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Relativistic Backward-Wave Oscillator with Parallel Wave Interaction
摘要： We study operation of a backward-wave oscillator (BWO) in the regime of parallel amplification of rotating waves with different azimuth indices. It is shown that in the linear case, the use of the parallel wave interaction principle allows one to enhance the BWO gain significantly as compared with a classical backward-wave oscillator. An option for using parallel amplification in high-power sectioned amplifiers is proposed.
关键词： backward-wave oscillator,parallel wave interaction,high-power amplifiers,relativistic sources,microwave band
更新于2025-09-04 15:30:14
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Micro-fabricated wideband band-stop filter using GaAs-based integrated passive device technology
摘要： This paper presents a new concept of implementing a micro-fabricated wideband band-stop filter on a gallium arsenide (GaAs) substrate using integrated passive device technology. The incorporation of an air-bridge structure was explored to enhance design flexibility and achieve excellent radio-frequency performance of the filter. A wideband band-stop filter was realized on a GaAs substrate, generating an insertion loss of ? 0.37 dB and a return loss of ? 38 dB with excellent attenuation of ? 28.78 and ? 22.27 dB, in the lower and the upper passband, respectively. The filter resonates at 10.72 GHz, occupying a die area of 2000 μm × 1540 μm. The selectivity of the filter is reflected by its tremendous suppression of out-of-band signals with the existence of attenuation poles in the vicinity of the resonance frequency. Experimental verification of the filter response demonstrates its potential use as an on-chip device operating in the X-band frequency spectrum.
关键词： Gallium arsenide (GaAs),Wideband band-stop filter,Radio frequency (RF),Integrated passive device technology,Micro-fabrication
更新于2025-09-04 15:30:14
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Band structure tuning and charge separation of MNX monolayers and MNX/GaS van der Waals heterostructures
摘要： Efficient charge separation and band structure tuning are very important for application to photoelectric device. Here, based on first-principles calculations, we propose two-dimensional (2D) MNX (M=Zr, Hf; X=Cl, Br, I) monolayers which can be exfoliated from the corresponding layered bulk phase due to the low cleavage energy. The phonon band structure and the mechanical analysis indicate that 2D MNX monolayers can form free-standing membranes. The calculated results suggest that 2D MNX monolayers are indirect band gap semiconductors with band gaps in the range of 1.55-3.37 eV. Among them, MNI (M=Zr, Hf) monolayers with effective charge separation and moderate band gaps would have potential application to photocatalytic water splitting. In order to realize the effective charge separation for the other MNX monolayers, MNX/GaS (X=Cl, Br) heterostructures are investigated. Our calculations reveal that MNX/GaS heterostructures are typical type-II band alignment, facilitating the separation of photogenerated carriers where electrons and holes are localized in MNX and GaS monolayers, respectively. Furthermore, the band gaps of the MNX/GaS heterostructures are obviously narrowed compared to those of the isolated constituent monolayers. In addition, the band gaps and band edge positions of MNX/GaS heterostructures can further be tuned by biaxial strain to match better with the redox potentials of water. These findings in this study not only enrich the family of 2D materials, but also demonstrate that MNI (M=Zr, Hf) monolayers and MNX/GaS heterostructures are promising candidates for photocatalytic materials.
