Nonlinear Optical Response Of Topological Semimetal In Terahertz Regime

The nonlinear optical response of a medium reflects its electron behavior to the external excitation field,which has played an important role in the modern optoelectronics,such as optical switcher,four-wave mixing,the up-conversion multiplication of terahertz electromagnetic radiation.Topological semimetals have attracted attention due to their unique energy band structure,topological surface state,anomalous electron transport characteristics and other physical properties.This thesis focuses on the study of the nonlinear optical response of topological semi-metallic systems,which provides a theoretical basis for its application in optoelectronic devices.The main topics of this thesis are listed below:1.We find that the intra-band excitation will lead to a strong nonlinear optical response in two-dimensional semi-Dirac system in the terahertz regime by applying the Boltzman transport theory.We predict that due to the anisotropic band structure,the difference of the third-order nonlinear conductance in different directions can reach about two orders of magnitude,which also shows that the third-order response strongly depends on the direction of the applied electric field.In particular,we also found that when the position of the chemical potential changes from below the saddle point to higher than the saddle point,the third-order nonlinear conductance undergoes a sign change.This sign change should originate from the opposite velocity of electrons below the saddle point.2.Due to the time reversal symmetry(TRS)breaking of electrons in the tilted Dirac cone,the second harmonic generation(SHG)can survive.In this work,we theoretically demonstrate the strong SHG in a two-dimensional(2D)tilted type-I Dirac metal in the terahertz regime.For a comprehensive understanding of SHG,we make a comparison with the first-and third-order responses by evaluating the dependencies on the tilting parameter,temperature and excitation frequency,respectively.Further investigating on the critical filed for the SHG and third harmonic generation(THG),we identify four regions in-_{(8) diagram for distinguishing the superiority of the three modes of optical responses.These results suggest that by varying the frequency and strength of applied field one can engineer the desired nonlinear response.Finally,we make a rough discussion on how the relaxation time affects the SHG in four types of6}-dependent scattering mechanisms.