Slow-wave Devices Research Articles

A Reconfigurable Three-Dimensional Electromagnetically Induced Transparency Metamaterial with Low Loss and Large Group Delay

In this paper, a solid-state plasma (SSP) metamaterial for an analog of the electromagnetically induced transparency phenomenon is designed and investigated. This electromagnetically induced transparency metamaterial has the ability to interact with both incident electric and magnetic fields, and its low-loss characteristics, slow-wave effect, band reconfigurability, and polarization-insensitive characteristics are researched and explored. According to the tunable SSP, we have successfully implemented two modes of operation (mode 1 and mode 2) by whether the SSP resonance unit is excited or not. Low-loss characteristics and polarization-insensitive properties are achieved by rotating the split-ring resonator (SRR) by 180° in the plane and rotating the overall plane framework 90° to form a three-dimensional structure. After that, the maximum group delay of 261.51 ps and 785.09 ps as well as the delay bandwidth product of 17.51 and 62.96 at mode 1 and mode 2, respectively, are discussed respectively. This indicates a good slow-wave effect as well as a high efficiency of communication devices. After all, in mode 1, a transmission peak at 0.541 THz is observed for a transmission ratio of 92.05%; and in mode 2, a transmission peak at 0.741 THz is observed for a transmission ratio of 93.01%, resulting in a bandwidth shift of 0.2 THz. Due to the uniqueness of the developed metamaterial, it holds potential for a wide range of applications in slow-wave devices, modulators, sensors, and communications equipment.

Translation of an existing implantable cardiac monitoring device for measurement of gastric electrical slow-wave activity.

Despite evidence that slow-wave dysrhythmia in the stomach is associated with clinical conditions such as gastroparesis and functional dyspepsia, there is still no widely available device for long-term monitoring of gastric electrical signals. Actionable biomarkers of gastrointestinal health are critically needed, and an implantable slow-wave monitoring device could aid in the establishment of causal relationships between symptoms and gastric electrophysiology. Recent developments in the area of wireless implantable gastric monitors demonstrate potential, but additional work and validation are required before this potential can be realized. We hypothesized that translating an existing implantable cardiac monitoring device, the Reveal LINQ™ (Medtronic), would present a more immediate solution. Following ethical approval and laparotomy in anesthetized pigs (n = 7), a Reveal LINQ was placed on the serosal surface of the stomach, immediately adjacent to a validated flexible-printed-circuit (FPC) electrical mapping array. Data were recorded for periods of 7.5 min, and the resultant signal characteristics from the FPC array and Reveal LINQ were compared. The Reveal LINQ device recorded slow waves in 6/7 subjects with a comparable period (p = 0.69), signal-to-noise ratio (p = 0.58), and downstroke width (p = 0.98) to the FPC, but with reduced amplitude (p = 0.024). Qualitatively, the Reveal LINQ slow-wave signal lacked the prolonged repolarization phase present in the FPC signals. These findings suggest that existing cardiac monitors may offer an efficient solution for the long-term monitoring of slow waves. Translation toward implantation now awaits.

Ideal acoustic quantum spin Hall phase in a multi-topology platform

Fermionic time-reversal symmetry ({T}_{f})-protected quantum spin Hall (QSH) materials feature gapless helical edge states when adjacent to arbitrary trivial cladding materials. However, due to symmetry reduction at the boundary, bosonic counterparts usually exhibit gaps and thus require additional cladding crystals to maintain robustness, limiting their applications. In this study, we demonstrate an ideal acoustic QSH with gapless behaviour by constructing a global Tf on both the bulk and the boundary based on bilayer structures. Consequently, a pair of helical edge states robustly winds several times in the first Brillouin zone when coupled to resonators, promising broadband topological slow waves. We further reveal that this ideal QSH phase behaves as a topological phase transition plane that bridges trivial and higher-order phases. Our versatile multi-topology platform sheds light on compact topological slow-wave and lasing devices.

