Study Of Intensity Laser Induced High Harmonics Generation In Solids

Because solids have a high electron density,solids are an effective way to obtain stable,efficient high harmonics generation(HHG).Recently,non-perturbative high harmonics have been observed using the interaction between ZnO crystals and midinfrared laser,which open the door for the study of high harmonics in solids.Although many experimental and theoretical studies have been conducted on high order harmonics in solids in the world,it is little known about the mechanism of high harmonics generation in solids.One important reason is that the solid system is more complex,it is difficult to theoretically solve the high-order harmonics spectrum in solids.In this paper,the highorder harmonics spectrum of the Kronig-Penny model under the driver of strong light is accurately solved by numerical methods.The mechanism of high-order harmonics generation and the effect of laser parameters on high-order harmonics are theoretically studied.The specific research content is as follows:(1)In theory,the mechanism of the odd harmonic generation in the high harmonic spectrum is analyzed.When the solid material is interacted with intensity laser,only the odd harmonic exist in the generated high order harmonics.By simulating the generation of high order harmonics,we found that the symmetry of the band plays a crucial role in generating odd order harmonics,and high orders harmonics contain even order harmonic components in the asymmetrical band structure.We studied the generation of high-order harmonics of intraband and interband,and found that the sum of electron motion velocity in the positive and negative directions of the wave vectors satisfies the odd harmonic function in a symmetric band.The sum of the electron distribution probability satisfies even harmonic function.The sum of the two parts of the modulus of the interband polarization between the bands satisfies the even harmonic function,and there is a corresponding relationship between phases.(2)The influence of laser parameters and dephasing time on the higher harmonic spectrum was systematically studied.We simulated the effects of laser parameters on the high harmonic spectrum and found that high harmonics are sensitive to dephasing times,laser pulse wavelength,and laser field strength.When the dephasing time is less than or equal to the laser pulse period,a clearly visible high harmonic spectrum can be obtained,and a larger dephasing time will cause the high harmonics to be annihilated by the environment;the wavelength of the laser pulse can be affects the plateau width of the non-perturbative region of the high order harmonic spectrum,and plateau width scales with laser-wavelength;the laser field strength can affect the intensity and cut-off of the high harmonic spectrum;the stronger the laser field,the high harmonic spectrum stronger,and the cutoff frequency and the laser field strength satisfy a linear relationship.(3)The high order harmonic spectrum in solids was studied in the two-color laser.Based on the basic drive laser,we added a laser beam with weaker intensity and twice the frequency of the base laser to form a two-color laser.Our research shows that even-order harmonics can be generated,and relative strength of the even-order harmonic and oddorder harmonics can be controlled by controlling the phase between the two laser fields under the interaction of the two-color field in solids.