High-order Harmonic Radiation Characteristics And Potential Applications Under Ultrashort Pulses

Due to potential applications as attosecond light sources,high-order harmonic generation(HHG)has attracted much interest in recent years.With the evelopment of laser technology,at present,ultrashort laser pulses with only a few optical cycles(o.c)can be obtained.In the third chapter of this paper,we study the HHG in ultrashort laser pulses,with focusing on the influence of pulse duration on HHG by exploring a wide laser-parameter region theoretically.Previous studies have shown that for high laser intensities close to the saturation ionization intensity,the HHG inversion efficiency is higher for shorter pulses since the ground-state depletion is weaker in shorter pulses.Our study shows that this high efficiency also appears for a moderate laser intensity at which the ionization is not very strong.A classical effect relating to shorter travel distances of the rescattering electron in shorter pulses is shown to contribute importantly to this high efficiency.At the same time,the effect can be amplified significantly as a two-color laser field is used,suggesting a potential approach in increasing the HHG efficiency and generating shorter and brighter attosecond pulses.In the fourth chapter of this paper,we study the HHG of asymmetric molecules in ultrashort laser pulses with different carrier-envelope phases(CEP).The calculated HHG spectra show a multiple plateau structure which is sensitive to the CEP.Our analyses show that the mechanism behind these phenomena is as follows.Firstly,due to the carrier-envelope phases(CEP)effectively determine the peak positions of the ultrashort pulse laser field,so field-dependent effects such as ionization and harmonic emission will be sensitive to the carrier-envelope phases.Secondly,because of the presence of permanent dipole,the change of laser phase will remarkably affect the ionization yield,and therefore affect the HHG which arises from the ionization of the electron.Thirdly,the classical effect in ultrashort pulses remarkably enhances the HHG yields associated with electrons returning in the fast falling part of the pulse.This will change the relative amplitudes of the electric rescattering orbits.The effects interplay in the HHG process and this interplay results in the phase dependence of multi-plateau structure of the HHG spectra.Molecular alignment has important applications in physics and chemistry.For polar molecules,besides the alignment,the orientation(arranging molecules in a head-to-tail order)is also needed.However,the degree of orientation of polar moleculars is difficult to measure in experiments.In the fifth chapter of this paper,we study the HHG of asymmetric molecules with a large permenant dipole,through numerical solution of the time-dependent Schrodinger equation.Through comparing the harmonic spectra at different laser phases(CEP),we find that there is important differences in the high energy part of the spectra.This phenomenon is closely related to the effect of the permanent dipole and is very sensitive to the degree of orientation of asymmetric molecules.A simple model is proposed to describe the HHG of asymmetric molecules with a large permanent dipole in ultrashort laser pulses.With this model,the relation between the HHG yields in the high energy part of the spectra and the degree of orientation is obtained.Using this relation,one can calibrate the degree of orientation indirectly through the measurement of the HHG yields in ultrashort laser pulses.Our simulations support the applicability of the calibrating approach.In summary,the classical effect in ultrashort laser pulses plays an important role in increasing the HHG yields,and the HHG of asymmetric molecules with a large permanent dipole in ultrashort laser pulses can be used to probe the degree of orientation.Our results give suggestions on relevant experiments.