It is found that the plasma waves driven by an electron beam rotating in a magnetized overdense plasma can be converted into terahertz electromagnetic radiation by new mechanisms. According to the particle-in-cell simulation results, the radiation modes include a dominant extraordinary (X) mode at twice the plasma frequency [Formula: see text] and a subordinate X mode at [Formula: see text]. The [Formula: see text] radiation can be generated by the coupling of a beam mode and a scattered upper-hybrid (UH) mode, or by the coupling of a beam mode and a right-handed X mode. Here, the beam mode can be a UH mode or a left-handed X mode driven by the beam under the Cherenkov condition, and the right-handed X mode can be induced by high-order electron cyclotron maser instability. The [Formula: see text] radiation is the right- or left-handed fundamental X mode escaping from the plasma boundary. This study also shows that the breakdown of beam modulation is responsible for the radiation attenuation. The scheme proposed in this paper can be applied in high-power THz radiation sources and diagnosis of magnetized plasmas.

Smith–Purcell radiation (SPR) is a phenomenon in that free electrons moving above a periodic structure deliver a part of their kinetic energies to electromagnetic waves. It plays an important role in the development of radiation sources and beam diagnosis. With the development of nanofabrication technology, various complex micro‐nano‐metallic periodic or aperiodic structures have been adopted to improve the efficiency and coherence of SPR. In general, when free electrons move above the metallic periodic structure, the localized surface wave and Smith–Purcell radiation wave are excited simultaneously. As the frequency of the surface wave is always lower than the threshold of Smith–Purcell radiation, the surface wave cannot be radiated. Here, the surface wave is transformed into SPR by using a novel composite subwavelength hole array, thus the SPR is enhanced and it becomes coherent and easy to tune. This work provides a competitive approach to achieve a coherent and high‐efficiency THz radiation source.

Photonic crystal structures have attracted significant attention because of their ability to confine light, especially outgoing waves in a bandgap. In this study, we investigate a layer that can control the Smith–Purcell radiation. The results show that transmission characteristics and quality factor of a 1D photonic crystal can narrow the radiation spectrum and enhance the radiation intensity. From another perspective, the entire structure can be treated as a composite grating, the radiation spectra of which are obtained by theoretical calculations and agree with that from the analysis based on the role of the 1D photonic crystal. Therefore, it can be concluded that the radiation from a composited structure can be simplified by the radiation controlled by the 1D metal photonic crystal, and it provides a fast way to reshape the radiation spectra by designing the transmission characteristic and quality factor of the photonic crystal. Furthermore, a layer with special transmission characteristics and quality factor above a reflection grating can be used to achieve coherent and tunable Smith–Purcell radiation, which is significant for the development of band-controllable light or terahertz radiation sources.

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In the particle-in-cell simulation study of this paper, it is found that the high-power terahertz radiation at twice the plasma frequency can be generated by the interaction between a weakly relativistic, large-radius, focused electron beam and an overdense plasma. The terahertz radiation is emitted from a multi-filament structure in the electron beam, which is caused by the coupling of the longitudinal two-stream instability and the transverse filamentation instability. The analysis of the k space of the plasma waves indicates that the radiation can be explained by the mode coupling among a forward oblique mode, a backward oblique mode, and a second harmonic radiation mode. The influences of the beam Lorentz factor and plasma density on the radiation power, efficiency, and coherence are also investigated. The present results not only contribute to the development of novel megawatt-level narrowband terahertz radiation sources, but also propose a potential terahertz radiation mechanism for the fundamental research of the beam-plasma system.