关键词： van der Waals heterostructures,photocatalytic water splitting,band structure tuning,MNX monolayers,charge separation
更新于2025-09-04 15:30:14
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Designing Conjugated Polymers for Molecular Doping: The Roles of Crystallinity, Swelling, and Conductivity in Sequentially-Doped Selenophene-Based Copolymers
摘要： Although chemical doping is widely used to tune the optical and electrical properties of semiconducting polymers, it is not clear how the degree of doping and the electrical properties of the doped materials vary with the bandgap, valence band level, and crystallinity of the polymer. We address this question utilizing a series of statistical copolymers of poly(3-hexylthiophene) (P3HT) and poly(3-heptylselenophene) (P37S) with controlled gradients in bandgap, valence band position and variable crystallinity. We dope the copolymers in our series with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) using solution sequential processing (SqP). We then examine the structures of the films using grazing incidence wide-angle x-ray scattering (GIWAXS), differential scanning calorimetry (DSC) and ellipsometric porosimetry, and the electrical properties of the films via the AC Hall effect. We find that the ability of a particular copolymer to be doped is largely determined by the offset of the polymer’s valence band energy level relative to the LUMO of F4TCNQ. The ability of the carriers created by doping to be highly mobile and thus contribute to the electrical conductivity, however, is controlled by how well the polymer can incorporate the dopant into its crystalline structure, which is in turn influenced by how well it can be swelled by the solvent used for dopant incorporation. The interplay of these effects varies in a non-monotonic way across our thiophene:selenophene copolymer series. The position and shape of the polaron absorption spectrum correlate well with the polymer crystallinity and carrier mobility, but the polaron absorption amplitude does not reflect the number of mobile carriers, precluding the use of optical spectroscopy to accurately estimate the mobile carrier concentration. Overall, we find that the degree of crystallinity of the doped films is what best correlates with conductivity, suggesting that only carriers in crystalline regions of the film, where the dopant counterions and polarons are forced apart by molecular packing constraints, produce highly mobile carriers. With this understanding, we are able to achieve conductivities in this class of materials exceeding 20 S/cm.
关键词： semiconducting polymers,conductivity,ellipsometric porosimetry,GIWAXS,solution sequential processing,valence band level,F4TCNQ,poly(3-heptylselenophene),AC Hall effect,poly(3-hexylthiophene),DSC,bandgap,chemical doping,crystallinity
更新于2025-09-04 15:30:14
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[Institution of Engineering and Technology 12th European Conference on Antennas and Propagation (EuCAP 2018) - London, UK (9-13 April 2018)] 12th European Conference on Antennas and Propagation (EuCAP 2018) - Dual-Band Self-Diplexed Modulated Metasurface Antennas
摘要： This paper presents a simple and effective technique to design dual-band modulated metasurface (MTS) antennas. In MTS apertures, an equivalent surface inductance supports the propagation of a transverse magnetic (TM) surface wave (SW). The SW power can be gradually radiated at one frequency by periodically modulating the surface reactance. Indeed, one can choose the period of the modulation so its interaction with the SW results in the (-1) indexed Floquet mode entering the visible region. Therefore, in order to achieve dual-band operation with a single aperture, one has to tailor a modulation able to radiate the SW power in the desired directions at both frequencies. The proposed approach consists in superimposing two modulations with different periods on the antenna aperture. One modulation produces beam #1 at frequency f1, whereas the other one generates beam #2 at frequency f2. The resulting inductive surface is then synthesized by means of sub-wavelength patches printed on a grounded dielectric slab. Moreover, it is possible to define a different position for the source of each modulation, which allows one to self-diplex the signals at f1 and f2. After providing basic design guidelines, we present numerical results for a design case consisting of an antenna with broadside pencil beams in the K and Ka bands.
关键词： leaky wave (LW),surface wave (SW),Dual-band,metasurface
更新于2025-09-04 15:30:14
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Band Gap Control in Bilayer Graphene by Co-Doping with B-N Pairs
摘要： The electronic band structure of bilayer graphene is studied systematically in the presence of substitutional B and/or N doping, using density functional theory with van der Waals correction. We show that introduction of B-N pairs into bilayer graphene can be used to create a substantial band gap, stable against thermal fluctuations at room temperature, but otherwise leaves the electronic band structure in the vicinity of the Fermi energy largely unaffected. Introduction of B-N pairs into B and/or N doped bilayer graphene likewise hardly modifies the band dispersions. In semiconducting systems (same amount of B and N dopants), however, the size of the band gap is effectively tuned in the presence of B-N pairs.
关键词： density functional theory,van der Waals correction,band gap,B-N doping,bilayer graphene
更新于2025-09-04 15:30:14
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[IEEE 2018 Iranian Conference on Electrical Engineering (ICEE) - Mashhad (2018.5.8-2018.5.10)] Electrical Engineering (ICEE), Iranian Conference on - Tunable Filter by Using Parity Time (PT) Symmetric Periodic Array Electronic
摘要： A circuit model of a periodic array of PT-symmetric (gain/loss coupled) waveguides is reported for the design of a novel tunable filter. This model consists of a pair of coupled circuits, one with amplification resistance and the other with an equivalent attenuation. By varying the value of the gain/loss, one can control the size of bandgap and hence the allowed frequencies propagating within the structure. For a critical value of gain/loss, the eigenvalues merge and the gap disappears. As a result, all frequencies can pass the structure i.e, an all pass filter. The Q-factor of reflected frequency can be varied such that, tunable Q from 20 to greatest has been successfully realized in the central frequency of 0.283 MHz. The remarkable feature of this design lies with its tunability, high speed, and relative simplicity.
