Research On The Generation Of Terahertz Radiation From Graphene Excited By An E-Beam

Terahertz(THz)radiation source is one of the main bottlenecks in the development of THz science and technology.The combination of electronics and photonics to generate terahertz radiation has gradually become a research hotspot in THz science.In this dissertation,based on the two-dimensional material graphene,some related theoretical researches and preliminary experimental studies on surface plasmon polaritons(SPPs)generated by an electron beam and transformation SPPs to THz radiation have been carried out.The main findings are as follows:1.The physical process of transforming graphene SPPs into Cherenkov radiation through a graphene/dielectric/substrate structure in the THz regime excited by a parallel electron beam is studied.The graphene SPPs can be converted into coherent Cherenkov radiation by loading an intermediate dielectric layer between the graphene and the dielectric substrate.The radiation frequency can be tuned by adjusting the chemical potential of graphene,the speed of the electron beam,and the dielectric and thickness of the intermediate dielectric layer.Compared with the Cherenkov radiation in the medium directly by parallel electron beam without graphene and without an intermediate dielectric layer,the coherent Cherenkov radiation from graphene SPPs excited by an electron beam is greatly enhanced.THz waves can be well modulated by adjusting the grating depth and the diffraction order of the dielectric grating.2.Based on graphene hyperbolic metamaterial composed of graphene-dielectric multi-layer films,the THz radiation generated by an electron beam and an electric dipole are studied.When the velocity of parallel electron beam satisfies some certain conditions,the graphene SPPs excited in the graphene hyperbolic metamaterial can be converted to THz radiation.The THz radiation comes from graphene SPPs couplings in the graphene hyperbolic metamaterial generated by the electron beam.In addition,studies on the generation of THz radiation from an electric dipole have been carried out.It shows that enhanced THz radiation with special angular distribution in graphene hyperbolic metamaterial can be excited by an electric dipole in comparison to conventional media.The enhanced THz radiation comes from graphene SPPs couplings in the graphene hyperbolic metamaterial generated by the electric dipole.3.Based on THz time domain spectroscopy,the conductivity of graphene in the THz band is measured by transmission and reflection measurements,and the spatial distribution of graphene conductivity is imaged.The two measuring methods are compared in detail by Drude-Smith formula fitting.Also,the change of spatial conductivity of graphene under laser excitation is studied.The results show that laser excitation can generate photo-generated carriers in graphene,and photo-generated carriers transport in graphene to neighboring regions.This is of great significance for studying the electrical and optical properties of graphene and designing graphene optoelectronics THz devices.In addition,a simple back gated graphene field effect transistor is fabricated and the preliminary researches on the basic electrical properties of graphene are conducted.The dirac point,carrier concentration,Fermi energy,and mobility of graphene are acquired.The results are of great significance for the subsequent research on the generation of THz radiation from graphene by an electron beam.4.Based on THz time-domain spectroscopy system and two-dimensional scanning control system,the effects on the THz conductivity of graphene under electron beam excitation are studied,and the changed spatial THz conductivities of graphene by different voltage electron beam excitations are imaged.The spatial conductivity imaging results show that under different current electron beam excitations,the electron doping speeds vary differently.The Drude fits of the decreased graphene THz conductivity under different current irradiations demonstrate that the graphene carrier densities and the carrier relaxation times are affected,where the larger current irradiation induces a greater change in carrier density and a shorter relaxation time.The decreased graphene THz conductivity induced by e-beam irradiations is the result of a combination of the changes in carrier density and relaxation time.Besides through e-beam irradiations a larger variance in graphene THz conductivity is achieved than through optical excitation,indicating a greater tuning of graphene properties.The findings are of significant value for understanding the carrier transport with non-equilibrium conditions in graphene under e-beam irradiations and for the development of graphene-based optoelectronics THz devices.