Propagation Of Femtosecond Strong Laser Pulse In Atomic Field-ionizing Media And Its Kinetics

The interaction of an intense femtosecond laser pulse with atoms and moleculesgives rise to a variety of nonlinear phenomena, including above-threshold ioniza-tion, the generation of high-order harmonics (HHG), etc. In this thesis, the prop-agation of an ultraintense femtosecond laser pulse through a feld-ionized medium,the laser energy transfer, the plateau extension of the frequency spectrum of HHGand the generalization of Keldysh adiabatic parameter are investigated. The mainachievements are as follows:1. By bringing the Coulomb potential into the laser propagation through a feld-ionized media, we investigate the role of the Coulomb efect plays in the en-ergy conversion processes. The results shows that the abstraction of Coulombpotential alters the local density distribution of ionized electrons, and a newmechanism of laser loss called laser-assisted inverse bremsstrahlung absorp-tion is introducing into the laser propagation equation.2. A recombination rate of electron-ion in the strong-feld atomic process is phe-nomenologically introduced into the ionization rate equation, and therefroman ionization and recombination rates equation (IRRE) is obtained. By usingthe extended IRRE, the propagation equation of an intense femtosecond laserpulse in the gaseous medium is re-derived.3. A manipulatable half-cycle light pulse, homochromy with the driving fem-tosecond laser, is applied to enhance the harmonic emission by optimizingthe delay time between the driving and manipulating light pulses in thestrong-feld atomic process. We fnd that, by numerical computation, thehalf-cycle manipulating light can signifcantly increase the atomic ionizationrate supplementarily and enhance the return kinetic energy of the electrons by a suitable phase delay. The results show that the harmonic order of themaximum or cut-of photon energy emitted in the presence of the half-cyclemanipulating light pulse is risen to the177th which is a signifcant increasecompared with the53rd order harmonics in the case of single driving lightpulse.4. By accounting for the Coulomb potential, we present a generalized version,which can divide the interaction into three domains that have evident bound-aries. The results show that the generalized parameter is equivalent to thetraditional Keldysh parameter in the weak feld limit.The corresponding computer programs are compiled using the FORTRAN languageand the data are processed by MATLAB.