【作者】 王超；

【导师】 徐文；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2020， 博士

【摘要】 太赫兹波(THz)作为在电磁波谱中被人们发现的新的电磁波,具有广阔的应用范围,其在电磁波谱中特殊的位置决定了它既具有光子学特性,又具有电子学特性,是连接长波长光学和电子学的桥梁。太赫兹光子能量较弱,非常适合用于无损检测,在医学、国防、航天、公共安全等方面具有重要的应用前景。在物理学中,应用太赫兹非接触式的探测特性,在科研中太赫兹时域光谱技术、泵浦探测技术等得到了较深入的应用和发展,帮助研究人员获得光电子材料的重要内在物理参数。例如通过太赫兹时域光谱技术可以直接得到材料光电导的实部和虚部,无需通过Kramers-Kronig变换。此外,红外光谱技术也是材料分析的一种重要手段。因此,本论文主要研究了半导体材料(如二硫化钼、锑化镓、石墨烯等)的太赫兹和红外光学特性,主要包括以下几部分内容:(1)首先介绍了目前常用的自由空间类和光纤类太赫兹时域光谱系统,描述了它们的组成部分、产生单元和探测单元,并分析了各自的优缺点。目前,太赫兹时域光谱系统已经取得了较为广泛的应用,我们在该光学平台的基础上进行了附加磁场和低温装置的集成,使该强场低温太赫兹时域光谱仪具有了更多的光学测量功能。此外,介绍了基于Labview程序编写的系统控制及测量软件和其操作方法。归纳总结了在零磁场和非零磁场两种条件下的样品数据处理方法以及THz TDS的应用。(2)通过太赫兹光谱测量单层二硫化钼中衬底诱导的电子局域化效应的研究。我们研究了不同衬底上(例如二氧化硅/硅,蓝宝石和石英)上单层(ML)二硫化钼(MoS2)的太赫兹光电特性。通过测量太赫兹傅里叶变换光谱(2.5-6.5 THz)和太赫兹时域光谱(0.2-1.2 THz),我们发现,二氧化硅/硅和蓝宝石衬底上的单层二硫化钼的实部光电导率随辐射频率的增加而增加,然而石英衬底上的实部光电导率随辐射频率的增加而减小。结果表明,由THz-TDS测量得到的ML MoS2的复光电导率可以通过Drude-Smith公式很好地拟合。因此,ML MoS2的光电导率与不同衬底的依赖关系可以通过电子局域化机理来理解,并且样品的电子浓度、弛豫时间和局域化因子也可以通过光学方法测定。此外,我们检测了温度对不同衬底ML MoS2的这些关键参数的影响。从中得到的结果表示太赫兹光谱是研究和表征基于ML MoS2电子系统的一种非常有效的工具,特别是用于检测不能直接通过传统的电输运测量获得的电子局域化效应,使我们对ML MoS2的光电特性和不同衬底诱导的邻近效应有了更深的理解。(3)单色皮秒太赫兹泵浦—皮秒探测光学系统的搭建及对n-GaSb电子能量弛豫时间的研究。中国工程物理研究院十所(应用电子学研究所)搭建的中国首台太赫兹自由电子激光(Chinese Terahertz Free Electron Laser,CTFEL)于2017年8月饱和出光且投入使用。所辐射的太赫兹激光具有宏—微式脉冲结构,微脉冲为皮秒量级,且太赫兹激光具有频率可调的特性。基于此特殊的光源,我们设计并搭建了单色皮秒太赫兹泵浦—皮秒探测光学系统,将太赫兹自由电子激光分为泵浦脉冲和探测脉冲,通过电控位移平台实现皮秒级时间延迟。利用单色皮秒太赫兹(THz)泵浦—皮秒探测实验,提出了一种测量电子气系统中电子能量弛豫时间的新方法。利用中国太赫兹自由电子激光器特有的太赫兹脉冲结构,实现了这一技术,可用于研究电子和光电子材料及器件的太赫兹动力学性质。在室温和自由空间中分别测量了高迁移率n-GaSb半导体在1.2 THz、1.6 THz和2.4 THz下的太赫兹动力学电子性质。得到的n-GaSb的电子能量弛豫时间与四波混频技术测量的电子能量弛豫时间一致。讨论了单色皮秒太赫兹泵浦—皮秒探测在电子和光电子材料研究中的主要优势,并与其它超快光电测量技术进行了比较。这项工作与脉冲太赫兹自由电子激光器的应用和先进的超快测量技术的发展有关。(4)石墨烯太赫兹磁光特性和中红外透射特性研究。应用Faraday光学配置的光纤太赫兹时域光谱系统研究了低温下双层和三层石墨烯在不同偏振角的太赫兹时域光谱随磁场的变化,通过傅里叶变换得到其频谱,进而得到了双层和三层石墨烯的纵向和横向光电导率。通过磁光Drude模型对实验数据的拟合,获得了石墨烯重要的材料参数,如电子浓度、电子弛豫时间、有效电子质量等。得到了弛豫时间与磁场的变化关系,还讨论了研究中使用的多层石墨烯的碳原子层堆叠方式。此外,我们测量了室温下可见光辐照可以显著地调节单层、双层和三层石墨烯的中红外(MIR)透射。以波长532 nm、功率110 mW的泵浦激光作为调制光源,利用傅里叶变换红外光谱仪在14～24 μm范围内测量石墨烯的光透射。我们发现石墨烯的中红外透过率随石墨烯碳层的增加而降低,与可见光到近红外波段的情况相似。而且,在可见光辐照下,不同层数石墨烯的中红外透射率显著降低。这种效应主要是由于可见光泵浦能够诱导石墨烯中产生光生载流子,从而导致光电导率的增加,光透射率的降低。这一有趣而重要的发现表明,可见光辐照可用于光学调制石墨烯光电器件的中红外响应,可适用于电子通信和红外探测等方面的应用。因此,论文研究内容主要包括光纤太赫兹时域光谱系统的应用,基于太赫兹自由电子激光的单色皮秒太赫兹泵浦—皮秒探测系统的搭建和应用,以及傅里叶光谱仪的应用,样品的制备和表征,实验测量,数据分析,理论拟合。论文获得了一些重要的原创成果。更多还原

【Abstract】 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 24μm.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.更多还原