Trapped mode resonances in symmetric rectangular-hole tetramers

High Q-factor trapped mode resonances are mostly supported by weakly asymmetric metamolecules. In this paper, we study theoretically and experimentally a planar all-metallic metamaterial comprising highly symmetric metamolecules (rectangular-hole tetramers in a freestanding metallic plate) and find that high Q-factor trapped mode resonances can also be realized. The effect comes from the destructive interference between two anti-phased excitations: the electric dipole modes of the two inner and two outer rectangular holes within individual tetramers. Here, the high Q-factor resonance is dominated by the hole separation that affects greatly the coupling and radiation. At the resonance, a huge enhancement of electric field in each hole appears, accompanied by a significant slow-wave effect in the reflection process. Our design has potential applications in constructing high Q-factor filters, highly sensitive sensors and slow-wave devices.

Simultaneous realization of electromagnetically induced transparency and electromagnetically induced reflectance in a metasurface.

The analogue of electromagnetically induced transparency (EIT-like) and electromagnetically induced reflectance (EIR-like) effects have been intensively studied and achieved by using metasurfaces. Nevertheless, previous designs could realize only one of them and were unable to support both effects in a metasurface. Here we numerically and experimentally demonstrate a metasurface simultaneously exhibiting EIT-like and EIR-like effects. Qualitative analyses and quantitative calculations based on the electromagnetic multipole decomposition method are performed to reveal their formation mechanisms. Our work offers a simple avenue for simultaneously realizing EIT-like and EIR-like effects in a metasurface, which may find potential applications in sensing, filtering, and slow wave devices.

A High-Current-Density Terahertz Electron-Optical System Based on Carbon Nanotube Cold Cathode

A high-current-density terahertz electronoptical system based on a carbon nanotube (CNT) cold cathode has been investigated in order to solve notable mode competition in high-order harmonic gyrotrons. Simulation results show that a near-axis electron beam can be generated, with a beam current of 160 mA at an accelerating voltage of 23.5 kV. The source supports the electron beam with a velocity ratio of 1.1 and a guiding center radius of 57 μm. A narrow beam spatial distribution is evidenced by adjusting control anode voltage, satisfying the need for high operating current density in slow wave devices. The current density of the electron beam is up to 620 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the output port and the velocity ratio is 0.44, with parallel energy accounting for 84% of the total energy.

Electromagnetically induced transparency metamaterial with polarization independence and multi-transmission windows.

In this paper, an electromagnetically induced transparency (EIT) metamaterial with the performances of polarization independence and multi-transmission windows is proposed. First, we design an EIT with a single transmission window, which is composed of an H-shaped structure and two split-ring resonators (SRRs). Then, by sequentially rotating the unit by 90°, a new EIT structure with rotational symmetry is obtained. The results show that the new EIT structure has the characteristics of polarization independence and multiple transmission windows, and each transmission window has a high maximum transmittance and group index. The first transmission window has a maximum group index of 88.9 and 98.9% maximum transmission. The maximum group indices of the second and third transmission windows are 117.9 and 215.3, and the peak transmissions are 89.9% and 97.4%, respectively. The multiple transmission windows with polarization independence widen the application scope of the proposed EIT metamaterial and are suitable for high-performance slow-wave devices with the above two requirements.

Tunable plasmon-induced transparency with a dielectric grating-coupled graphene structure for slowing terahertz waves.

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

A planar metamaterial based on metallic rectangular-ring pair for narrow electromagnetically induced transparency-like effect

We design numerically and demonstrate experimentally a planar metamaterial with a narrow band electromagnetically induced transparency (EIT)-like resonance at microwave frequencies. The meta-molecule, consisting of a pair of metallic rectangular rings connected by a top metallic strip, is mirror symmetric with respect to the exciting electric field of the normally incident wave. The coupling between a broad bright mode (electric dipole) of the whole meta-molecule and a sharp dark mode (electric quadrupole) belonging only to the rectangular-ring pair can efficiently induce an ultra-narrow EIT-like resonance. By using a coupled-resonator model that shows good accordance with the numerical and experimental results, the observed effect can be well explained. Our design provides another way to realize the sharp EIT-like behavior with strong dispersion and the slow-wave effect, which may find potential applications in constructing the slow-wave devices, filters, and sensors.

