Characteristics Research Of Free Electron Radiation Based On Phase Gradient Metasurface

Metasurfaces are artificially engineered two-dimensional interfaces formed by a periodic or aperiodic array of subwavelength meta-atoms.And metasurfaces have become a very important research focus in nanophotonics because they offer a new way for realizing highly compact planar optical devices.Unlike conventional optical devices that rely on phase accumulation in bulk materials,metasurfaces present an unprecedented capability to manipulate the frequency,amplitude,phase,and polarization of electromagnetic waves over a subwavelength thickness.Additionally,the fabrication of metasurfaces is less cumbersome,and their associated losses with respect to bulk metamaterials are lower.In recent years,several research efforts have been focused on exploiting the ability of metasurfaces to comprehend the interactions between structured matter,electrons,and photons.Since the radiation in charged particles is highly dependent on the surrounding matter,artificially fabricated metasurfaces provide the opportunities to facilitate the interaction between charged particles and matter,resulting in the efficient manipulation of radiated light.The development of high-sensitivity and small-sized detectors provides new sources of electromagnetic radiation.Utilizing metasurfaces to fully control the degrees of freedom of electron-radiated light,such as frequency,polarization and amplitude,will further promote the application of on-chip electromagnetic radiation sources and detectors.This thesis is mainly based on the phase gradient metasurface to control the wavelength,direction,polarization and wavefront of free electron radiation.A waveguide-integrated metasurface composite structure is designed to modulate the polarization,wavelength,and direction of Cherenkov diffraction radiation.First,the multidirectional flexible control of Cherenkov diffraction radiation is realized by using waveguideintegrated supergratings.By changing the period of the grating and the propagation direction of the electron beam,the flexible adjustment of-75° to 75° is achieved.Then,in order to control the polarization of the Cherenkov diffraction radiation,the grating is replaced with a phase gradient metasurface.And the polarization and diffraction quantity of the Cherenkov diffraction radiation can be adjusted based on the geometric phase gradient metasurface.At the same time,Cherenkov diffraction radiation with linear polarization,left-handed or right-handed circular polarization can be produced in the separate radiation directions,which promotes the development of on-chip free electron radiation sources.In order to modulate the wavefront of free electron radiation,a simple cascaded metasurfaces are designed to generate vector vortex Smith-Purcell radiation.By controlling the local orientation and geometric parameters of the first nanograting,the linearly polarized and circularly polarized electron radiation is regulated respectively.Then the structure arrangement of the second metasurface is changed to generate a vector vortex beam.The tunable vector vortex Smith-Purcell radiation is realized by cascading two metasurfaces.At the same time,the chirality conversion of vector vortex beam circular polarization is accompanied by the change between positive and negative vortices.This work provides a new method for generating vortex electron radiation and is beneficial to advance the development of free electron driven devices.