Research On Terahertz Radiation Generation From Metamaterials Excited By Free-electron

The interaction between the Coulomb field of a moving charged particle and the surrounding matter can produce radiation.This phenomenon has a wide range of applications in the fields of physics,electron microscopy,optics,biology and materials science.This free-electron-excited radiation depends heavily on the properties of the surrounding matter.Artificial electromagnetic metamaterials have many properties that are not found in natural materials and have unique advantages in the field of manipulating radiated light.Therefore,the study of combining vacuum electronics and artificial electromagnetic metamaterials to produce coherent,tunable,high power density THz radiation is of great scientific value for THz radiation sources.Based on the above considerations,dissertation investigates electronically excited metamaterials to generate THz radiation from both theoretical and experimental aspects.The main studies are as follows:1.When electrons are passing through the surface of a periodic structure,excitation radiation,called Smith-Persell radiation,can be produced.Smith Purcell radiation,whose radiation frequency is dependent on the angle of observation and the velocity of the electron beam,is incoherent.With the discovery of this effect for more than 60 years,Smith Purcell radiation has been an important method for generating THz radiation in electronics,but they all had to solve the problem of incoherence.Many inverse physical phenomena such as negative refraction,reverse Cherenkov and inverse Doppler can be realized in left-handed materials.Therefore,dissertation realizes the reverse Smith Purcell radiation phenomenon with a photonic crystal with negative refraction index and achieves the modulation of Smith Purcell radiation angle by designing the photonic crystal structure.In this dissertation,the reverse Smith Purcell radiation effect excited by free electron is realized,revealing another inverse physical phenomenon in negatively refractive materials.The study of reverse Smith Purcell radiation is important for the development of free-electron based radiation sources.2.Meanwhile,dissertation proposes that anomalous Smith Purcell radiation can also be achieved by a free electron beam in a homogeneous medium substrate.Theoretical calculations and electromagnetic simulations show that when the electron velocity exceeds the phase velocity in a homogeneous medium,two types of anomalous Smith Purcell radiation exist: 1)special Smith Purcell radiation generated by the superlight normal Doppler effect frequency shift of the electron beam;2)reverse Smith Purcell radiation generated by the superlight inverse Doppler effect frequency shift of the electron beam.The results of dissertation show that the electron velocity can be adjusted to simultaneously excite normal Smith Purcell radiation,special Smith Purcell radiation,and reverse Smith Purcell radiation.The reverse Smith Purcell radiation effect in a positive refractive index medium breaks the traditional physical concept and provides new ideas for the generation of monochromatic multi-output radiation sources by free electrons.3.Dissertation achieves efficient threshold-less in-plane Cherenkov radiation using a periodic graphene hyperbolic grating structure.Numerical theoretical calculations show that due to the biaxially induced hyperbolic property in graphene hyperbolic gratings,the phase velocity in graphene hyperbolic gratings is always lower than the electron velocity,and thus is capable of generating threshold-less Cherenkov radiation.According to the simulation results,the power of in-plane Cherenkov radiation in graphene hyperbolic gratings is nearly two orders of magnitude larger than that of out-of-plane Cherenkov radiation in conventional graphene-based hyperbolic metamaterial.The Cherenkov radiation angle and intensity can be completely controlled by adjusting the structural size,graphene chemical potential and free electron energy of the graphene hyperbolic grating.4.In order to further investigate the terahertz properties of free-electron excited graphene metamaterials experimentally,a SEM-TDS system based on scanning electron microscopy and terahertz time-domain spectroscopy was constructed.It is able to realize the precise modulation of terahertz graphene metamaterials by free electrons.The terahertz absorption rates of graphene metamaterials with different graphene chemical potentials were also studied by electromagnetic simulations,which were in high agreement with the experimental results.In this system,the electron beam size of the scanning electron microscope can be tuned to a small size,so that the SEM-TDS system has good advantages for precisely tuned experiments on electrode-free metamaterials.This creative approach through free-electron modulation of materials paves the way for the study of free electrons combined with photonics research and the realization of onchip integrated and precisely tunable terahertz modulated devices.5.Metal grating structures are capable of generating local surface plasmon polaritons(LSP)in the optical frequency band by free-electron excitation.In this dissertation,the LSP-enhanced Smith Purcell radiation produced by a free-electron excited gold grating is investigated by electromagnetic simulations and experiments.In order to investigate the relationship between different observation angles and radiation frequencies,the Zeiss EVO 18 scanning electron microscope is modified.The Smith Purcell radiation produced by the free electron parallel excitation gold grating and the LSP-enhanced Smith Purcell radiation effect produced by the free electron tilted incidence excitation gold grating were experimentally verified.The LSP-enhanced Smith Purcell radiation spectra of different sample rotation angles at different viewing angles and electron voltages were also analyzed.The experimental verification of the LSP-enhanced Smith Purcell radiation produced by the free-electron excited gold grating further explains the electron-matter interaction mechanism.And this experimental verification lays the foundation for the development of free-electron-generated enhanced terahertz radiation sources.