The Theoretical Study On High-order Harmonic Generation In Solid With Initial Superposition State

In the past few decades,based on the rapid development of laser technology,the interaction between laser and atoms and molecules has become a research field of great concern.Many new phenomena have been found in experiments,such as Above threshold ionization,tunneling ionization,multiphoton ionization,high-order harmonic generation,Coulomb explosion and so on.In recent years,people pay more attention on the interaction between laser and condensed matter,in which the high-order harmonics in solid have attracted much attention.At present,the main issues of the high-order harmonic generation in solids are the emission and control mechanism,and how to enhance the emission efficiency of high-order harmonic plateau has become an issue which people pay much attention.Based on the above motivation,we adopt the scheme of changing the weight of the conduction band in the initial state to regulate the generation of high-order harmonics,and analyze its generation mechanism theoretically.We have theoretically investigated the physical mechanism of high-order harmonics generation in solid from the superposition of the conduction band minimum and valence band maximum states.Through the calculation,we find that the difference of the highorder harmonics generation from the superposition state and the valence band maximum states alone is not obvious in the lower order,but the intensity of the highorder harmonics may be enhanced by two to three orders in the higher-order plateau region.The above results are analyzed by calculating the contribution of intraband current,the interband current and the current generated by interband transitions between different energy bands to high-order harmonics.The results show that the enhancement of high-order harmonic generation in the plateau region is mainly caused by the enhancement of interband transitions between different energy bands.The loworder of high-order harmonic generation is mainly due to the small effect of changing the initial state on the high-order harmonics generated by the intraband current.In addition,we make a time-frequency analysis of the high-order harmonics caused by the intra-band current,the inter-band current and the current generated by interband transition between different energy bands,respectively,to explain its physical mechanism more vividly.