental conditions such as the initial beam radius,optical media,laser pulse duration,the polarization states of the driver laser.For example,it has been experimentally demonstrated that the critical power changes as either the pulse duration or the optical medium changes.Especially,although it is commonly accepted that the critical power should be higher for the self-focusing of a circularly polarized light than that of a linearly polarized light,this conjecture has not been experimentally convinced yet.On the other hand,it was demonstrated that femtosecond filament induced nonlinear fluorescence in optical media is also influenced by the polarization states of the driver laser.Based on the above discussions,in this thesis,we mainly focused our attention on the polarization effect on critical power and luminescence in an air filament,and discuss the underlying mechanisms of filament-induced nitrogen molecular fluorescence in air.The main content of this thesis is as follows:1.We experimentally measure the critical power of different polarized lasers in air by observing the shift of the laser focal position as a function of the driver laser energy.Based on the measured critical powers,the second order nonlinear index coefficients of the air for the used linear and circular polarization pulses are determined.Our results show that when the laser polarization changes from linear to circular,the critical power for self-focusing of a Ti:Sapphire laser?800 nm,40 fs?in air increases from about 9.6 GW to 14.9 GW,while the second nonlinear refractive index of air decreases from n₂=9.9×10^-20 cm²/W to n₂=6.4×10^-20 cm²/W.Our results agree well with the theoretical analysis.2.We investigate the variation of the luminescence signals of nitrogen molecules produced during the filamentation of different polarized pulses in air under a single filament condition.Our results indicate that the neutral and ionic nitrogen molecules induced by linear and circular polarized pulses show different behaviors.For the three input energies?0.7 mJ,1.2 m J,1.8 mJ?that are all higher than the critical power,the laser pulse with linear polarization are always more efficient to induce the luminescence from the B²?_u⁺-X²?_g⁺ transition of N₂⁺at 391nm.While for the luminescence from the C³?_u–B³?_g transition of N₂ at 337 nm,there exists a reversal in the relative intensity between the linear and circular polarizations as the laser energy increases.These results demonstrate that femtosecond filament induced nonlinear fluorescence is not only rely on the clamping intensity but also influenced by the plasma density inside the filament.