| This article focuses on two topics in strong field physics.One is high-harmonic generation in crystal materials under strong laser fields,and the other is Floquet band engineering based on the Houston basis under strong laser fields.With the rapid development of high-intensity laser technology,various nonlinear optical phenomena have been extensively studied.Among them,coherent radiation produced by the interaction between strong laser and matter-high-order harmonic generation（HHG）-is one of the current research hotspots.In this study,the complete formula for laser-induced currents in solids is re-derived,and a new mechanism for mixed current is proposed.By simulating the semiconductor Bloch equation and performing Fourier transforms,we obtained the harmonic spectra of four types of currents,including inter-band currents,intra-band currents,mixed currents,and anomalous currents.Subsequently,we studied the crystal orientation dependence of the harmonic spectra induced by the four types of currents.By rotating the incident laser360°,we found that the harmonic spectra induced by mixed currents and inter-band currents have very similar properties,the Berry curvature-induced anomalous current only generates even-order harmonics with perpendicular polarization,while the intra-band current has a unique symmetry due to the crystal orientation dependence of electron group velocity.This study will contribute to a better understanding of high-harmonic phenomena in solids.In the past ten years,with the rapid development of strong laser technology,including Q-switching,mode-locking and chirped pulse amplification,the combination of strong laser field and Floquet theory has attracted widespread attention in the academic community.This work studies the changes of physical quantities such as quasi-energy and average energy of Mo S2 under a periodic driving field.The conclusions are as follows:First,under the near-resonance condition,the quasi-band avoids the crossover phenomenon.When the transition dipole moment is constant,the quasi-band remains constant with the increase of the scanning driving field strength.However,when the transition dipole moment is considered as a function of the momentum k,the quasi-band at some specific k points（e.g.k=±0.2 and k=±0.9）The band gap increases,that is,avoids the crossover phenomenon.Secondly,under the periodic driving field,we found periodicity in both k-space and energy space,that is,the Floquet-Bloch Brillouin zone.When the Keldysh parameter is much smaller than 1,the incident driving field is a strong field.At this time,with the increase of the driving field strength,the average energy band will undergo a process of being gradually compressed and then stretched again.Our research provides a new idea for the study of Floquet theory under strong laser field. |
