Ultrafast Carrier Dynamics Of Three-dimensional Dirac Semimetal Cd₃As₂ Investigated With THz Spectroscopy

Ultrafast optical spectroscopy based on the interaction between ultrafast laser pulse and matter is the main research method to reveal the dynamic behavior of nonequilibrium photocarriers.In recent years,ultrafast spectroscopy technology is introduced into topological quantum materials to uncover the mechanisms of carrier scattering and relaxation dynamics of nonequilibrium states protected by topological invariance,which information is not available from general transport measurement.In this thesis,the photocarrier dynamics of a Dirac semimetallic,Cd₃As₂ film is studied systematically by means of ultrafast spectroscopy,which is represented by time-resolved terahertz spectroscopy.Because terahertz photons have very low photon energy?1 THz?4.1 Me V?,the time-dependent e Volution dynamics of the carrier scattering and the relevant relaxation around Fermi surface can be obtained.At the same time,a traditional semiconductor material Cd Te nano film is also studied in this thesis.It is found that the response time of terahertz signal after photoexcitation can be effectively shortened in a nano-structurized film,and a new method to realize the subpicosecond response speed of optical switch is proposed.The main contents of this thesis include:?1?The ultrafast dynamics of three-dimensional Dirac semi-metallic Cd₃As₂ thin films was studied systematically at room temperature by using the optical pumping terahertz probe spectroscopy.The rise time of THz photoconductivity caused by pump is about 1.0 ps,which increases slightly with the increase of pump power.In such a time scale,the electrons and holes establish their quasi Fermi distribution with electrons in the conduction band and holes in valence band,respectively via carrier-carrier scattering and carrier-phonon coupling.The subsequent relaxation of photoconductivity lasted about 6.0 PS,and was independent of the pump power.We believe that the relaxation process is mainly dominated by the electron-hole recombination mediated with phonons.The dispersion relation of THz photoconductivity can be well reproduced with Drude Smith model.Based on these experimental phenomena,we propose a physical model of carrier dynamics in a three-dimensional Dirac semi metallic material,Cd₃As₂.The experimental results are helpful to understand the dynamics of photocarriers in the 3D Dirac semi metallic and the related materials.?2?By employing optical pump and THz probe spectroscopy,we also investigated the photocarrier dynamics in Zn-doped Cd₃As₂ films at both room temperature and low temperature.In contrast to the undoped samples,the Zn-doped Cd₃As₂ films exhibit a faster THz relaxation signal at room temperature,and the relaxation time is relevant to the pump fluence:the higher fluence has a slower relaxation time.Based on the Drude-Smith model,the photoconductivity dispersion relation is reconstructed.The simulation results show that there is a strong carrier backscattering phenomenon in the doped samples,which is due to the introduction of a new carrier-phonon scattering channel,and therefore accelerate the carrier relaxation process.In addition,the temperature dependent experimental results further prove the enhanced scattering occurring in of Zn-doped Cd₃As₂ films.Finally,THz time-domain spectra at different temperatures show that the THz conductivity in a Zn doped Cd₃As₂ films increases significantly with the decrease of temperature,indicating that the samples with high dopant show obvious metallic character.?3?The ultrafast light modulation of THz switch in 10 nm Cd Te nanostructured films was studied.In order to achieve ultrafast THz modulation,the mobility of photocarriers can be reduced by introducing the surface defect states which can effectively capture the photocarriers.We also demonstrate that the ultrafast transient THz signal response is almost independent of the temperature down to 100 K.On the other hand,the results of transient absorption spectra show that the lifetime of photocarriers in Cd Te nanostructured films is several nanoseconds.Therefore,the photoconductivity response with time constant of 1.3 ps is caused by the sharp decrease of the mobility of photocarriers in the Cd Te nanostructure,which is due to the increase of the scattering of photocarriers in the surface state of the Cd Te nanostructure film.The experimental results provide a new scheme for the design of optical driven ultrafast THz response devices,such as THz switches and THz modulators.