Gun‐Sik Park

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from <; 0.1 to >; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundament

We report an effective method to enhance and modify the magnetic plasmon (MP) resonance in three-dimensional (3D) optical metamaterials consisting of periodic arrays of silver vertical split-ring resonators (VSRRs) for high-sensitivity sensing. By positioning the 3D metamaterials above a thick silver film separated by a silica dielectric spacer layer, the strong coupling between the MP resonance in the VSRRs and the surface plasmons polaritons (SPPs) propagating on the silver film can be realize

Coating building envelopes with a passive daytime radiative cooling (PDRC) material has attracted enormous attention as an alternative cooling technique with minimal energy consumption and carbon footprint. Despite the exceptional performance and scalability of porous polymer coating (PPC), achieving consistent performance over a wide range of drying environments remains a major challenge for its commercialization as a radiative cooling paint. Herein, we demonstrate the humidity vulnerability of

The authors show that evanescent tunneling transmission of effective surface plasmon polaritons between two counterstreaming electron beams noticeably increases Smith-Purcell radiation (SPR) intensity by about two orders of magnitude as well as lower its transition threshold from a spontaneous emission to a stimulated one. An emission mechanism of the superradiant SPR is theoretically analyzed by the dielectric conversion of the structured metal surface and the boundary matching condition of Max

This paper presents a study of coherent and superradiant Smith-Purcell (SP) radiation with the help of a two-dimensional particle-in-cell (PIC) simulation. The simulation model supposes a rectangular grating with period length of $173\text{ }\ensuremath{\mu}\mathrm{m}$ to be driven by a single electron bunch, a train of periodic bunches and a continuous beam, respectively. We chose 40 keV as the initial energy of electrons and therefore the SP radiation frequency falls in the THz regime. From ou

The circuits for millimeter wave vacuum electron devices with all circuit elements including an electron beam tunnel are microfabricated by two-step deep-etch x-ray lithography (x-ray LIGA). The discrepancies of eigenfrequency between experiment and simulation are within 1.1% in a coupled-cavity structure and 1.4% in a folded waveguide structure when the operating frequency is about 100GHz. Furthermore, a measured tolerance of below 2μm, and a measured surface roughness of 20–70nm, of LIGA-fabri

Fingerprints are unique to primates and koalas but what advantages do these features of our hands and feet provide us compared with the smooth pads of carnivorans, e.g., feline or ursine species? It has been argued that the epidermal ridges on finger pads decrease friction when in contact with smooth surfaces, promote interlocking with rough surfaces, channel excess water, prevent blistering, and enhance tactile sensitivity. Here, we found that they were at the origin of a moisture-regulating me

We present an atomistic simulation scheme for the determination of the hydration number (h) of aqueous electrolyte solutions based on the calculation of the water dipole reorientation dynamics. In this methodology, the time evolution of an aqueous electrolyte solution generated from ab initio molecular dynamics simulations is used to compute the reorientation time of different water subpopulations. The value of h is determined by considering whether the reorientation time of the water subpopulat

Abstract 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

An analysis of the effects of electromagnetic pulses from a high-power microwave (HPM) radiation technique is conducted using a relativistic backward-wave oscillator (RBWO) which uses relativistic electron beams in vacuum circuits. The application described here is based on a relativistic electron device and uses relativistic electron beams to generate high-power electromagnetic radiation. The RBWO was fabricated to operate in a relativistic region with a gamma factor (γ) of 2 at an acceleration

The field emission properties of the controlled emission edge of a vertically aligned graphene-based thin film are presented. A current and current density of above 7 mA and 200 A/cm2, respectively, with uniform electron emission, are achieved. Uniform high current and current density emissions can be realized by the pre-mechanical shaping and post electrical conditioning of reduced graphene oxide (rGO) film emission, owing to the robustness, thinness (<1 μm), and well-defined uniform film thick

The cell-type continuous electromagnetic radiation system is a demonstration device capable of generating high-power millimeter electromagnetic waves of a specific wavelength and observing their effects on living organisms. It irradiates a biological sample placed in a 30 × 30 × 50 cm3 cell with electromagnetic waves in the 3.15-mm-wavelength region (with an output of ≥1 W) and analyzes the temperature change of the sample. A vacuum electronic device-based coupled-cavity backward-wave oscillator

Characteristic x rays generated by a collision between a tungsten target and relativistic electron beams generated from a compact electrostatic accelerator based on a Marx generator and a Blumlein pulse-forming line can induce transient ionizing radiation in electronics. While the decelerated electron beam is close to a collector out of range of an external magnetic field, Bremsstrahlung x rays occur at the same time. Compton scattering also occurs through secondary reaction interactions with qu

The advances in THz VEDs adopting novel photonic concepts such as photonic crystals or plasmonics for efficient beam-wave interaction between convection electrons and THz waves are explored.