Optoelectronic And Magnetic Optoelectronic Physical Properties Of New Electronic Materials Under Terahertz Laser

Terahertz wave(THz),as a new electromagnetic wave found in the electromagnetic spectrum,holds a wide range of applications due to the special position in the electro-magnetic spectrum,which shows the properties of both photonics and electronics.It is a bridge between long wavelength optics with electronics,with relatively small photon energy.Therefore,THz is used for non-destructive detection.Moreover,it displays an important application prospect in medicine,national defense,aerospace,public security and so on.More importantly,THz non-contact detection,a widely used technique in physics,such as THz Time-Domain Spectroscopy(THz TDS)and pump-probe tech-nology,are applied to obtain important intrinsic physical parameters of optoelectronic materials.For example,the real part and the imaginary part of photoconductivity can be directly obtained by THz TDS without Kramers-Kronig transformation.Meanwhile,infrared(IR)spectroscopy is also an important method of material analysis.Therefore,in this paper I mainly studies the THz and IR optical properties of semiconductor ma-terials,such as MoS2,GaSb,graphene,etc.,with the following four parts:(1)We introduce the free-space and optical fiber THz TDS systems,describe their components,generation units and detection units,in order to analyze their advantages and disadvantages.Based on the widely used THz TDS,we built up the THz TDS with the magnetic field and low temperature devices,which exhibits more optical mea-surement functions.In addition,the system control and measurement software based on Labview program and its operation method are introduced.The analysis method of sample data at the condition of zero magnetic field(non-zero magnetic field)and the application of THz-TDS are also investigated.(2)The THz photoelectric properties of monolayer(ML)MoS2 placed on the dif-ferent substrates,such as SiO2/Si,sapphire and quartz,have been studied.Through the measurements of THz Fourier transform spectroscopy(2.5-6.5 THz)and TDS(0.2-1.2 THz),we find that the real part of optical conductivity increases(decreases)for ML MoS2 on SiO2/Si and sapphire(quartz)substrate with increasing radiation frequency.It is found that the complex optical conductivity data for ML MoS2,obtained from THz TDS measurements,fit very well with that obtained by the Drude-Smith formula.Thus,the dependence of optical conductivity of ML MoS2 on different substrates can be un-derstood via a mechanism of electronic localization.The electron density,relaxation time,and localization factor of the sample are determined optically.Furthermore,the influence of temperature on these key parameters in ML MoS2 on different substrates were examined.The results obtained from this study indicate that THz spectroscopy is a very powerful tool in studying and characterizing ML MoS2-based electronic sys-tems,especially in examining the electronic localization effect which cannot be directly measured in conventional electrical transport experiment.This study is relevant to an in-depth understanding of the optoelectronic properties of ML MoS2 and of the prox-imity effect induced by different substrates.(3)We study of the electron energy relaxation time of n-GaSb,based on the estab-lished monochrome picosecond(ps)terahertz pump-ps probe optical system.The first Chinese THz free electron laser(CTFEL),built by the Institute of Applied Electronics of Chinese Academy of Engineering Physics,was successfully developed and put into use in August 2017.The THz laser has a macro-and micro-pulse structure.The pulse width of micro-pulse is picosecond,and the frequency of terahertz laser is tuneable.Based on this special light source,a monochrome ps terahertz pump-ps probe optical system is designed.The terahertz free electron laser is divided into pump pulse and probe pulse,and the ps time delay is realized through the electronic displacement plat-form.A new technique to measure the electron energy relaxation time in an electron gas system via monochrome ps terahertz(THz)pump and ps probe experiment is devel-oped.The special THz pulse structure of Chinese THz free-electron laser is utilized to realise such a technique which can be applied for the investigation into THz dynamics of electronic and optoelectronic materials and devices.The THz dynamical electronic properties of high-mobility n-GaSb wafer at 1.2 THz,1.6 THz and 2.4 THz was mea-sured at room-temperature and in free-space.The obtained electron energy relaxation time for n-GaSb is in line with that measured via,e.g.,four-wave mixing techniques.The major advantages of monochrome ps THz pump-ps probe in the study of electronic and optoelectronic materials are discussed in comparison with other ultrafast optoelec-tronic techniques.This work is relevant to the application of pulsed THz FELs and to the development of advanced ultrafast measurement technique for the investigation on dynamical properties of electronic and optoelectronic materials.(4)The THz magneto-optical properties and mid-infrared(MIR)transmission prop-erties of graphene are achieved.The THz TDS of bilayer and trilayer graphene with magnetic field were studied in Faraday optical configuration.Through the Fourier trans-form to its spectrum,the longitudinal and transverse photoconductivity of bilayer and trilayer graphene were acquired.By fitting experimental data with the magneto-optical Drude model,the important material parameters of graphene,such as the electron den-sity,the electron relaxation time and the effective electron mass,were obtained.The relationship between the relaxation time and the magnetic field was obtained and the type of carbon atom layer stack of the multi-layer graphene used in the study was also considered.Moreover,it is demonstrated that the visible light irradiation can marked-ly modulate the MIR transmission of mono-,bi-,and tri-layer graphene at the room temperature.The optical transmission was measured via Fourier transform IR spec-trometer within 14 to 24?m.The pumping laser with 532 nm wavelength and 110 mW power was taken as modulation light source.We find that the MIR transmittance of graphene decreases with increasing carbon layer of graphene,similar to the case in the visible to near-infrared bandwidth.Furthermore,the MIR transmittance of graphene with different carbon layers decreases pronouncedly in the presence of visible light ir-radiation.This effect is induced mainly by the fact that the visible light pumping can induce photon-generated carriers in graphene,which leads to an increase in the optical conductivity,and to a decrease in optical transmission.This interesting and important finding indicates that the visible light irradiation is able to be employed to modulate optically the MIR response of graphene based optoelectronic devices for applications such as telecommunication and IR detection.This paper mainly includes the application of optical fiber THz TDS system,the construction and application of picosecond THz free electron laser pump-picosecond probe,the application of Fourier spectrometer,sample preparation and characterization,experiments measurement,data analysis and theoretical fitting.The related important original achievements have been acquired.