Research On Macroscopic Propagation And Phase Matching Of High-order Harmonic Generation

High-order harmonic generation(HHG)is an extremely nonlinear process in the interaction of intense femtosecond laser field with atoms and molecules.Attosecond pulses generated based on HHG have the unprecedented time resolution and excellent spatial coherence,which provide a powerful tool for the ultrafast detection of microscopic dynamics and have greatly promoted the development of attosecond metrology.The HHG process includes the microscopic single-atom/molecule response as well as the macroscopic propagation in the gas media.The macroscopic propagation and phase matching play important roles in the spectral features of harmonics and the generation of attosecond pulses.Based on the traditional phase matching theory,we further develop the time-dependent phase-matching theory in this thesis.We have investigated the influence of the time dependence of phase matching on the spectral features of HHG.Moreover,we have also extended the macroscopic propagation model to the metal nanostructure system,and have studied the macroscopic effect of HHG in the metal nanostructure.The main content and innovation points of this thesis are as follows:(1)We theoretically investigate the effect of time-dependent phase matching on HHG.The results show that the phase-matching degrees of the short and long paths are different at the leading and falling edges of the driving laser pulse.By using the strong-field approximation model and solving the Maxwell equations,we find that the time-dependent phase matching of the short and long quantum paths will lead to the harmonic splitting and frequency shift in the HHG spectra.Furthermore,we systematically investigate the influence of the laser intensity,the laser duration,and the position of the gas medium on the time-dependent phase matching as well as the spatial and spectral properties of the HHG spectra.This study gives a deep insight and is beneficial for understanding the spatio-spectral features of HHG.(2)We investigate the influence of the time-dependent phase matching on broadband extreme ultraviolet(XUV)supercontinuum generation with an intense laser field.By solving Maxwell's equations,we show that the time-dependent phase matching can lead to the broadening of the XUV supercontinuum by manipulating the quantum path of HHG.Moreover,we also find that the bandwidth of the XUV supercontinuum can be controlled by adjusting the beam waist of the laser field and the position of the gas medium.This result provides a new way for generating XUV supercontinuum and ultrashort attosecond pulses.(3)We establish a macroscopic propagation model of HHG in metal nanostructures.Based on this model,we find that the macroscopic harmonic yields drop dramatically in the high-energy region.This result well interprets the disagreement in the cutoff between the single-atom prediction and the experimental detection.Moreover,we also find that the harmonic cutoff difference induced by a ?-shift in carrier-envelope phase(CEP)of laser pulses depends sensitively on the spatial position.However,when the collective effect of plasmon-driven HHG is considered,this cutoff difference is eliminated.This work provides a comprehensive understanding of HHG in metal nanostructures.(4)We present a plasmon-shaped polarization gating for high-order harmonic generation by using a linearly polarized laser field to illuminate two orthogonal bow-tie nanostructures.The results show that when these two bow-tie nanostructures have nonidentical geometrical sizes,the transverse and longitudinal components of the incident laser field will experience different phase responses,thus leading to a time-dependent ellipticity of laser field.For the polarizing angle of incident laser field in the range from 45° to 60°,the dominant harmonic emissionis gated within the few optical cycles where the laser ellipticity is below 0.3.Then sub-50-as isolated attosecond pulses(IAPs)can be generated.Such a plasmon-shaped polarization gating is robust for IAP generation against the variations of the carrier-envelope phases of the laser pulse.Moreover,by changing the geometrical size of one of the bow-tie nanostructures,the electron dynamics can be effectively controlled and the more efficient supercontinuum as well as IAP can be generated.