Low Ohmic Losses Research Articles

Investigation of ultra‐wideband Bowtie antennas for phased array feed application

In this article, two new ultra-wideband (UWB) dual-polarized Bowtie antennas are investigated as the elements for a phased array feed for reflectors. In addition to its UWB impedance matching characteristic, the Bowtie antennas have stable large beam-width and a low cross-polar level over a wide frequency band with a compact size, which is an essence for phased array applications. The simulated and measured results state a low ohmic loss, good impedance matching (S11 below −15 dB) and good radiation performance, with a simple structure for easy manufacturing. The proposed antennas can be good candidates for phased array feed (PAF) in FAST and the SKA (square kilometer array) pathfinder PHAROS2 projects, and massive MIMO antennas in wireless communication systems.

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls.

SummarySpin waves offer promising perspectives as information carriers for future computational architectures beyond conventional complementary metal-oxide-semiconductor (CMOS) technology, owing to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices have been proposed previously, scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin-wave network can be extended to two dimensions and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit, and detect spin waves independently in the chiral domain wall, and spin-wave logics are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls

Spin waves offer promissing perspectives as information carriers for future computational architectures beyond conventional complementary metaloxide-semiconductor (CMOS) technology, due to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices are proposed previously, the scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin wave network can be two-dimensionally extended and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit and detect spin waves independently in the chiral domain wall and spin wave logic computings are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Enhanced optical absorption of graphene by plasmon

The plasmons in graphene have the superior properties to metal surface plasmons, such as high field confinement, low Ohmic loss and long wave propagation, highly tunable via electrostatic. More importantly, the frequency of plasmons ranges from terahertz to infrared which indicates that graphene is an ideal candidate for terahertz plamsonics. On the other hand, the strong coupling between incident photons and plasmons in graphene can lead the optical absorption to be enhanced. However, it is difficult for light to couple directly with plasmons in graphene, for the momentum of incident photons cannot match the plasmons in graphene. A metal grating can be used to compensate for the momentum of photons so that it can match that of plasmons in graphene. In this work, we theoretically investigate the effect of plasmons on the terahertz optical absorption of graphene with grating based on finite difference time domain. A great enhancement of electric field component of light field can be obtained near the gold grating strip in the sheet of graphene. Thus, the photons, of which the momentum is compensated for by the grating, can strongly couple with plasmons in graphene. An obviously decrease of the transmission of the graphene structure can be seen at the resonant frequency. The transmission peak corresponds to the resonant frequency spliting into two peaks due to the fact that two plasmon polariton modes are formed by the coupling of photons and palsmons. So we also study the plasmon polariton modes made by coupling photon with palsmon based on the many-body self-consistent method. Two plasmon polariton modes are obtained and an obviously splitting at the resonant frequency can be seen due to the coupling between photons and plasmons. The work conduces to deepening the understanding of the photoelectric properties of graphene and the terahertz plasmonics based on graphene.

A High-Efficiency Reconfigurable Element for Dynamic Metasurface Antenna

A high-efficiency reconfigurable element for dynamic metasurface antennas (DMA), consisting of a complimentary electric-LC (CELC) resonator fed through the cavity from a waveguide slot, is proposed in this paper, with positive- intrinsic-negative (PIN) diodes loaded to realize the coding performance. To begin with, the low efficiency problem of a traditional CELC element is demonstrated before the introduction of the proposed element. Subsequently, an equivalent circuit is illustrated to analyse the resonant performance, with extensive parametric study for verification. The proposed element has a low ohmic loss when the diode is switched on, 25 times less than the traditional element. Furthermore, the radiation efficiency of a DMA with 10 proposed elements is compared with a traditional design, showing a significant efficiency improvement from 25% to 75% when half diodes are switched on. Finally, measurements are conducted to verify the feasibility of the dynamic element to achieve radiation ON and OFF states, showing a 12 dB radiation gain difference between two states.