Microscopic Analyses of Electrical Conductivity of Micromachined-Folded Waveguides Based on Surface Roughness Measurement for Terahertz Vacuum Electron Devices

Manufacturing slow-wave vacuum electron device (VED) components operating in a terahertz (THz) region required the use of proper techniques and detailed plans. Moreover, how to fabricate components and evaluate manufactured ones are essential in identifying defects, checking their performance, and finding what needs to be improved. This study introduces estimated conductivity based on the surface roughness and evaluations of fabricated folded waveguides (FWGs), which is one of the THz slow-wave VED components, using nanoscale precision computer numerical control (nano-CNC) machining. Produced circuits operating in the G-band (0.11–0.3 THz) and Y-band (0.325–0.5 THz) were diagnosed in terms of the RF transmission loss, achievable surface roughness, tolerance, and flatness. During testing, our devices exhibited the highest reported $R_{a}$ (arithmetic mean surface roughness) at less than 11.17 nm in the Y-band. The conductivities based on the measured surface roughness were estimated in the G-band and the Y-band. Through a simulation and an experiment, we analyzed the reasons for the losses caused by the reduced electrical conductivity. The main contribution of this study is the demonstration of an achievable conductivity of the THz FWGs fabricated with nano-CNC machining and an improved surface finishing capability of the fabrication technique using lubricants.

Slowing designer surface plasmons in a surface-wave photonic crystal.

We propose and experimentally demonstrate a broadband slow-wave system in a surface-wave photonic crystal based on a non-uniform line-defect waveguide with graded metallic pillars, whose dispersions and cutoff frequencies vary gradually along the waveguide. Since the group velocity of designer surface plasmons at the cutoff frequencies is zero, we show that surface waves can be slowed and stopped at different positions under different frequencies. Experimental evidence including transmission measurements and direct near-field profile imaging performed in the microwave frequencies validate the stopping of surface waves in a broadband range within the forbidden bandgap of the surface-wave photonic crystal. This proposal is a promising candidate for slow-wave devices implemented on a single metal surface in both the microwave and terahertz frequency ranges.

Design of mm-wave slow-wave devices with sheet and hollow electron beams

The frequency increase of the slow-wave electron devices is accompanied by inevitable increase of the electron current density and ohmic losses which strongly restricts the attainable power. In recent years, the use of the spatially-developed sheet electron beams is considered as the major way to develop the medium-power slow-wave devices at mm and sub-THz waves. Hollow electron beams is another configuration which could be used for this purpose. The designs of the oscillators and amplifiers in Ka-band and W-band with both sheet and hollow electron beams are considered and compared.

A Dielectric-Thickness-Adjusting Method for Manipulating Graphene Surface Plasmon Polariton

We propose and numerically investigate a dielectric-thickness-adjusting method to manipulate the graphene surface plasmon polariton (SPP). The dispersion relationships of graphene SPP at different dielectric thickness are derived by solving the analytic equations. In addition, the SPP effective index at cutoff dielectric thickness is obtained according to different dielectric permittivity and working frequencies. As a typical application, a plasmonic Bragg reflector is designed by alternately depositing dielectric gratings along the transverse direction of the SPP propagation. The performance of the Bragg reflector is analyzed at different grating thickness, and the effective index at cutoff thickness is verified by numerical simulation. The proposed method will have important potential prospects in designing graphene-based wave trapping and slow wave devices in future.

Interaction of an Ultrarelativistic Electron Bunch Train with a W-Band Accelerating Structure: High Power and High Gradient.

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to the interference of the wakefields from the two bunches, was measured as a function of bunch separation. Measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.

Trapping surface plasmon polaritons on ultrathin corrugated metallic strips in microwave frequencies.

It has been demonstrated that an ultrathin uniformly corrugated metallic strip is a good plasmonic waveguide in microwave and terahertz frequencies to propagate spoof surface plasmon polaritons (SPPs) with well confinement and small loss (Shen et al., PNAS 110, 40-45, 2013). Here, we propose a simple method to trap SPP waves on the ultrathin corrugated metallic strips in broad band in the microwave frequencies. By properly designing non-uniform corrugations with gradient-depth grooves, we show that the SPP waves are slowed down gradually and then reflected at pre-designed positions along the ultrathin metallic strip when the frequency varies. We design and fabricate the ultrathin gradient-corrugation metallic strip on a thin dielectric film. Both numerical simulation and measurement results validate the efficient trapping of SPP waves in broadband from 9 to 14 GHz. This proposal is a promising candidate for slow-wave devices in both microwave and terahertz regimes.