关键词： RLC Circuit Model,Parity-Time (PT) Symmetry,Grating Waveguides,Optical Waveguides,Band Stop Filter (BSF)
更新于2025-09-04 15:30:14
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Polarization and incident angle independent metasurface absorber for X-Band application
摘要： In the article, polarization and incident-angle independent metasurface absorber for X-band application has been proposed. Proposed absorber unit cell composed of a circular sector attached to a cross-shaped structure. The proposed absorber is symmetrical along the horizontal and vertical axis, which supports polarization angle insensitivity. Metasurface absorber shows the very good impedance matching with free space at 10.48 GHz along with 97% absorptivity. The full width at half maximum (FWHM) bandwidth is 480 MHz (10.24-10.72 GHz). In transverse electric and magnetic (TE and TM), modes the absorptivity of the proposed absorber is maintained above 90% up to 60o incidence angle. The absorber has an ultrathin thickness of 0.028λo, where λo is the wavelength of absorption peak. The absorption phenomenon is explained by normalized impedance, E-field distribution and surface current density. To analyze the metasurface properties effective permittivity, permeability and dispersion diagram has been demonstrated. The metasurface absorber has been fabricated and it shows the good accord between simulated and measured results.
关键词： X-band,polarization independent,absorber,Metasurface
更新于2025-09-04 15:30:14
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An L-Band Relativistic Magnetron With Cathode Priming
摘要： An L-band relativistic magnetron (RM) that adopts stepped-mode conversion diffraction output (DO) is presented and investigated by the 3-D particle-in-cell simulations. The cathode priming technique is introduced to quicken startup of the π-mode and suppress unexpected modes during startup. The length and width of the emission region, the deviation angle between the emission region and the anode vane, and the extended length of the cathode are four important parameters for cathode emission. The simulation results indicate that these four parameters have significant impact on the performance of the RM with DO (MDO). With fixed voltage and magnetic field, all these four parameters have the optimum values which lead to the highest power efficiency. Typical simulation results are as follows: when the applied voltage and magnetic field are 520 kV and 0.3 T, respectively, the L-band A6 MDO radiates microwave of 2.19 GW at 1.577 GHz, and the corresponding power efficiency is 61%.
关键词： L-band,MDO,Cathode priming
更新于2025-09-04 15:30:14
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Effect of Deposition Method on Valence Band Offsets of SiO <sub/>2</sub> and Al <sub/>2</sub> O <sub/>3</sub> on (Al <sub/>0.14</sub> Ga <sub/>0.86</sub> ) <sub/>2</sub> O <sub/>3</sub>
摘要： There are often variations in reported valence band offsets for dielectrics on semiconductors and some of the reasons documented include metal or carbon contamination, interfacial disorder, variations in dielectric composition, thermal conditions, strain, and surface termination effects. In this paper we show that there are differences of up to 1 eV in band alignments for the common gate dielectrics SiO2 and Al2O3 on single crystal (Al0.14Ga0.86)2O3, depending on whether they are deposited by sputtering or Atomic Layer Deposition. In the case of Al2O3, this changed the band alignment from nested (type I) to staggered gap (type II). The valence band offset at each heterointerface was measured using X-Ray Photoelectron Spectroscopy and was determined to be ?0.85 ± 0.15 eV for sputtered Al2O3 and 0.23 ± 0.04 eV for ALD Al2O3 on β-(Al0.14Ga0.86)2O3, while for SiO2 it was 0.6 ± 0.10 eV for sputtered and 1.6 ± 0.25 eV for ALD. These results are consistent with recent results showing that the surface of Ga2O3 and related alloys are susceptible to severe changes during exposure to energetic ion environments.
关键词： Al2O3,(Al0.14Ga0.86)2O3,SiO2,valence band offsets,Atomic Layer Deposition,X-Ray Photoelectron Spectroscopy,sputtering
更新于2025-09-04 15:30:14