Study On Optimization And Quasi-Phase-Matching In Generation Of High-Order Harmonics And Attosecond Pulses

The laser is a great invention of the20th century which is following the atomic energy, computers and semiconductors. It provides an unprecedented new type of light source which has great direction, high brightness, pure color and large energy. The application of laser technology has greatly expanded man’s vision and promoted the development of productivity.Since the appearance of the first ruby laser in1960, the wonderful ultrafast, high-energy physical world has stimulated people to continue to pursue a shorter and stronger laser. The history of the development of laser technology is also a laser pulse compression history and laser energy enhancement history. Technological progress of more than50years has compressed the time width of the laser pulse to a few femtoseconds (10-15s) which is enough to track the ultrafast dynamics of atoms in a molecule; has enhanced the power density to the magnitude of1021W/cm2which is enough to inspire extreme nonlinear physical phenomena. Up to now, the commercial miniaturization femtosecond laser systems have become widespread. However, attoseconds (1018s) laser systems exist only in the most advanced laboratories in the world because they demand harsh technology supports. The technical challenge of the next generation is show to people, that is the introduction of stable and cheap attosecond laser systems.High-order harmonics are catching more and more attention as light sources of coherent extreme ultraviolet (XUV) light which could support attosecond pulses. In recent years, the use of commercial lasers instead of special ultrashort lasers to generate high-order harmonics and attosecond pulses has made great progress (such as the polarization-gate technology, the two-color-field technology and etc.). They reduced the threshold of the access of attosecond ultrashort pulses to some extent.In this dissertation, we propose some new improved schemes which are based on the traditional technology. We study how to use a combination of lasers to access to attosecond ultrashort pulses with high quality, and how to use a fiber to enhance the turning efficiency of high-order harmonics.We study the fast optimization problem for parameters in the model of the generation of attosecond pulses with a combination of lasers, give a clear objective function, introduce the simulated annealing algorithm and improve it, optimize multiple parameters of the laser system simultaneously, select the best value in the solution space quickly, improve efficiency of the experiments.We combine with the traditional ellipticity-time-gate technology and the two-color-field technology, use two commercial30-fs femtosecond lasers (the polarization angle between them is45degrees) to compose suitably. In this way, we destroy the periodicity of the driving field and generate isolated60-as pulses.We propose a new three-color ellipticity-gate scheme, introduce two control fields (the polarization angle between them is90degrees) to modulate the electric field and the ellipticity of the many-cycle50-fs driving field. In this way, we generate intense isolated100-as pulses with many-cycle laser fields.We use aperiodic-modulated hollow fiber to achieve multiple quasi-phase-matching for any selected high-order harmonics, enhance the turning efficiency.