Radiation Mechanism Of MgO High-order Harmonic Generation Under Different Laser Field Intensity

The development of ultra-fast and ultra-intense laser makes it possible to study the interaction between laser and medium.Based on this,the scientists have discovered the high-order harmonics generation which is an extreme nonlinear effect.After several decades of research into high-order harmonic generation since it was first discovered in atomic medium,the mechanism of high-order harmonics generation from atoms have laid a solid foundation for further research.However,the high-order harmonic generation from solids is not detected until 2011.Subsequently,more attentions have been paid to the study of high-order harmonic generation in solids.The mechanism of high-order harmonic generation in solids can be described simply as follows:the electrons are excited to the conduction band by the laser field at the top of the valence band,and then move to the conduction band under the action of the laser field and obtain more energy.Finally,the electrons of conduction-band and holes of valence-band compound and radiate high-order harmonics.However,it has been found that the high-order harmonic generation differs in different laser conditions,so as corresponding radiation mechanism of high-order harmonics.Thus,we study the solid high-order harmonics generation from solid by changing the laser field conditions,which can help us to understand the spectrum structure of high-order harmonic in solid,and further control the generation of high-order harmonic in solid.In this paper,the mechanism of high-order harmonic generation in solids under different laser intensity is studied.The results are as follows:First,the relationship between the generation mechanism of high-order harmonic generation and the phase of the two-color field is investigated under different intensity of the two-color field.Previous studies have found two mechanisms for the generation of higher-order harmonics in solids.The first mechanism is that the reverse motion of electron and hole pairs comes from the reverse of electric field.The second mechanism is that the high-order harmonic radiation of the solid comes from the back scattering of electrons and hole pairs colliding with neighboring atoms,which makes the electrons and hole pairs move in the opposite direction.There’s a chance to re-compound the radiation to produce higher harmonics.In this thesis,we use the two-color field to detect the resonance radiation structure and the radiation time of high-order harmonic generation in different laser intensity.It is found that the variation of the relative phase of even-order harmonics in different fundamental frequency field intensity can detect the recombination time of electron-hole pair in different mechanism.Thus,The two-color field can be used to detect the time delay of Mg O high-order harmonic radiation under different radiation mechanism.Furthermore,it is found that the resonance radiation structure of high-order harmonic in solid state varies periodically under different intensity of fundamental frequency field.Second,the radiation mechanism of high-order harmonic in solid under the condition of lower laser field intensity is investigated.It is found that the mechanism of harmonic generation is changed at low laser intensity:the mechanism of low-order harmonic radiation is mainly based on multi-photon transition,and the energy of low-order harmonic radiation does not change with time.The mechanism of higher-order harmonic emission comes from the three-step model of valence-band electron transition at k₀≠0（initial velocity is not zero）under 0.6V/?laser field intensity.Due to the initial velocity,electrons and hole pairs may move to the boundary of Brillouin zone and combined to radiate higher-order harmonics.Under the condition of low intensity laser field,the Mg O high-order harmonic generation mechanism is transformed from multi-photon radiation to"three-step model"radiation.