Research On Physical Mechanism Of Intense Laser-induced High-order Harmonic Generation In Solids

In recent decades,with the continuous development of laser technology,research on the interaction of intense laser with matter has gradually became one of the most active and cutting-edge fields in physics.High-order harmonic generation?HHG?,a typical nonlinear physical phenomenon resulting from the interaction of intense laser with matter,is a preferred solution for synthesizing highly efficient attosecond pulses and acquiring soft X-ray,high-brightness coherent polar ultraviolet light sources.Due to the high electron density of the solid materials,it is an important way to obtain stable and efficient high-order harmonics.In recent years,lots of experimental and theoretical research has been carried out in the field of high-order harmonic generation in solids.However,few related research on accurate numerical calculation of high-order harmonic generation in solids and analysis of odd order harmonic generation were carried out.Based on the research status mentioned above,intense laser-induced high-order harmonic generation in solids by theoretical research and numerical calculation were studied in this thesis.The specific research includes the following aspects:?1?The high-order harmonic spectrum in solids induced by intense laser was obtained by numerical calculation,and the symmetry of current density was analyzed.We started from the lattice parameters of Cu₂S crystal in combination with the K?nig-Penny potential to build the three-band model,and high-order harmonic spectrum in solids was obtained by the density matrix theory under the velocity gauge,then the high-order harmonics with only odd harmonic components was obtained.We studied the generation process of high-order harmonics,and studied the generation of intra-band and inter-band harmonics respectively:the symmetry relationship between intra-band density matrix elements and energy band structures in wave vector space lead in the intra-band current meet the odd harmonic function;the amplitude and phase of inter-band density matrix elements and polarization components meet the corresponding relationship so that inter-band current also satisfied the odd harmonic function,causing the intra-band and inter-band harmonics were all odd harmonics.?2?The influence of different initial values on the evolution of density matrix and the influence of laser power density on the contribution of different energy bands to the high-order harmonic spectrum in solids of the three-band model were studied.We found that the solution of the density matrix equation was not sensitive to the initial value.When laser power density was low,almost only the valence band and the low-energy conduction band in the three-band model made contribution to the high-order harmonics.At this time,the calculation result of high-order harmonics in solids is basically equivalent compared with the two-band model.With the increase of laser power density,the contribution of the high-energy conduction band to the high-order harmonics was also increasing.Therefore,the three-band model can be more accurate in describing the generation of high-order harmonic in solids.?3?The spectrum characteristics of high-order harmonic in solids under the action of the external field were studied systematically.The electrostatic field introduced on the basis of monochromatic laser field changed the energy band structure of solid materials,causing the dual platform structure of the high harmonic spectrum disappear,and increasing the cut-off frequency of high harmonic spectrum.The spectral characteristics of high-order harmonic in solids under the interaction of electrostatic field and two-color laser field was also analyzed,by adding a secondary laser field which is weaker compared with the main laser field to form the two-color laser field.Our study result showed that high-order harmonic spectrum with both odd and even harmonic in solid materials were produced under the action of two-color laser field.The intensity of high-order harmonics was affected by the periodic modulation of the initial phase delay of the two-color laser field,and the influence of the initial phase delay on the even harmonics was significantly stronger than that of the odd harmonics.