Dispersion properties of plasma-filled metallic photonic crystal slow-wave structure

Plasma-filled slow-wave devices provide a new way to develop high efficiency and high power vacuum-electron microwave sources, but their theoretical analysis and simulation is difficult. This paper introduces the wheel spoke antenna to excite signals for analyzing the dispersion characteristics of resonant cavity with plasma-filled metallic photonic crystal slow-wave structure (SWS). Influences of parameters of the SWS and plasma density on dispersion characteristics of the SWS are studied. Results show that there is little difference in dispersion characteristics obtained by wheel spoke antenna excitation of signals and other methods without plasma filling. When plasma fills in the SWS, the frequency of zero mode is consistent with the previous results obtained by other methods. Hence, both the results with and without plasma filling demonstrate that the wheel spoke antenna signal-excitation method is effective. Moreover, decreasing the thickness of wheel spoke antenna properly and the distance between the antenna and reflection surface of the metal plate can reduce the wheel spoke antenna influence on the cavity resonance frequency. Furthermore, thicker antenna can excite the slow wave field easily, while thinner antenna can excite the resonant mode easily. Besides, the outer radius and thickness of the SWS plate have little influence on the dispersion characteristics, while the period length and the inner radius of the SWS plate have greater influence on the dispersion characteristics. In addition, the dispersion curves of frequency and phase velocity will move to higher frequency regions with the increase of plasma density. Further, the influence of plasma filling on low-order modes is greater than that on higher order modes. It is also found that the higher-order mode operation can reduce the size of cavity and the velocity of the electron beam.

Design and Numerical Analysis of W-band Oscillators With Hollow Electron Beam

The use of hollow electron beams permits a significant increase in the diameter of the beam tunnel in comparison with the conventional pencil-beam slow-wave electron devices. As a result, the current density and the heating of the microwave structure are decreased, which allows increasing the average radiation power at high frequencies. To demonstrate this capability, two versions of W-band oscillators, namely, an axisymmetric orotron and a backward-wave oscillator (BWO), have been designed. The electron beam with an outer diameter of 1.6 mm and a 0.1-mm thick wall could be produced in a 30 kV/1 A Pierce-like electron gun with the hundredfold magnetic compression. Oscillators use microwave structures with an azimuthally symmetric corrugation, sinusoidal for the orotron and rectangular for the BWO. Simulations based on the averaged equations and 3-D PIC-code predict an output power up to 1 kW for the orotron and 0.7 kW for the BWO, whereas the thermal regime in the BWO is easier for continuous-wave operation. Wideband frequency tuning of the BWO is simulated. To reduce ohmic losses, a smooth downtaper of the corrugation in the orotron and an abrupt cutoff with a specially optimized last tooth in the BWO were designed.

The Gyrotron at 50: Historical Overview

Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.

THz Gyrotron and BWO Designed for Operation in DNP-NMR Spectrometer Magnet

Dynamic nuclear polarization (DNP) in high-field nuclear magnetic resonance (NMR) spectroscopy requires medium-power terahertz radiation, which nowadays can be provided basically by gyrotrons with superconducting magnets. As the electron cyclotron frequency is very close to the frequency of electron paramagnetic resonance for the same magnetic field, under certain conditions the gyrotron can be installed inside the same solenoid used for NMR spectrometer. This eliminates the need for an additional superconducting magnet, results in a shorter terahertz transmission line, and can make DNP systems practical. In addition to an extremely low-voltage gyrotron (“gyrotrino”), we analyze also advantages of strong magnetic field for a slow-wave electron device as an alternative terahertz source.

Miniaturized Microstrip Ring Hybrid with Defected Microstrip Structure

ABSTRACTA method for miniaturization of microstrip rat‐race hybrid with the aid of “defected microstrip structure (DMS)” is proposed. Each transmission line section is loaded with cascaded DMS sections. This results in slow‐wave characteristics which in turn increases the electrical length of the line. In the frequency range below the bandgap, the phase of S21 is negative and DMS behaves as a slow wave device; allowing a miniaturized structure. A reduction of 50% in the occupied area of the circuit and no bandwidth reduction is achieved using this technique. The miniaturized rat‐race operates at 3.4 GHz and can be used in LTE systems. The circuits are fabricated and the measured and simulated results are in excellent agreement. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2245–2248, 2013