Investigation About The Physical Mechanism Of The High-Order Harmonic Generation From Solids

Due to the innovation of laser technology,the pulse width has been reduced,and the laser intensity has been improved.The interaction of strong lasers with atoms and molecules can generate high-order harmonics.The high-order harmonic generation(HHG)can be used to obtain the tunable extreme ultraviolet(XUV)and probe the ultrafast dynamics of atomic and molecular systems.Moreover,due to the high-density,the HHG from the solids has a potential for improving the harmonic efficiency.In addition,it also can provide a useful tool to probe the energy-band structures of crystals.Therefore,the research on the generation of solid harmonics in strong fields has attracted widespread attention.Based on the one-dimensional periodic potential,the physical mechanism has been explored by numerical simulation about the solid HHG.The main research contents include the following two aspects:(1)The physical mechanism of the HHG from solids is theoretically investigated based on the one-dimensional periodic potential.The results show that there are two different channels for the electron transition from the low conduction band to the high conduction band,so that the adjacent trajectories contributing to the second harmonic plateau exhibit different intensities.The trajectories with higher intensity originate from the superposition of two electrons channels.In addition,by increasing the laser intensity,the electrons are excited to the second Brillouin zone,and four quantum trajectories contribute to the harmonic generation.Based on the time-dependent population imaging,these four quantum trajectories are analyzed and identified to make deep understanding of the physical mechanism of solid HHG.(2)Based on the one-dimensional periodic potential,the electron dynamics process of solid HHG is numerically simulated and the influence of doping rate on the harmonic efficiency is discussed.The results show the corresponding doping rate dependence changes from enhanced to suppressed rather than a linear tendency.The harmonic efficiency of the second plateau is enhanced almost six orders of magnitude for the doping rate of 0.04 compared with the doping rate of 0.Combined the variations of the band structure and the electrons population in different bands,the improvement of harmonic intensity is attributed to two excitation channels,i.e.the direct excitation channel and the step-by-step excitation channel.