Coherent Terahertz Radiation from Multiple Electron Beams Excitation within a Plasmonic Crystal-like structure

Steering Smith-Purcell radiation angle in a fixed frequency by the Fano-resonant metasurface

Smith-Purcell radiation (SPR) is a kind of electromagnetic wave radiation that happens when an energetic beam of electrons passes very closely parallel to the surface of a ruled optical diffraction grating. The frequency of radiation waves varies in the upper and lower space of the grating for different electron velocity, satisfying the SPR relationship. In this study, a Fano-resonant metasurface was proposed to steer the direction of the SPR waves at the fixed resonant frequency by changing the velocity of the electron beam without varying the geometric parameters or adding extra coupling structure. The maximum emission power always locates at the resonant frequency by utilizing the integration of the Poynting vector. The relative radiated efficiency can reach to a maximum value of 91% at the frequency of 441 GHz and the efficiency curve has a dip when the direction of SPR is nearly vertical due to the high transmission. There is a great consistence of steering radiation angle from 65 degrees to 107 degrees by altering the velocity of electron beam from 0.6c to 0.95c both in analytical calculation and PIC (particle-in-cell of CST) simulation at terahertz frequencies, where c is the speed of light in vacuum. Furthermore, the destructive interference of Fano resonance between the magnetic mode and the toroidal mode shows the underlying physics of steering SPR in a fixed frequency. Our study indicates that the proposed structure can produce direction-tunable THz radiation waves at resonant frequency by varying the velocity of the electron beam, which is promising for various applications in a compact, tunable, high power millimeter wave and THz wave radiation sources.

Free-electron-driven beam-scanning terahertz radiation

A beam-scanning terahertz (THz) radiation mechanism in a free-electron-driven grating system is proposed for THz applications. By loading a period-asynchronous rod array above the grating, the spoof surface plasmon (SSP) originally excited by the electron changes its radiation characteristics owing to the rod-induced Brillouin zone folding effect. The rod array functions as an antenna and converts the SSP into a spatial coherent THz radiation. The radiation frequency and direction can be precisely controlled by the electron energy. The field intensity of the radiation is increased approximately 20 times compared with that of the conventional Smith-Purcell radiation in the same frequency range. In addition, a microwave-band scaling prototype is fabricated and the frequency-controlled radiation is measured. Excellent agreement between the experimental and simulated results is obtained. This study paves the way for the development of on-chip THz sources for advanced communication and detection applications.

Terahertz Radiation from Combined Metallic Slit Arrays

We report an approach to efficiently generate terahertz radiation from a combined periodic structure. The proposed configuration is composed of two metallic slit arrays deliberately designed with different periodic length, slit width and depth. We found that the combination of the two slit arrays could provide special electromagnetic modes, which exhibit nonradiative property above the surface of one slit array and radiative property inside the other one. An electron beam holding proper energy could resonate with those modes to generate strong and directional electromagnetic radiations in the terahertz regime, indicating that the approach has the potential in developing high-performance terahertz radiation sources.