Dynamics Of Nitrogen Molecules And Molecular Nitrogen Ions During The Filamentation Of Femtosecond Laser Pulses

The propagation of a high-power femtosecond laser pulse in gas induces a filament plasma because of the balance between nonlinear Kerr effect and plasma defocusing.The laser intensity inside the plasma is clamped at 10¹³-10¹⁴ W/cm~2,where the interaction between ultrafast intense lasers and matter transfers from the traditional perturbation regime to nonperturbation regime.Much interest has been devoted to the researches of kinetic energy distribution of free electrons and luminescence to character the plasma.The forward laserlike radiations from molecular nitrogen ions were found as new effects from filaments in 2011,and have attracted wide attention in the past ten years.The cavity-less amplification effects have distinct features of narrow bandwidth,high brightness,and coherence.Taking the atmospheric molecules as the gain medium,the radiations show significant potential applications in remote monitoring supported by the long-distance transmission of the femtosecond lasers.However,the special time evolution of the lasing effects from molecular nitrogen ions makes the mechanism behind hard to understand for a long time.This thesis studies the dynamics of nitrogen molecules and molecular nitrogen ions inside filaments by the experimental measurement and numerical simulation.To explore the physical mechanism,the side luminescence and forward emissions of the plasma are measured through the pump-probe scheme,and the theoretical calculations of relevant interaction processes are performed based on tunnel ionization and optical Bloch equations.The main contents are summarized below:1.The distribution of free-electron energy under different polarized femtosecond lasers is investigated.The simulation shows that the majority of electrons own high energy for circularly polarized laser,while most electrons are left with energy around0 e V in the case of linear polarization laser.The experimental measurements show that the intensity of the luminescence from N₂(C~3(?)_u) to N₂(B~3(?)_g)) at 380 nm under circularly polarized laser field is stronger than that under linearly polarized laser field.Our work verifies that electron impact excitation is an efficient channel to generate N₂(C~3(?)_u) with circularly polarized laser pulses by the combination of the simulated and experimental results.2.The population information of both the excited and ground states of molecular nitrogen ions are simultaneously acquired by using a pump-probe fluorescence-measurement.The nitrogen molecules are populated to N₂⁺(X~2?_g⁺) and N₂⁺(B~2?_u⁺) by the 800-nm pump pulse.The 391-nm fluorescence from the transition N₂⁺(B~2?_u⁺)?N₂⁺(X~2?_g⁺) is enhanced after the interaction between the molecular nitrogen ions and the weak 400-nm probe pulse.We find that the 400-nm probe laser transfers the ions from N₂⁺(X~2?_g⁺) to N₂⁺(B~2?_u⁺)efficiently with solving the near-resonant interaction between the probe pulse and the two-level system.The relative population difference can be related to the enhancement factor of the fluorescence.The population distribution of both excited and ground state as a function of pump energy are extracted.Compared the experimental result with the calculated one by PPT model,they agree well at low pump energy,but diverge at high pump energy.The divergence is explained by multi-state couplings of molecular nitrogen ions induced by the tail of the 800-nm pump pulse with high energy.3.The superradiant nature of the 391-nm forward“lasing”emission in molecular nitrogen ions is confirmed.The time evolution of energy and polarization of the two-level system of N₂⁺(X~2?_g⁺) and N₂⁺(B~2?_u⁺)is investigated in ultrafast coherent laser fields by solving the optical Bloch equations.The correlation of the molecular ions is established by the ultrafast coherent laser fields,and the radiation becomes superradiance.When the population of N₂⁺(X~2?_g⁺) equals to that of N₂⁺(B~2?_u⁺),the correlation reaches its maximum and the strongest supperadiance is achieved.An analytical formula for the temporal evolution of supperadiance intensity is given,and the difference and boundary between superradiance and stimulated emission are figured out.By changing the experimental conditions,the good agreements between experimental results and theoretical calculations on the time profile,time delay,characteristic duration,radiated peak intensity and total emitted energy of the 391-nm forward“lasing”emission in molecular nitrogen ions reveal that the emission is superradiance.4.The mechanism of the disappearance of 391-nm forward“lasing”emission at high nitrogen pressure is studied.The 391-nm forward emission vanishes when the nitrogen pressure is higher than a certain value.Based on the balance formula of refractive index inside filament,we reproduce the clamped effect of laser intensity under self-focusing condition,and further obtain the clamped intensity as a function of nitrogen pressure with the external focusing.The clamped intensity is no more independent but decreases with the gas pressure,causing the failure of the population inversion inside the molecular nitrogen ions.Hence,the 391-nm forward emission disappears at high nitrogen gas pressure.