THz Studies Of Van Der Waals Two-dimensional Materials

With human beings stepping into the information age from the traditional electric one,the demand for information is increasing dramatically,which puts forward higher requirements for the development of materials and devices in corresponding industries.Traditional electrical materials and devices have encountered great challenges.Therefore,people are trying to explore new materials to break the existing technical bottlenecks.Since graphene was successfully discovered in 2004,it has attracted wide attention from researchers of all over the world for its excellent electrical,optical and mechanical properties,which are usually different from bulk graphite.Since then,van der Waals two-dimensional electronic materials and devices with atomic layer scale are regarded as the most promising candidates for breaking through the current technical limit,and one of the key research directions in this area.However,in study of the two-dimensional materials,there exist some issues that cannot be solved by previous traditional research approaches.For example,these two-dimensional electronic materials and devices are usually very sensitive and vulnerable to be easily damaged in the testing and fabrication processes.Moreover,infrared spectroscopy,Raman spectroscopy and other traditional optical testing methods are difficult to be employed to effectively identify and detect the weak interactions in such materials.Therefore,a low-energy,high-resolution,and non-invasive measurement approach is required in study of these materials.Terahertz(THz)spectroscopy is a sharp tool for studying of two-dimensional electronic materials and micro/nano structure devices for its special frequency band,low energy(meV),high sensitivity and ultra-fast detection capability.In addition,some breakthroughs have been made in the research of some typical two-dimensional electronic materials.Terahertz Waves(THz),also known as sub-millimeter wave,refers to the electromagnetic waves with frequency in the band of 1011 to 1013 Hz,namely between the bands of microwaves and far-infrared range.In recent decades,with the numerous development and maturity of the terahertz technology,corresponding terahertz gap in the whole spectral range has been filled.The terahertz band is between macro-electromagnetics and micro-photo-electronics.It possesses some unique properties and advantages that other electromagnetic waves do not have.Therefore,it possesses very important theoretical value and application potential and has attracted more and more attentions from scientific researchers.Predictably,it will bring revolutionary influence on the fields of future communications,military,security,medical imaging,astronomical remote sensing,and biochemical detection,etc.Terahertz time-domain spectroscopy(THz-TDS)is a coherent detection technology and a typical example of the development of terahertz technologies.This technology takes on the characteristics of strong penetration,high sensitivity,high spectral resolution,high time resolution and non-destructive.In recent years,it has become some burgeoning research areas,especially in the studies of most sensitive and vulnerable two-dimensional electronic materials and devices.It has been one of the important technical methods.In this thesis,with the support of the National Natural Science Foundation of China and the National Key Research and Development Project,a magneto-optical terahertz time-domain spectroscopy system was built.And choose the currently hot two-dimensional electronic materials,propose the scientific problems in these materials,and exert research on them mainly with terahertz time-domain spectroscopy system.This paper consists of the following six parts:The first chapter gives a general introduction on terahertz technology and its applications in two-dimensional material research.In the first section,the development of two-dimensional electrons and the necessity of terahertz research are briefly introduced.The second section gives a brief introduction to the development history of terahertz waves.The third section first illustrates the characteristics and advantages of terahertz waves.Afterward,it systematically introduces the development and application of terahertz technology in various fields.The third section follows the development history of two-dimensional materials in recent years and introduces the development of two-dimensional zero band gap materials,two-dimensional non-zero band gap materials,and two-dimensional non-zero bandgap magnetic materials.Combined problems and difficulties encountered with the research of two-dimensional electronic materials,it introduces the opportunities and current progresses of terahertz spectroscopy in the research of two-dimensional electronic materials.The second chapter introduces the construction and expansion process of the terahertz time-domain spectroscopy system in detail.Specifically,it firstly introduces the generation and detection methods of terahertz waves that are currently widely used.The second section introduces the construction process of the optical path of the terahertz time-domain spectroscopy system.The third section realizes the drive,signal collection and preliminary data processing of the terahertz time-domain spectroscopy system with the NI Labview program.The last section gives a systematic introduction to the analysis of the terahertz signal and the method of extracting the optical,electrical and magnetic information of the terahertz band with the subsequent data processing.In Chapter 3,based on graphene,a two-dimensional electronic material with zero band gap,a prototype of strain driven terahertz modulator is designed and prepared.A self-built terahertz time-domain spectroscopy system was applied to systematically study its strain modulation characteristics.Detailed studies have shown that the device possesses excellent modulation effects.Firstly,the modulation depth is large.With the deformation of～1%,corresponding modulation depth at 1 THz can reach 26%.Secondly,it can realize both positive and negative modulation effects.Thirdly,benefiting from its low insertion loss,the device possesses the feature of low power loss.Finally,it also demonstrates excellent repeatability and stability.In Chapter 4,the two-dimensional ferromagnetic magnetic semiconductor material,Cr2Ge2Te6,is selected as the research object.Its responses under different temperatures and magnetic fields were characterized by THz-TDS system.As a result,an efficient monochromatic terahertz wave irradiation has been observed.And based on the analysis of its features,we found that the effect is an electromagnetic wave radiation mechanism based on the phonon-polaritons.The study in chapter 5 is based on the two-dimensional antiferromagnetic semiconductor material MnPS3.We studied its ultrafast nonlinear optical effect induced by the terahertz pump.Corresponding results indicate that for MnPS3 materials,the ultrafast optical effect mainly originates from the non-linear Kerr-type electro-optic effect and has nothing to do with its magnetic properties.In addition,the ultrafast optical effect possesses significant anisotropy that strongly correlated with the lattice symmetry of the MnPS3 crystal structural.Finally,the difference between the results of THz pump and infrared pump was also compared discussed.Chapter 6 is the summary and follow-up prospect of the full text.