Teraherz Wave Emission From The Weyl Semimetals

In the past decade,the research of topological material is one of the hot topics in the fields of physics,material and electronic science.Topological material generally includes the topological insulator and the topological semimetal,where the topological quantum states are protected by specific symmetries,and hence are immuned to the external environment perturbations.This makes the topological materials potentially for applications in the spintronics and quantum computing in the future.Topological semimetals are based on the topological properties of electronic structures.They have unique energy bands and spin structures in their bulk and surface states.This thesis demonstrates experimentally the generation of coherent broadband terahertz(THz)waves in the Weyl semimetals,and introduces the principle of the generation.Weyl semimetal has distinct chiral fermions and spin structures.The chiral fermions,upon light excitation,can cause novel carrier responses,leading to novel optical phenomena.The generation and manipulation of the photocurrent are essential for the research and development of photo-electronic devices,where participation of the optical transition selection rules makes it possible to utilize the charge and spin freedom of electrons in a nonthermal way.This thesis focuses on the investigation of the ultrafast photocurrents and their associated THz wave emission in the topological semimetals.The microscopic mechanisms have been revealed by careful time-resolved ultrafast photocurrent measurements and theoretical calculations.At the same time,we have studied systematically the properties of emitted THz wave depending on the light polarizationand photon energy.This thesis illustrates that the Weyl semimetal TaAs can generate stable and controllable coherent chiral ultra-broadband THz wave,and thus can be used as a chiral broadbandTHz emitter.The ultrafast photocurrent can be controlled by the polarization of excitation optical pulses,and eventually lead to the chiral THz radiation.The chiral properties of the ultrafast photocurrent or the associated THz wave originate from two components: one related to the nonthermal photogalvanic effect and the other due to the thermal effect.These two different physical mechanisms result in the time delay between the corresponding current or THz field components in the time domain,leading to the phase difference and the chirality.Specifically,in the near infrared region,the dominant circular phtotgalvanic effect is found to arise from the relative band velocity changes of the chiral Weyl fermions undergoing selective optical transitions between the tilted anisotropic Weyl cones and the bulk energy bands.The related sheet current density or the THz amplitude is dependent on the photon energy of the excitation light: J(?)?1/?.Our work thus reveal that in the Weyl semimetal TaAs,upon excitation of high energy photons,the linear Weyl bands involved in the optical transitions play a key role in the generation of colossal ultrafast photocurrents and the strong THz wave emission.Our discovery provides a new design concept for making chiral photon source using quantum materials,and opens up a new way for developing ultrafast optoelectronic devices using Weyl physics.