The Lasing Emission During Femtosecond Laser Filamentation In Nitrogen

The propagation of an intense femtosecond laser beam in transparent media will experience filamentation when its peak power exceeds a critical value,due to the self-focuses of the optical Kerr effect and the defocusing effect of the plasma generated by the multiphoton ionization or tunneling ionization.Laser filament has been employed in the development of technologies for lightning guiding and suppression,remote sensing,combustion diagnostic,and so on.Recently,it is noticed that the coherent amplified lasing emission can be emitted in both forward and backward directions,which is quite universal in various gas medium.The lasing effect,especially the backward lasing,provides a route for high-precision atmospheric remote sensing in kilometer to 100s kilometer range,and lasing generation and basic characteristics have emerged as an attractive topic in recent yearsIn this thesis,we investigate lasing effect during the filamentation of 800nm laser pulse in the two-color laser field.With a center wavelength of 400nm,the spectral attenuation of probe pulse caused by the 800nm pump filament is studied with a relative high gas pressure in air.The far-field pattern,the energy loss of the 400 nm probe pulse and the fluorescence emissions of the interaction area are measured.It is the plasma diffraction effect caused by the pump filament and the energy loss of the 400nm pulse through near resonant absorption process result in the spectral attenuation of probe pulse.Then,with the center wavelength of the probe pulse set to be 354 nm,the vibrational dynamic of N2+ is investigated by measuring the spectral signals at 353.3 nm,353.8 nm,354.9 nm and 356.4 nm,corresponding to the transitions among the vibrational levelson the states B2?u+ and X2?g+ of N2+ ions.The dependences of these signals are displayed on the time delay between the probe and pump pules,the gas pressure,the relative position of the probe focus point on the pump filament and the polarization of the probe pulse.The coexistence of lasing emission and absorption at different wavelengths is found.The coupling effect among these vibrational levels on states B2?u+ and X2?g+ in strong laser filed can induce population inversion between specific levels.Meanwhile the absorption signal demonstrate that the population inversion will not happen to every vibrational level.The study of lasing effect from femtosecond laser filamentation is a fascinating topic,not only because of its potential applications in remote sensing techniques but also fruitful physics to discover.