Harmonic Enhancement By Combining XUV Pulse With Infrared Lasers

High-order harmonic generation(HHG)is a nonlinear process in which an intense infrared(IR)laser pulse interacts with gas medium.It has become an important tool for providing tabletop subfemtosecond or a few tens of attosecond light sources with great brightness and coherence from the extreme ultraviolet(XUV)to X-rays.However,the low conversion efficiency of HHG has limited its wide applications in science and technology.In this thesis,we investigate how to enhance the harmonic yields with the assistance of XUV attosecond pulse.Based on the strong-field approximation(SFA),we simulate the single-atom high harmonic spectra under different conditions.We compare the harmonic spectra obtained by the two-color laser waveform consisting of the 800-nm laser and its second or third harmonic field with those obtained by the single-color 800-nm laser.We also check the enhancement of harmonics generated by the 800-nm laser by adding the XUV pulse.The mechanism of XUV parametric amplification(XPA)indeed works for different atomic targets and different applied XUV wavelengths.We next propose a scheme of the two-color waveform combining with the XUV attosecond pulse to further enhance the harmonic yields.The two-color waveform can be modified by varying the relative phase between the two colors,and the intensities of individual colors.We thus investigate the effect of harmonic enhancement with the assistance of XUV pulse under a number of different conditions.Especially,we take a close look of the enhancement factor as a function of the time delay between the XUV pulse and IR lasers.Time-frequency analysis of harmonic emission is also shown to uncover the physical mechanism of harmonic enhancement.We find that the XPA in the suggested scheme is valid,in which the harmonic radiation is emitted by forward scattering of the XUV field from the electron wave function.This scheme can be used to improve the harmonic yields in the XUV or in the soft X-rays.We then study the effect of macroscopic propagation for high harmonics with an alternative approach of intensity averaging,which can be a substitute of solving the three-dimensional Maxwell's equations numerically if the gas medium is located after laser focus.Finally,we discuss possible ways of retrieving the information of attosecond pulses by using the modulated harmonic spectra as a function of time delay between the IR laser and XUV pulse.