Studies On Femtosecond Laser Filamentation And The Associated Effects

The phenomenon of femtosecond laser filamentation in transparent nonlinear media,especially in air,and the associated nonlinear effects were experimentally investigated.When the intense femtosecond laser pulse propagates in air,the dynamical balance between Kerr self-focusing and plasma de-focusing induces the formation of long and intense plasma filaments.Much nonlinear effects accompany with the propagating of laser filaments,likes super-continuum,third harmonic,THz radiation and air lasing,etc.Therefore,studies on femtosecond laser filamentation and the associated effects would be helpful to understand the physics mechanisms and guide the potential application in lightning trigging,microwave-guide and remote sensing.Long plasma filaments were formed both in laboratory with length from several centimeters to tens of centimeters by pre-focusing,and at outside with length of tens of meters by free propagating.We adjusted the pulse energy,focal length and chirps to control the location and length of the femtosecond laser induced plasma filament.Multiple methods are available for the diagnosis of plasma filaments,including the traditional methods like shadow photography,transverse interferometry,quantitative fluorescence emission measurement and other characteristic methods like electrical conductivity measurement,THz scattering and microwave disrupting method,etc.In the experiments,we chose appropriate diagnostic method as necessary.Among these methods,the electromagnetic induction method was developed by us,based on the electrical conductivity measurement,combining to B-dot detection,for detecting the decay of plasma filament in a single shot.Based on these diagnostic methods,we investigated the effect of the carrier-envelope-phase of a few-cycle femtosecond pulse,it had a modulation of over 20%to the electron density of plasma filament;A homemade femtosecond pulse sequence laser was built and it's verified to had prolonged the lifetime of the quasi-steady plasma channel for tens of times;A TW femtosecond laser generated a long filament with over30 m length,its electron density was at 10¹¹/cm³ level.The diagnostic of plasma filament is aims at unveiling the formation,propagation and characteristics of filaments,while the modulation of filament is for further applications,for example,lightning triggering has nearly unreachable requirements to the length and lifetime of the femtosecond laser induced plasma filament.Remote sensing is another valuable application of femtosecond laser filamentation.We investigated the third harmonic generation,THz radiation and air lasing in the laser filamentation process.With two circularly polarized dichromatic pump beams,we obtained third harmonic generation with easily modulated intensity and polarization.These associated radiations can be generated from a remote femtosecond laser filamentation.Therefore,it's significantly to study the mechanism,the modulation of the intensity and polarization of these nonlinear radiations,for the remote sensing application.