Studies On Coherent Control Of Few- Or Sub-cycle Pulses And Its Numerical Calculation

The interaction of high-intense, few-cycle laser pulses with matter is one of the most important frontiers in modern physics. Theoretically, there is the breakdown for the traditional perturbative theories, and new non-perturbative theories have been emerged. Technically, the recent developments in laser technology have made it possible to produce high-intense laser pulses with few optical cycles, and it lacks the essence of wave. A series of new physical phenomena open up the newest frontier of the extremely nonlinear interactions, and much attention has been focused on.For few or sub-cycle laser pulses, the traditional approximation and calculation methods for the case of many-cycle pulses are almost breakdown, and what's more the analytical solution is impossible. So, it is necessary to establish an efficient and steady numerical calculation method with high accuracy and numerical dispersion free. In this thesis, we are focused on the construction of practical numerical calculation method for few and sub-cycle laser pulses. High-harmonic generation and attosecond soft X-ray pulse in the hollow fiber, the propagation properties of few-cycle laser pulses in multilevel systems, the dynamics of sub-cycle laser pulses in the two-level systems, and the optimal coherent control in multilevel systems are investigated. This thesis includes the following innovative results:1. High harmonic generation from Ar and He atom by few-cycle laser pulse in the periodic and chirped hollow fibers is investigated theoretically by self-consistent model. Based on enhanced high harmonics in the periodic hollow fiber, a chirped hollow fiber is proposed to improve quasi-phase matching for the generated harmonics near the cutoff. The results show that the extended and enhanced harmonics near the cutoff are well phase- matched, and a single x-ray pulse with the duration of 279 attosecond and wavelength of 8.69 nm in Ar gas, and 255 attosecond and wavelength of 4.66nm in He gas can be achieved by frequency synthesizing of high harmonics in the well-selected cutoff bandwidth. Theresults show that it is potential candidate to generate intense isolated attosecond pulse in the "water window" spectrum by this technique.2. A high-order and without numerical dispersion FDTD method was established by moving window technique, and it is steady and efficient for few and sub-cycle laser pulses. The influences of ionization on the propagation and spectrum of a few-cycle circularly (elliptically) polarized laser pulse in an open two-level medium (two-level plus continuum model of multiphoton ionization) was investigated based on FDTD method. Here, the propagation dynamics of an arbitrary elliptically polarized laser pulse may boil down to that of a right and left circularly polarized laser pulse. When the laser intensity is high enough to cause ionization, its propagation behavior and corresponding property is different from the closed model, and it is different for the different pulse duration.3. The dynamics for the interaction of a sub-cycle laser pulse and a two-level system was investigated. The results indicate that there is significant blue shift in the carrier frequency of a sub-cycle laser pulse, and the fewer cycle laser pulses result in more blue shift. The carrier envelope phase has an important role in the dynamics for the interaction of the sub-cycle laser pulse and the two-level systems, and RWA appears invalid for the sub-cycle laser pulse based on analytical and numerical analysis.4. In order to improve the convergence of genetic algorithms for multi-variable problems, a search region retraction method has been introduced. The population transfer in three-level Λ system is simulated numerically and optimized in this paper. Almost complete population transfer from |1> to |3> is achieved by genetic algorithm while the population in state |2> reached minimum over the entire evolution at the same time. The results is shown that the optimal pulse sequence is the well-known Stimulated Raman adiabatic passage (STIRAP) scheme, the detuning Δ_P,Δ_S of pump pulse and Stocks pulse with opposite sign and chirps with the same sign is in favor of the complete and robust population transfer. Rabi frequency Ω_p and Ω_s have insensitive effects on the population transfer during a large scope of their ratio, so do the phases. In addition, the software for experiments was developed.