Research On Some Key Issues Of High-order Harmonics In Solid

The development of ultra-intense and ultra-short laser technology makes it possible for us to observe the changes of matter at the microscopic scale.Because the intensity of the intense laser would be higher than the Coulomb field of the atom(10¹⁶W/cm²),when the intense laser interacts with the matter,the electrons can be ionized,resulting in rich nonlinear effects.The high-order harmonic generation（HHG）is of great significant.It can produce coherent extreme ultraviolet light and attosecond pulses,which is helpful to research the ultra-fast electron motion.The solid high-order harmonics expands the target material for obtaining high-order harmonics.Due to the natural periodicity of atoms and high density of solid-state materials,the HHG in solid shows great potential in improving the intensity of attosecond light sources.At the same time,it is possible to study the ultrafast dynamics of carriers in materials.In this paper,the carrier dynamics of HHG in solid is studied theoretically.The information of energy band structure in high-order harmonic spectra is decoded.And we also explored the technology of long wavelength intense laser pulse.The main results and innovations are listed as follows:1.Interference effect between electron trajectories on solid HHG is analyzed.Based on the solid three-step model,by solving semiconductor Bloch equations（SBEs）,it is found that the constructive interference between transient electron trajectories generated by the oscillation of the intense laser field,resulting in the emergence of a plateau structure in high frequency region and enhancement of yield of HHG.The research shows that the plateau comes from the interband Bloch oscillations,which is contributed by electrons at the boundary of Brillouin zone（BZ）.When the number of electron trajectories with constructive interference decreases under the modulation of vector potential,the plateau disappears.Based on above analysis,by controlling the laser field strength and the two-color field respectively,we realized the phase modulation of electron trajectories in the full k-space and enhanced the yield of HHG in the high-frequency region.This discovery can improve the yield of harmonics in high-frequency region and expand cut-off region.It is helpful to generate high brightness attosecond pulses.2.A high-precision scheme for reconstructing the energy band structure by solid high-order harmonic spectrum in full momentum space is proposed.Due to the energy of harmonics contributed by interband mechanism comes from the band gap energy,the information of band structure is encoded in the harmonic spectra.When the electric field strength is increased continuously,the cut-off energy of HHG is mapped to the bandgap energy at the farthest position of electrons in k-space.By the classical model of electrons motion in momentum space,we successfully predicted the field-strength dependence of cut-off energies of the HHG,proving the accuracy of the model in predicting the motion of electrons tunneling at high-symmetry points.By calculating the trajectory of electrons tunneling at high symmetry point,the maximum momentum can be extracted.This momentum is mapped to the cut-off energy obtained from the SBEs.And the reconstruction of the full k-space bandgap structure can be realized by fitting the relation between momentum and cut-off energy.It is proved that the error of the reconstruction is smaller under long-wavelength light driven.3.An optical system of broadband mid-infrared laser pulse is built.We preliminarily proved the feasibility of this system.Mid-infrared ultra-short intense laser pulses play an important role in promoting the frontier research on intense-filed optical properties of materials.The work is based on two three-stage optical parametric amplifiers with center wavelengths of 1.7μm and 2μm output,and then the broadband mid-infrared light pulse is generated by spectral compression and differential frequency generation.By simulating the self-phase modulation in the hollow-core fiber and the coupled wave equations of the difference frequency generation,the gas pressure,crystal angle and thickness are selected,and angular dispersion is compensated.The analysis shows that the system can generate broadband mid-infrared ultrashort pulse with a center wavelength of 10μm.The feasibility of the system is also preliminarily proved by experiments.