Ellipticity Dependence Of High-order Harmonic Generation In Solids

The laser-matter interaction has long been one of the most advanced and active fields in physics.In recent years,due to the rapid development of ultra-short-intense laser technology,people observed a series of novel ultra-fast dynamic phenomena in experiments of laser-condensed matter interaction.Solid high-order harmonic generation(HHG)is the most attractive one.Currently,the main issues in the field of solid HHG are to understand and control its generation process.As we know,the elliptical polarized laser is the most common and effective tool which can control the harmonic emission and study the generation mechanism.Therefore,the relationship between solid HHG and laser ellipticity has been investigated by a large amount of experiments in recent years.However,the lack of theoretical researches seriously hindered the understanding of these experimental results.Based on this motivation,we theoretically studied the ellipticity dependence of solid HHG in this thesis.In the first part,we investigate the ellipticity dependence of HHG using a new ZnO model.In the driving of a relatively weak field,we successfully reproduce the ellipticity dependence as observed in the HHG experiment of wurtzite ZnO.It shows that the intensity of harmonics monotonously decreased with increasing the ellipticity,and the intensities of higher-order harmonics drop more rapidly.When increasing field strength,the HHG shows interesting features in which the lower-order harmonics are suppressed rapidly and the higher-order ones are enhanced with increasing ellipticity.This is similar to the anisotropic ellipticity dependence that was observed in the singlecrystal MgO.Subsequently,we also find the transition of ellipticity dependence of HHG as the laser wavelength increases.These results show a strong correlation between different types of ellipticity dependence in solids.In the second part,we investigate the mechanism of ellipticity dependence transition with the help of time-frequency analysis and the electron-hole recollision model.It is found that the key mechanism inducing the change in ellipticity dependence is the interplay between the dynamical Bloch oscillation and the anisotropic band structure.Firstly,the strong lasers could drive the electrons cross the edges of Brillouin zone which makes the dynamical Bloch oscillation possible,and then the dynamical Bloch oscillation contributes additional quantum paths,which are less sensitive to ellipticity.Secondly,the anisotropic band structure makes the driving pulse with finite ellipticity capable of driving the pairs to the band positions with a larger gap,which extends the harmonic cutoff.The combination of these two effects leads to the anomalous ellipticity dependence.Our analysis reveals the importance of dynamical Bloch oscillations for the ellipticity dependence of HHG from bulk ZnO.