Dual-electron-beams Steering Direction Tunable THz Radiation Waves at a Fixed Frequency

Terahertz (THz) waves commonly refer to the electromagnetic (EM) waves between millimeter microwaves and infrared waves. Among the three main ways of generating THz waves, vacuum electronics, especially Smith-Purcell radiation (SPR), have been considered as a popular way due to their unsubstitutability to produce relatively high emission power. 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 changes in the upper and lower space of the grating for different electron velocity, satisfying the SPR relationship. In this study, a Fano resonance metasurface was proposed to steer the direction of the SPR waves at the fixed resonant frequency by changing the velocity of the electric 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 absolute efficiency is normalized by the kinetic energy of the electrons. There is a great consistence of steering radiation angle about 40 degrees by altering the velocity of electron beam from 0. 6c to 0. 95c both in theoretical analysis and effective surface current simulation, where c is the speed of light in vacuum. Our study indicates that the proposed structure can produce direction-tunable THz radiation waves at resonant frequency by varying the velocity of the electric beam, which is promising for various applications in compact, tunable, high power millimeter wave and THz wave radiation sources.