Submitted For The Degree Of Doctor Of Philosophy

Generation and application of ultrafast high intensity laser pulses are one of the frontiers in laser technology development and laser physics. The shortest laser pulse generated by state-of-the-art laser technology has approached one optical-cycle. And the laser intensity at the focus is comparable or even much higher than relativistic intensity. The development of ultrafast laser technology makes the study of interaction between laser fields and matters come into nonperturbative and highly nonlinear domain and uncover a lot of new physical effects and phenomenon, one of which is the high harmonic generation. Generally, harmonic spectra generated by interaction between high-intensity laser field and atomic or molecular gases contain three parts. There is a sharp decline in conversion efficiency with orders in the lowest order harmonics, then followed by a broad plateau where the harmonic conversion efficiency varies weakly with orders. And the highest-order harmonics whose conversion efficiency decline rapidly to zero form a cutoff. Experimental studies show that the several-hundredth-order harmonic can be generated. For example, with 800nm wavelength driving laser pulse, harmonic photon energy higher than keV was observed.High-order harmonic generated in high-intensity laser field is a candidate for tunable, high-intensity, tabletop, femtosecond/attosecond coherent extreme-ultraviolet (XUV) source, which is currently one of the hottest research areas in high-intensity ultrafast laser physics. However, as a highly nonlinear process, high-order harmonic generation is intrinsically inefficient. How to push the high harmonics to much shorter wavelength and how to improve its energy conversion efficiency, and based on this point, how to generate isolated attosecond pulse with high energy, have become the hottest topic in current research.The main focus of this thesis is about improving the energy conversion efficiency of high-order harmonic generation, as well as the optimization control of the single attosecond pulse through the XUV supercontinuum generation in the cutoff region of high harmonics. The main contents are the following:1. We made detailed illustration on the theory of high-order harmonic generation and their propagation in the medium, and theoretically analyzed how to improve the high harmonic radiation from single atom. The main schemes for phase matching of high harmonics propagation in macroscopic media are discussed.2. For the first time, we optimized multiple parameters of two-color laser field simultaneously, and obtained broadened XUV supercontinuum in the cutoff region. As an example, we optimized the two-color laser field synthesized by an intense 5fs pulse at 800nm and a relatively weak, subharmonic pulse at 2400nm. Simulated annealing algorithm is applied to optimize the pulse energy, pulse width of the control 2400nm laser pulse, and the time delay between the two pulses. The width of supercontinuum in cutoff region was broadened to 115eV after optimization, nearly two times broader compared with simple superposition of two color fields, and eleven times broader than 10eV in single color field. The idea of simultaneous optimization of multiple parameters of two-color field can fully utilize the current laser facilities to broaden the XUV supercontinuum in cutoff region, thus support isolated attosecond pulse generation with much shorter pulse width.3. For the first time, we brought forward the idea of multiple quasi-phase matching of high harmonic generation, numerically demonstrated the harmonic propagation signal using a simplified model. Inner-diameter nonperiodically modulated hollow-core fiber can be designed by applying the simulated annealing algorithm. The phase mismatch of different high harmonics can be compensated by the multiple reciprocal vectors given by the modulated laser field, thus multiple quasi-phase matching of high harmonics can be implemented. The energy conversion efficiency of arbitrary HHG can be improved by designing different nonperiodically modulated hollow-core fibers. This idea provides a new viewpoint for attosecond pulse generation.4. Experimentally, we prepared the inner-diameter nonperiodically modulated hollow-core fibers using the method of laser micromachining, for the multiple quasi-phase matching of HHG experiment. And then we designed and machined a set of HHG experimental apparatus. In addition, we proposed several feasible experimental schemes for MQPM HHG experiments.