Generation And Optimizational Control Of High-order Harmonic

With the continuous development of the laser technology,the pulse duration is increasingly narrow.From the early continuous wave,nanosecond pulse,picosecond pulse,until now the femtosecond pulse,and the attosecond pulse in study, which marks the interaction between the laser and the material are always in the new areas.Due to that the ultrashort laser pulse,not only plays an important role in the physics research field(such as the tracking of atomic fast relaxation processes),but also has wide applications in other fields(such as the device detection of ultra-high-speed semiconductor,the tracking of chemical reaction process and the surgery,etc.).Therefore,the development of ultrashort pulse has great significance both for the scientific research and the application areas.At present,the attosecond pulse is mainly obtained from the high-order harmonic generation.The main focus of this thesis is about improve the energy conversion efficiency of high-order harmonic generation,as well as the optimizational control of the single attosecond pulse through the supercontinuum generation in the high-order harmonic plateau region.Mainly include the following:1.Experimentally investigate the phase-matching control of high-order harmonic generation.According to the principle that the electric field can effectively control the electron ionization,acceleration and the recombine with the nuclear core,the phase-matching condition of the high-order harmonic generation driven by intense few-cycle pulses could be controlled by adding second-harmonic pulses to change the ionization fraction of the gaseous medium.The harmonic generation efficiency could be improved by moving the phase-matching point with an all-optical control of the ionization fraction or a proper change of the confocal parameter.Such an all-optical,precise and real-time controlled new program can also be used for the optimization and control of the frequency comb in the extreme ultraviolet.2.Numerically demonstrate that an XUV supercontinuum in the high-order harmonic plateau region(SCP)can be generated by using a two-color laser field with an intense second or third harmonic pulse added to the fundamental-wave pulse.The generated supercontinuum can be controlled by changing the pulse durations,intensities and frequencies of the driving fundamental and harmonic waves as well as their initial relative phase delay.Additionally,the advantages and disadvantages of these two two-color laser fields have been discussed in detail,which provides a solid foundation for the practical application of SCP in the future.3.Numerically demonstrate that the XUV supercontinuum in the high-order harmonic plateau region could be further raised to the high-frequency region by shaping a fundamental-wave laser pulse with its second and third harmonic pulses. At the same time,the energy conversion efficiency of the supercontinuum can be greatly enhanced(about five orders of magnitude as compared with the supercontinuum generated in the cutoff region).This is a new and effective method for the practical application and optimizational control of the single attosecond pulse.4.We present experimental results that the chirped few-cycle laser pulse can produce a supercontinuum in the high-order harmonic plateau region effectively. The frequency chirp and phase-matching pressure together determine the SCP generation,intensity and spectral span.In addition,by adjusting the chirp of few-cycle femtosecond laser pulses,we can obtain a variety of different forms of high-order harmonic generation,including the supercontinuum,even order spectrum,interference spectrum,discrete spectrum and so on.The relationship between pulse chirp and various forms of high-order harmonic generation is confirmed according to the time-domain phase measurement.This provides a convenient and effective method for the practical application of the high-order harmonic generation.