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development of such CR-based applications is limited by thermal and dielectric breakdown issues. [3,4] Another limitation is that, due to the velocity threshold, large facilities are required to generate highly energetic particles. These limitations restrict the upper and lower size and performance boundaries in vital applications. Nowadays, metallic metamaterial devices have been proposed, which overcome the limitations of conventional Cerenkov devices to realize reversed CR from the negative-refractive index, [5] the ultralow threshold of kinetic energy in hyperbolic metamaterials, [6] and various metallic metamaterials comprised of subwavelength slits. [7][8][9] However, the realization of a high quality factor (Q) is still an outstanding roadblock for practical Cerenkov devices. Previous works on Cerenkov lasing (CL) using mirrors were limited by a lower electron beam impedance due to the interaction device and the radio-frequency (RF) coupling mechanism. [4,10] Here, to achieve a high Q for highly efficient CL, we focus on maximally trapping the electromagnetic Cerenkov radiation wave to extend its interaction with the electron beam and on controlling the radiation damping, as has been described in Rayleigh scattering to maximize the efficiency of CL. In this paper, we demonstrate that the interplay between an extremely low group velocity from the infinite transverse permittivity of the anisotropic metamaterials and the subradiant A high-quality-factor (high-Q) metallic Fano metamaterial is demonstrated both experimentally and theoretically. This material is suitable for highly efficient Cerenkov lasing in which subwavelength metallic slits are arranged to form asymmetric unit cells. In contrast to conventional dielectric Cerenkov or Smith-Purcell devices, in the proposed device, convection electrons traverse the high-Q metallic metamaterial. The interplay between an extremely low group velocity from the infinite transverse permittivity of the anisotropic metamaterial and the subradiant damping from a Fano-type slit mode is shown to be responsible for the high-Q value, which is measured to be unprecedented at 700. The resulting improvement in the Cerenkov lasing efficiency is estimated to be more than two orders of magnitude using a particle-in-cell simulation.

electronics. The flexible and rechargeable solid-state Zn-air battery (ZAB) has attracted significant attention as a nextgeneration power source given its advantages of its unique "half-open" cell with a high energy density utilizing inexhaustible atmospheric oxygen, affordability, and eco-friendliness. One of the most critical factors related to flexible solid-state ZABs is the use of a highly deformable solid electrolyte. Thus far, there have been extensive studies of elastomeric hydrogel electrolyte substrates. These hydrogels are coated onto an anode metal framework as a thin film [1][2][3][4][5] or constructed as freestanding blocks [6][7][8][9] and molded architectures. [10][11][12] However, despite much effort, resolving certain critical issues related to hydrogel-based electrolytes for use in ZABs remains challenging. Among these issues, interfacial engineering is important to enlarge the triple-phase boundaries (TPBs) and optimize the wettability so as to facilitate ion transport and suppress the flooding phenomenon in the gas-diffusion layer. [13,14] Recently, Tang et al. demonstrated a landmark cycle life up to 120 h at 5 mA cm −2 due to their use of a compact contact between carbon cloth and a PAA-Fe 3+ -CS/NH 4 Cl hydrogel. [15] In addition, a quaternary ammonia-functionalized cellulose electrolyte membrane with a high degree of hydration due to its hydrophilicity and good dimensional stability in a water-swollen state was recently reported. [2] A thermo-reversible alkaline hydrogel electrolyte was also proposed as a material capable of effectively mitigating the poor electrolyte/electrode contact and mechanical degradation caused by deformation. [16] Another critical problem with ZAB electrolytes is that the water in the hydrogel evaporates through the open cathode, thereby reducing the ZAB performance and lifespan. Thus far, many studies have attempted to improve water retention using hydrogels with high hydrophilicity, such as polyacrylic acid (PAA), sodium polyacrylate (PANa), polyacrylamide (PAM), and bacterial cellulose (Table S1, Supporting Information). Though studies have mainly focused on the development of materials to inhibit water evaporation in the hydrogel when the relative humidity (RH) exceeds 40%, there has been few reports of hydrogel for a dry atmosphere with a RH less than 30%. In order to solve this problem, the development of a hydrogel Flexible and rechargeable Zn-air batteries (ZAB) are promising candidates for attaining the high energy density required for next-generation energy storage devices given their half-open cathode configuration. However, the poor interfacial contact between the immobilized solid-state electrolyte and porous cathode, and drying-out of water in the electrolyte at low relative humidity (RH) of 30% or less, cause low performance and short lifespan. Herein, a novel hygroscopic dough-type electrolyte with water-absorbing and malleable free-standing configuration originating from deliquescent KOH and severe gelation of sodium alginate (SA) by facile knead...