Intensive vertical orientation Smith-Purcell radiation from the 2D well-array metasurface

Rigorous coupled-wave analysis of unilateral Smith-Purcell radiation from asymmetric resonators

The Smith-Purcell radiation produced by electrons moving closely to a grating can be enhanced by resonances. Here, we show a method to manipulate the directionality of the resonance-enhanced radiation. Using the rigorous coupled-wave analysis method, we compare the radiation from symmetric and asymmetric gratings, showing that the enhanced Smith-Purcell radiation can become unilateral with a perturbation that breaks the structural symmetry. Our work provides an effective method for frequency-domain calculation of Smith-Purcell radiation and also an approach to realize more efficient use of the radiation.

Engineering tunable terahertz radiation from an electron bunch using graphene metasurfaces

We propose an approach to generate tunable terahertz (THz) radiation from an electron bunch passing over the unique graphene metasurface. We not only control the frequency of the THz radiation but also tune the amplitude and direction of the radiation by varying the chemical potential of the graphene. Several new phenomena are observed. The radiation has the same frequency with the resonant frequency of the graphene metasurface at normal incidence. The radiation frequency meets the linear relationship with the chemical potential. The radiation magnitude is the inverse to the reflection magnitude, and the sum of them is close to being a constant. The strong Smith-Purcell radiation on the graphene metasurface is due to the interaction between the electron bunch and periodic surface plasmon polaritons (SPPs). The stronger the SPP, the higher is the radiation magnitude that is obtained. These results would provide a promising way for developing tunable radiation in the THz band.

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.

Smith-Purcell radiation based on the transmission enhancement of a subwavelength hole array with inner tunnels

The use and control of the extraordinary optical transmission through subwavelength hole arrays has enormous application potential in photonic devices. In this paper, we propose a subwavelength hole array with inner tunnels, for which the Smith-Purcell radiation (SPR) with this enhanced transmission phenomenon in THz is excited when the transmission peak locates in the SPR band. The SPR is monitored using particle-in-cell simulations in order to analyze the mechanisms responsible for improving the radiation coherence. Analysis of the electron energy loss reveals that the proposed subwavelength hole array with inner tunnels outperforms a conventional subwavelength grating array with respect to SPR generation efficiency. As SPR plays a significant role in research on particle diagnosis and terahertz radiation sources, the performance of the proposed structure suggests that it has high application potential.