| Terahertz radiation is an electromagnetic wave band between microwave and infrared waves,with the advantages of low photon energy,strong coherence,excellent spectral resolution,etc.It has become a powerful tool in condensed matter physi cs to study the free carrier dynamics properties of materials such as metals,oxides,superconductors,and other materials.The terahertz time-domain spectroscopy(TDS)and laser pump-terahertz TDS probe(OPTP)techniques based on terahertz radiation can be used for the study of terahertz optoelectronic/magneto-optoelectronic properties of materials.For theoretical studies,this dissertation generalizes the Drude-Smith model,which was proposed for studying the frequency-domain optical conductivity of non-Drude types,to the time-domain(i.e.,the time-domain Drude-Smith model),which can be applied to the theoretical study of experimental results obtained from ultrafast pump-probe.The effectiveness of the time-domain Drude-Smith model was verified using experimental results obtained from measurements on gallium antimonide(GaSb)wafers and polycrystalline diamond(PCD).For experimental studies,monolayer molybdenum disulfide(MoS2),carbon dots(CDs)with different chemical modifications,and CDs-monolayer hexagonal boron nitride(hBN)-sapphire substrate heterostructures were prepared on the substrates;the free carrier dynamics properties and terahertz optoelectronic/magneto-optoelectronic properties of these material systems were thoroughly studied by applying terahertz TDS.1.Terahertz TDS systems and data processing models.In this dissertation,three kinds of TDS systems are debugged and used,namely,the terahertz TDS system based on photoconductive antennas,the OPTP system based on optical rectification effect,and the picosecond terahertz free-electron laser pumped-picosecond terahertz probe(TPTP)system.The constituent units and measurement principles of the terahertz TDS systems are presented.The latest experimental data processing models are summarized for obtaining important data such as complex optical conductivity,complex refractive index,etc.of films.The theoretical models of free carrier optical conductivity,such as Drude and Drude-Smith formulas and their magneto-optical forms,are summarized.By fitting the theoretical results with the experimental data,we optically determined the important physical parameters such as electron density,the electronic relaxation time,the electronic localization factor,and so on,of the samples.Finally,the program for data processing written in LabVIEW software in this paper is presented.2.Time-domain Drude-Smith model for pump-probe experiments and its application.In response to the current processing of time-domain data obtained from ultrafast pump-and-detect experiments,which is usually based on a purely exponential decay model that may be inaccurate,this dissertation generalizes the Drude-Smith model,which was proposed for the study of non-Drude types of optical conductivity in the frequency domain,to the time-domain Drude-Smith model.The time-domain Drude-Smith model can process time-domain data obtained from pump-probe experiments when considering the presence of optically induced electron localization or backscattering effects.It is found that:The time-domain Drude-Smith model can be well used to fit the terahertz time-domain data of n-type gallium GaSb obtained by the TPTP system and PCD film obtained by the OPTP system;Under the effect of strong terahertz free-electron laser pulses,the free electron localization/backscattering effect of GaSb decreases with the increase of terahertz radiation frequency;The phonon bottleneck effect that occurs as a result of the weakening of the effective electronphonon scattering strength in PCD film can lead to an increase of the electronic relaxation time with increasing pump intensity,and the related carrier localization/backscattering effect weakens accordingly.3.Terahertz magneto-optical properties of monolayer MoS2.Similar to conventional semiconductor devices,monolayer MoS2 usually needs to be placed on a substrate for device applications,and the substrate affects the device performance through proximity effects and by providing extra charges and scattering centers.It is therefore necessary to study the basic physical properties of monolayer MoS2 on the substrate,such as free carrier dynamics and optoelectronic/magneto-optoelectronic properties.In this dissertation,terahertz TDS system was applied to study terahertz magneto-optical properties of monolayer MoS2 on SiO2/Si substrate at liquid nitrogen temperature.The main results of this study are as follows:The real and imaginary parts of the longitudinal magneto-optical conductivity of monolayer MoS2 under different magnetic fields(0-8 T)were obtained experimentally,and it is found that the real part curves wavily with the increase of terahertz frequency under higher magnetic fields;The magneto-optical conductivity of monolayer MoS2 was fitted by applying the magneto-optical Drude-Smith model obtained from the previous theory of our group.The parameters such as the free electron density,the free electronic relaxation time,and the electronic localization factor of monolayer MoS2 and their dependence on the magnetic field were extracted by fitting the theoretical equations to the experimental data;With the increase of the magnetic field,the electronic relaxation time of the sample increases linearly,while the localization factor of the electrons decreases gradually,which indicates that the presence of the magnetic field can effectively weaken the optically induced localization/backscattering effect of free electrons in monolayer MoS2.4.Terahertz optoelectronic properties of CDs-monolayer hBN heterostructures.Uniformly dispersing CDs on the substrate is essential to realize their device applications.During the dispersion process,due to the hydrophilicity or hydrophobicity of the substrate to the CDs-containing solution,CDs tend to agglomerate on the substrate,thus weakening or even losing their properties.In this dissertation,the technical challenge of CDs agglomeration on the substrate in device preparation was effectively solved by introducing monolayer hBN between the sapphire substrate and the CDs layer,and CDs-monolayer hBN van der Waals heterostructures(vdWHs)were obtained for the first time.The free carrier dynamics properties and terahertz optoelectronic properties of CDs-monolayer hBN heterostructures were investigated using a terahertz TDS system.The conclusions of the study are as follows:Different chemical solvents can achieve different chemical modifications at the edges of carbon cores of CDs through solvation effects;The relevant chemical modifications strongly affect and modulate the electronic and optoelectronic properties of CDsmonolayer hBN vdWHs,especially the vdWHs prepared using green CDs containing more nitrogen-based functional groups and C=O bonds at the edges of the carbon cores have stronger terahertz optoelectronic response properties.By analyzing the temperature dependence of carrier density and relaxation time,we found that the features of carrier transport in CDs-hBN vdWHs are verified to be similar to those of conventional semiconductors.This dissertation focuses on the free carrier dynamics properties and terahertz optoelectronic/magneto-optoelectronic properties of low-dimensional electronic materials,such as monolayer MoS2 and CDs-monolayer hBN heterostructures,by using a terahertz TDS system with transmission spectroscopy measurements.The relevant experimental studies are combined with theoretical studies and theoretical fitting to obtain the key electronic parameters of the material systems under study.In addition,this paper also contains the construction of the optical path of the experimental measurement systems,the preparation and characterization of low-dimensional electronic materials,and the writing of the experimental measurement software and experimental data processing programs.Many of the research results obtained in this paper have important reference value for in-depth understanding of the basic physical properties of the relevant low-dimensional and nano electronic materials,laying the foundation of scientific research for the device applications of these new electronic materials. |
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