Theoretical Study On High Harmonic Generation And Attosecond Transient Absorption Spectrum In Solids By Intense Laser Pulses

In recent years,with the development of laser technology,the study of ultrafast dynamics of atoms and molecules in laser fields has been developed rapidly,and the interaction between laser and solid materials has appealed to more and more attention.Many laser-induced strong field effects occur when the laser intensity is equivalent to the Coulomb field.Studying the resulting high order harmonics,ionized electrons,etc.,can realize the detection of the dynamic process in the materials.In addition,ultrashort pulses in attosecond scale can be used as probes to control the movement of electrons or molecular reactions.With the fast development of ultrafast optics and ultrafast laser technology,it lays a foundation for people to learn and understand the real time detection of strong field processes and dynamics in crystals,especially in semiconductors.Based on the current research interest,we have done the following work in this paper:（1）We theoretically investigate high-order harmonic generation（HHG）in Zn O crystal driven by linearly polarized laser pulse by numerically solving the semi-conductor Bloch equations.Firstly,we briefly explain the differences between the atomic recollision model and the solid recollision model.Secondly,we investigate the electron dynamic in different crystal momentum channels by the classical model with and without the preacceleration process.Finally,the preacceleration process is revealed and visualized by the crystal-momentum-resolved harmonic spectrum and the transient conduction-band population.It is shown that an electron initially located at nonzero crystal-momentum in the valence band undergoes the so called preacceleration process before tunneling excitation.The contributions of the crystal-momentum channels away from the minimum band gap via the pre-acceleration mechanism are non-negligible.Our results verify the recently proposed four-step model for HHG in solids.（2）The attosecond transient absorption spectrum（ATAS）of two-band and three-band Zn O crystals are firstly studied based on the principle of pump-probe technology and the numerical solution of the semiconductor Bloch equations.The differences between the absorption spectrum are described briefly.The absorption spectrum of Zn O crystals with two or three-band have some common characteristics.When two laser pulses interact with the crystals together,the absorption spectrum show fringes structure of periodic oscillations.and the tilted fringes indicate different absorption positions and have different slopes,resulting in a V-shaped structure.then,since the photon absorption depends on the Fourier spectrum of the current induced by the probe field.By changing the dephasing time,for longer dephasing time（T₂（29）2T₀）,the currents can last several optical cycles and maintain the characteristics of plane wave.As a result,the absorption structure remains almost the same.For smaller the dephasing time,the reduction in the number of optical cycles by the probe induced current will only lead to a broadening of the Fourier spectrum,but it does not significantly affect the shape of the absorption spectrum.Thus,the dominant structure of the delay-dependent absorption spectrum is preserved.