Second-order And High-order Harmonic Generation And Their Manipulation In Semiconductor Materials

Nonlinear responses,such as second-order harmonic generation(SHG)and high-order harmonic generation(HHG),induced by the interaction between ultrastrong and ultrafast pulse laser and semiconductor materials,have attracted much attention.As a spectroscopy technique,SHG and HHG pave the way to study the fundamental strong light-matter interaction processes happening at femtosecond and attosecond time.In terms of application,the observation of the SHG and HHG in semiconductor makes it possible to engineer and control the laser-matter interaction by changing the structures of matters or shapes of the laser fields.The study on the SHG and HHG and their manipulation in semiconductor in ultrafast timescale will promote the development of solid coherent extreme violet source and novel compact ultrafast photonic devices.In this thesis,the underlying mechanism and manipulation methods of SHG and HHG in semiconductor materials are discussed and analyzed from two perspectives: controlling the internal structures of matters and controlling the laser fields.The main contents of this paper are as follows:(1)A method to improve the efficiency of HHG based on a doped semiconductor is proposed.The model of doped semiconductor is built based on a 1D model in periodic potential and the HHG is simulated with the single-electron time-dependent Schr?dinger equation.The results show that the high-order harmonic spectra of doped and undoped semiconductors have two-plateau structure.The intensity of the second plateau generated from the doped semiconductors is about one to three orders of magnitude higher than that from the undoped semiconductor.By controlling the species of dopants and the doping rates,HHG can be modulated efficiently.The results are explained based on the analysis of the energy-band structure and the time-dependent population imaging.It shows that the doping changes the band structure of the semiconductors and controls the HHG.In this work,band engineering is introduced into the study of ultrafast optics,which provides a new idea for the generation and control of high-order harmonics in solids.Meanwhile,this method points out the potential of the doped semiconductor to generate efficient solid attosecond laser pulses.(2)A method to modulate the HHG of monolayer hBN under two counter-rotating circularly polarized fields is proposed.The results show that the 3nth(n is nonnegative integer)order harmonics of hBN HHG intervene destructively,and the polarizations of(3n+1)th order harmonics are opposite to that of(3n+2)th orders.When improving the intensity ratio of the two fields,the intensity of HHG is modulated obviously.The appearances of the peak and valley values of higher orders also shift to higher ratio.Based on the reciprocal-space-trajectory analysis,the right and left circular polarization harmonics are primarily contributed by the electron trajectory ensembles with the ionization channel around the high symmetry points K and K’,respectively.The modulation of HHG is determined by the interference between electron trajectories.In this work,the control of the hBN HHG by using two counter-rotating circularly polarized fields is achieved.Besides,it offers a quantitative and intuitive perspective to understand the polarization of HHG in the view of electron reciprocal space trajectory.(3)The modulation of second-order harmonic generation(SHG)in <11-20> ZnO crystal driven by strong near-infrared laser pulse is investigated experimentally.The results show that the anisotropic structure of the total SHG yield changes from “8” to the double-pesk butterfly shape when increasing laser intensities.An extended bond-charge model(EBCM)and a generalized second-order susceptibility(GSOS)are proposed to explain the results.It shows that the interbond electron hopping induces a novel source of nonlinear polarization and contributes to the component of SHG in the direction perpendicular to the optical axis.When increasing laser intensities,the anisotropy of parallel component keeps the “8” shape unchanged.The peaks of butterfly structure of perpendicular component changes from 45°and 135° to 60° and 120°.And the intensity of the perpendicular component becomes dominant.It results in the change of anisotropy of SHG.This work reveals the nonlinear responses of materials at the electron scale and paves the way for realizing controllable nonlinearity on an ultrafast timescale.