| The high-order harmonic generation(HHG)by the interaction of an intense ultrafast femtosecond laser and the gas medium can provide with a new light source,whose wavelength range covers the extreme ultraviolet(XUV)to X-ray.It has been used as an ideal table-top light source because of its good temporal and spatial coherence.Compared to the traditional synchrotron radiation and the free-electron laser,the HHG has become an important tool for the ultrafast time-resolved pump-detection spectroscopy and the detection of electron dynamics.In this thesis,we theoretically study the genetic optimization of the two-color laser waveform in the frequency domain to extend the cut-off energy of the single-atom HHG and the generating of the isolated attosecond pulse(IAP)by considering the effects of macroscopic propagation.This includes the following items.First,we study the optimization of two-color laser waveform by using the genetic algorithm.With the “temporal gate” formed by the optimized two-color chirped laser pulse in the frequency domain,the extension of the HHG cut-off energy is achieved compared to the single-color 800-nm laser with the same input energy.By considering the macroscopic propagation effects,the contribution of long-trajectory electron in the HHG emission burst is suppressed,leading to the reduction of its duration,thus the IAP with the duration of about 200 as is generated in the X-rays.Second,we study the phase-matching factors in the macroscopic propagation process in HHG.By changing the macroscopic parameters,the HHG by the two-color optimized laser pulse can be phase matched and its conversion efficiency can be enhanced.Finally,we optimize the two-color laser waveform with longer wavelengths to extend the HHG cut-off energy into even higher photon energy region.The simulation results show that although the long-wavelength lasers can effectively extend the HHG cut-off energy,the overall high-order harmonic yields are decreased. |
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