Theoretical Investigations On The Control Of Ultrafast Electron Dynamics In Solid High-order Harmonic Generation

With the development of ultra-short and ultra-strong laser pulse technology in the mid-infrared band,the solid high harmonic generation has gradually become a heated issue over the world.The wavelength range of the mid-infrared laser is 2-5 ?m.Com-pared with the band gap of the semi-conductor,the long wavelength and high intensity of the mid-infrared laser(MIR)make the ionization process to be in the tunneling regime when the MIR laser acts on the semi-conductor.The study of solid harmonics is very important:first,because the time scale of solid harmonic generation(HHG)process is on the order of sub-femtosecond or attosecond,therefore,it has ultra-short time resolu-tions.The solid high harmonic generation contains the physical information of ultrafast electron dynamics and lattice dynamics inside the solid.Some new physical ultrafast dynamics phenomena have been discovered and their laws need to be revealed.Sec-ond,the crystal target in solid HHG has natural orientated atomic arrangement and high atomic density.Thus it is expected to produce higher yield harmonics.The solid HHG is expected to become the carrier of bright high order harmonic and attosecond light sources.It has been found that many crystals can be used as target materials to generate solid HHG in experiments,such as wurtzite zinc oxide(w-ZnO),solid argon,magne-sium oxide,quartz(SiO2),sapphire(Al2O3),there are also two-dimensional materials like graphene,gallium selenide,molybdenum disulfide,and so on.These experimen-tal results reveal a series of new phenomena different from the HHG in gaseous atoms and molecules.For example,the HHG spectra of solid argon exhibits a double-plateau structure;the cutoff energy of the HHG spectra of w-ZnO crystals is linearly depen-dent on the amplitude of the driving laser field;the HHG spectra of graphene exhibits an unique ellipticity dependence,and so forth.In this thesis,we combine the semi-conductor Bloch equation combined with the classical model of solid HHG to study theoretically the control of ultrafast electronic dynamics during the nonlinear processes inside the crystal driven by strong ultrafast lasers.The main achievements are as follows:1.We have discovered the interference between the long and short quantum trajec-tories in interband transitions which can be used to reconstruct the band structure.We also propose a scheme for selecting short or long trajectories by a two-color laser field.We collaborated with Kim's group who works on experimental HHG in South Korea to study sapphire(Al2O3)as a target material for solid HHG.The experimentally measured spectra have shown that the 7th harmonic will have spectrum broadening and splitting with the increase of the incident light intensity.Through our theoretical simulations,it was found that the abnormal splitting observed in this experiment came from the inter-ference of the long and short trajectories in the interband transition.The broadening of the frequency spectrum comes from the modulation of the laser waveform.Interference between long and short trajectories reflects the electron dynamics of the crystal on the time scale of attosecond.We also theoretically studied the interference between the long and short trajec-tories in interband transitions in MgO crystals and observed similar interference phe-nomena as that in sapphire.Since the interference mechanism relies sensitively on the band structure of the crystal,we propose to reconstruct the band structure of the target material of the solid HHG based on this interference.We also use a two-color laser field to select short or long trajectories.By superposing a weak third-harmonic control pulse with a relative intensity ratio of 0.1,the harmonic yields in the plateau and the cutoff region can be enhanced by around one order of magnitude.By using a two-color field to select long or short trajectories,solid high-order harmonics can also produce an isolated attosecond pulse.2.We studied the control of solid HHG by using strain effects.This scheme can significantly improve the harmonic yield and change the time-frequency characteristics of harmonics by changing the external conditions.First,we compared the effects of four commonly used strains(shear strain,uniaxial strain,biaxial strain,and isotropic strain)on the lattice constant,band structure,and transition dipole moment of the piezoelectric material w-ZnO.We find that its band structure and transition dipole moment have a strong dependence on the strain.When isotropic tensile strain is applied,the piezoelec-tric effect is the most significant.Second,we compared the effect of strain on the solid harmonic spectra.Isotropic strain improves harmonic yield better than other strains.Under the isotropic tensile(compressive)strain,the harmonic yield can be strongly en-hanced(suppressed).This is because the tunneling ionization rate has an exponential dependence on the bandgap of the material.Under the tensile strain,the cutoff frequency of harmonics is almost unchanged compared to unstrained zinc oxide.In addition,we also found that under the biaxial tensile strain of+3%,the harmonic radiation in the time domain shows a smaller chirp,which can effectively reduce the full width at half maximum of the harmonic emission,and can be used to synthesize the solid attosecond light source.3.We cooperated with Legare's group who works on experiments to study the de-pendence of the ablation of lithium niobate(LiNb03)on the crystal orientation and the phase of the two-color field.We found the ablated area can be used as a macroscopic observable to reflect the phase characteristics of the laser field and the microstructure information of the crystal.The experimental results show that,for every 2? rad of phase modulation in the two-color field,the LiNbO3 crystal has a maximum ablation area and a minimum value.When the c axis of the LiNbO3 crystal sample is rotated by 180°relative to the laser polarization direction,a completely opposite phase dependence of the two-color field appears.Our theoretical simulations show that the LiNbO3 crystal has spontaneous polarizations.When the driving light polarization direction is consis-tent with the spontaneous polarization direction of LiNbO3,the bandgap decreases,and the ionization probability increases.Conversely,when the laser polarization direction and the spontaneous polarization of LiNbO3 are reversed,the bandgap increases and the ionization probability decreases,resulting in the ablated area of the LiNbO3 crystal with completely opposite phase dependence curves of the two-color field.We also studied the laser-induced ablation area dependence of the ferroelectric y-cut and x-cut LiNbO3.The experiment used 1800 nm linearly polarized femtosecond laser and the laser-induced ablation area of LiNbO3 sample is measured.Under tunnel-ing ionization,the crystal orientation angle is closely related to the crystal symmetry of the material.When the c axis of the LiNbO3 sample rotates around the direction of the laser propagation,the ablated area shows a strong crystal orientation angle dependence.When the laser polarization direction is parallel to the Nb-O bond,the ablation area is the largest.Density functional theory calculations show that Nb-4d orbit contributes to the conduction band near the Fermi-energy,and O-2p orbit contributes to the valence band near the Fermi-energy.When the laser polarization direction is along the Nb-O bond,electrons are effectively ionized and driven along the Nb-O bond.We used the two-band semi-conductor Bloch equation combined with density functional theory to explain the experimentally measured ablation area dependence on the crystal orientation.