Highly efficient and controllable surface polariton beam splitters

We propose a scheme to realize highly efficient and controllable surface polariton beam splitters with a hybrid system via the electromagnetically induced transparency (EIT) of quantum emitters doped near the interface between a metamaterial and a dielectric. By means of the destructive interference effects contributed by the EIT of the quantum emitters and by the electric-magnetic response of the metamaterial, we show that such beam splitters can work at very low light level and they have small quantum decoherence and low Ohmic loss; moreover, the propagation loss of the surface polaritons can be reduced further by using weak microwave fields and the dispersion effect of the surface polaritons can be balanced by using the EIT-enhanced Kerr effect. Therefore, the surface polariton beam splitters obtained here have high splitting efficiency and may be controlled actively. Our study opens a route not only for coherent and active control of low-loss surface polaritons but also for promising applications in optical information processing based on quantum hybrid systems.

Multi-frequency near-field enhancement with graphene-coated nano-disk homo-dimers.

In this paper, a 3D sub-wavelength graphene-coated nano-disk dimer (GDD) is proposed for multi-frequency giant near-field enhancement. We observed that the dual-band operation originates from the excitation of hybridized localized surface plasmons on top and bottom faces of the disks along with the mutual coupling from the adjacent particle. Due to the sub-wavelength nature of the disks, the excited localized surface plasmons on the sidewalls are weak but they still can affect the dual operating bands. On the other hand, the strength and resonance frequency of the enhanced fields can be simply modulated by tuning the relative distances of 2D graphene disks on top and bottom faces. Adjustable dual-band performance is hardly attainable using simplified 2D graphene disks, however, it naturally comes out through modal hybridization in the subwavelength 3D structure containing multiple resonant units. Our suggested configuration has better optical properties than its noble metal counterparts because of its higher field enhancement and lower ohmic losses. Moreover, the electromagnetic response is reconfigurable by varying the bias voltage. The influence of graphene quality, chemical potential, and dimer gap size on the electric field enhancement and the resonance frequency of the surface plasmons are investigated, as well. To further improve its performance, a double negative metamaterial core is considered. This mechanism of the performance improvement by the core material is feasible thanks to the 3D nature of the structure. Two possible applications of the presented design are in Surface-Enhanced Raman Spectroscopy (SERS) and optical absorbers.

All-metallic geometric metasurfaces for broadband and high-efficiency wavefront manipulation

Abstract Geometric metasurfaces have shown superior phase control capacity owing to the geometric nature of their phase profile. The existing geometric metasurfaces are generally composed of metal-dielectric composites or all-dielectric subwavelength structures. Here, a novel configuration, all-metallic structure, is proposed to achieve broadband and high-performance electromagnetic wavefront manipulation based on the geometric phase. A catenary model is built to describe the optical dispersion and guide the design of metasurfaces. Two metadevices including a beam deflector and a hologram are designed and experimentally demonstrated in the infrared regime, with the measured optical efficiency up to 84% (the simulated efficiency reaches 93%). Compared to previous metal-insulator-metal structures, this approach can realize higher efficiency and broader operating bandwidth owing to its lower ohmic loss. This design strategy is universal and can be easily scaled to any other spectra without complex optimization. Moreover, since metals have excellent mechanical and physical properties, such as good thermal and electrical conductivity, this all-metallic structure may provide a new thinking on interdisciplinary research.

Designing Optimal Perovskite Structure for High Ionic Conduction.

Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3- δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes.

Dielectric nanoantennas to manipulate solid-state light emission

Thanks to their enhanced and confined optical near-fields, broadband subwavelength resonators have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters at room temperature. Over the last few years, high-index dielectrics have emerged as an alternative platform to plasmonic materials in order to design nanoresonators/optical nanoantennas with low ohmic losses. In particular, the excitation of electric and magnetic multipolar modes in dielectric resonators provides numerous degrees of freedom to manipulate the directivity and radiative decay rates of electric or magnetic quantum emitters. We review recent theoretical and experimental applications of dielectric nanoantennas to enhance or control decay rates of both electric and magnetic emitters but also to manipulate their radiation pattern through the coherent excitation of electric and magnetic modes; before discussing perspectives of this emerging field.

Novel Helical Groove Rectangular Waveguide Slow Wave Structure for 0.2 THz Traveling Wave Tube

A novel helical groove rectangular waveguide slow wave structure (HGRW-SWS) which evolves from the double staggered grating waveguide (DSGW) SWS is put forward to develop the high-gain and high electron efficiency terahertz (THz) traveling wave tube (TWT). The major advantages of the HGRW SWS are the higher interaction impedance, lower ohmic loss, and lower phase velocity especially in higher frequency band compared with the DSGW SWS. The gain and electron efficiency of TWT using HGRW-SWS have been significantly improved, respectively, compared with the TWT using DSGW-SWS.

