The Experimental Investigation Of High-order Harmonic Generation Using Ellipticity-modulated Laser Pulse

The ultrafast evolution processes of the electronic quantum states in atoms,molecules and solids are involved in the picosecond,femtosecond to sub-femtosecond time scale.High harmonic generation?HHG?from atoms or molecules is a highly nonlinear process.HHG provides high frequency light source extended into the extreme ultraviolet?XUV??10-100 eV?or even the soft x-ray?100-200 eV?energy region.HHG is an essential technique to develop attosecond science.Using the ellipticity dependence of HHG on laser field combining with polarization gating technology could generate isolated attosecond pulses.The mechanism of semi-classical three-step model can explain that the HHG yield decreases as ellipticity increases.In this thesis,we use the pump-probe technique combining with self-made flat field grating spectrometer and measure the HHG yields with ellipticity dependence produced by 800nm femtosecond laser pulse with gasous sample.We present a comparison between the ellipticity dependence of high harmonic generation in argon and nitrogen gas.It is found that the ellipticity dependence of harmonic yield from the atomic and molecuar on the ellipticity of the driving field is different.The dependence of the molecule on the ellipticity of the driven laser is weaker than that of the atom.This phenomenon is due to the fact that the effective cross-section for recombination is larger in the molecule than in the atom.And we investigate the ellipticity dependence of high-order harmonics generated from aligned N₂ and CO₂ molecules.The experimental results show that the dependence of harmonic intensity from aligned N₂ and CO₂ molecules on the ellipticity of the driving laser field is sensitive to the molecular alignment and the structures of valence orbitals.Since the highest occupied orbital?HOMO?of N₂ is a?_g orbital and the highest occupied orbital?HOMO?of the CO₂ is a?_g orbital.The ellipticity dependence of high-order harmonic generated from aligned N₂is obviously different from the aligned CO₂ molecules.The tendency that??for the perpendicular configuration is larger than??for the parallel configuration is observed for all the harmonics higher than the 21st harmonics generated from N₂ molecules(??_perp>??_para).For CO₂molecules,the tendency that??for the perpendicular configuration is smaller than??for the parallel configuration is observed for all the harmonics higher than the 13th harmonics(??_perp<??_para).CO₂molecules are affected by the destructive interference during the HHG process.There is a competitive relationship between the destructive interference and the effective photorecombination cross section.Ellipticity-dependent studies in CO₂ clearly show that increasing the degree of ellipticity can remove the destructive interference.