Reversed Cherenkov radiation is the exotic electromagnetic radiation that is emitted in the opposite direction of moving charged particles in a left-handed material. Reversed Cherenkov radiation has not previously been observed, mainly due to the absence of both suitable all-metal left-handed materials for beam transport and suitable couplers for extracting the reversed Cherenkov radiation signal. In this paper, we develop an all-metal metamaterial, consisting of a square waveguide loaded with complementary electric split ring resonators. We demonstrate that this metamaterial exhibits a left-handed behaviour, and we directly observe the Cherenkov radiation emitted predominantly near the opposite direction to the movement of a single sheet electron beam bunch in the experiment. These observations confirm the reversed behaviour of Cherenkov radiation. The reversed Cherenkov radiation has many possible applications, such as novel vacuum electronic devices, particle detectors, accelerators and new types of plasmonic couplers.

—We review the research progress in reversed Cherenkov radiation in double-negative metamaterials (DNMs) starting from the first experimental verification of the DNMs reported in 2001, including theories, numerical computation and simulation and experiments. We also discuss the potential applications to particle detectors and high- power microwave or millimeter-wave devices, including the oscillators and amplifiers, and the formidable challenges needed to be resolved before the benefits of using such artificial materials can be harvested. Corresponding author: Z. Y. Duan (zhyduan@uestc.edu.cn). 76,Duan et al.

Metamaterials (MTMs) are structured materials with subwavelength features that can be engineered to have some unique properties not found in nature, such as negative refractive index, reversed Doppler effect, and reversed Cherenkov radiation. Based on these novel MTMs, several important research groups have made great attempts to develop novel MTM-inspired vacuum electron devices (VEDs) and accelerators. Just as solid-state power devices are innovated by incessant emerging of new semiconductor materials, VEDs can also be inspired by MTMs to have very remarkable advantages, such as smaller size, higher power, higher efficiency, and/or larger gain relative to conventional VEDs, such as traveling-wave tubes, backward-wave oscillators, and klystrons. Similarly, relative to conventional accelerators, MTM-inspired accelerators have obvious advantages, such as smaller size and higher accelerating gradient. Furthermore, MTM-inspired devices have promising applications in areas such as radar, communication, electronic warfare, microwave heating, and imaging.

In this paper, we have proposed a metamaterial (MTM) which is suitable for the compact high-power vacuum electron devices. For example, an S-band slow-wave structure (SWS) based on the all-metal MTMs has been studied by both simulation and experiment. The results show that this MTM SWS is very helpful to miniaturize the high-power vacuum electron devices and largely improve the output power and the electronic efficiency. The simulation model of an S-band MTM backward wave oscillator (BWO) is built, and the particle-in-cell simulated results are presented here: a 2.454 GHz signal is generated and its peak output power is 4.0 MW with a higher electronic efficiency of 31.5% relative to the conventional BWOs.

We theoretically study reversed Cherenkov radiation (CR) in anisotropic double-negative metamaterials (DNMs) in general, and particularly in detail for one of the most practical cases, i.e., CR in a waveguide partially filled with anisotropic DNMs. The theory presented here provides a theoretical basis for possible experiments and potential applications. As an example, we discuss the physical properties of CR and the potential applications such as particle detectors and high-power sources.

The physical properties of Cherenkov radiation (CR) are theoretically investigated for a charged particle traveling along the axis of a cylindrical waveguide filled with anisotropic double-negative metamaterials (DNMs). The reversed CR and CR conditions are obtained using analytical method. The influence of the particle velocity, the waveguide radius, and the constitutive parameters of the anisotropic DNMs are discussed. A numerical example illustrates that the total radiated energy increases with increasing particle velocity, the radiated energy spectral density has different poles at the different frequencies for different anisotropic DNMs when the loss of the anisotropic DNM is smaller, and when the radius has the same order as the operating wavelength, the influence of the waveguide radius on the total radiated energy is smaller on the whole. Since most of the metamaterials realized so far are anisotropic, our theoretical work based on anisotropic DNMs will be helpful for future experimental realizations.