Impedance Matching of THz Plasmonic Antennas

Plasmonic antennas with subwavelength gaps work as sensing devices for molecules for the optical and terahertz (THz) frequency range. In such a configuration, the sensing gap creates a high impedance, in contrast to the antenna itself, which is designed for low ohmic losses. Besides metals, highly doped semiconductors can be used as plasmonic materials for the THz range, which increase the impedance of the antenna while keeping the strong electromagnetic intensity enhancement in the sensing gap. Still, a substantial impedance mismatch remains. In this paper, an approach for matching the gap to the antenna impedance is proposed. First, a germanium semiconductor slab antenna is designed for THz operation, with a dipole resonance at 4 THz. Based on an impedance analysis, an impedance matching element consisting of a graphene sheet is inserted in parallel to the gap. Employing impedance matching, a strong intensity enhancement occurs and the normal dipole mode of the plasmonic antenna is split into a symmetric one at 0.67 THz and an antisymmetric one at 4 THz. The symmetric mode provides a very high-quality factor and a substantial enhancement; furthermore, its resonance can be tuned by the Fermi level adjustment in the graphene. First designs of this structure are computed analytically with a circuit model and are verified by 3D full-wave simulations.

Analytical design of all-dielectric grating as a narrowband absorber

All dielectric grating absorbers due to the high potential of providing reliable applications in modern devices and technology are being investigated. Periodic structures based on high-index all dielectric materials are particularly attractive due to the low ohmic loss. In this work, we have proposed and investigated all dielectric grating absorbers by an analytical model. Firstly, we have compared the results of the proposed analytical method with the existent experimental outcomes. Secondly, using the impedance matching concept we have proposed another structure of a hemisphere shape and compared the results of the proposed method with those of the full-wave simulations. With this novel structure, we have achieved high absorptance of approximately 100% and high-quality factor (Q=14.23). Additionally, our proposed method reduces the calculation time and needs less memory compared to the full-wave simulations. The results of the analytical method have an acceptable agreement with the experimental data and those of the full-wave simulations.

The Characteristics of Cermet-Supported Tubular Solid Oxide Fuel Cells Manufactured by Thermal Spraying

Tubular solid oxide fuel cells (SOFCs) with cells in series assembled on a porous cermet-support tube with self-sealing were designed for easy manufacturing by thermal spraying. Such design has distinct features of high power output, good thermal matching of support tube with different functional layers, and low ohmic loss from two electrode leaders. Tubular SOFCs with different numbers of cells (6, 10, and 20) in series were manufactured on the flame-sprayed NiCr-Al2O3 support tube with an insulating interlayer, YSZ electrolyte, Ni-YSZ anode, and La0.8Sr0.2MnO3-based cathode. All components in the tubular SOFC were produced by thermal spray processes. The performance test showed that tubular SOFC presented an open circuit voltage and maximum output power being proportional to numbers of cells in series. For tubular SOFC with 10 cells with a total length of 200 mm, the test yielded a peak power output over 20 W at 800 °C with a maximum power density over 200 mW/cm2. Moreover, it was also found that the tubular SOFC with 20 cells presented a comparable peak powder density. Therefore, it is clearly demonstrated that the cermet-supported tubular design is suitable for thermal spraying manufacturing with a high output performance.

Plasmonic versus All-Dielectric Nanoantennas for Refractometric Sensing: A Direct Comparison

In comparison to nanoplasmonic structures, resonant high-index dielectric nanoantennas hold several advantages that may benefit nanophotonic applications, including CMOS compatibility and low ohmic losses. One such application area might be label-free refractometric sensing, where changes in individual antenna resonance properties are used to quantify changes in the surrounding refractive index, for example, due to biomolecular binding. Here, we analyze and compare the sensing performance of silicon and gold nanodisks using a common and unbiased testing framework. We find that the all-dielectric system is fully capable of effectively monitoring small changes in bulk refractive index and biomolecular coverage, but the sensitivity is five to ten times lower than the plasmonic counterpart. However, this drawback is partly compensated for by a more linear response to adsorbate layer thickness changes and an approximately four times smaller susceptibility to photothermal heating. Finally, dielectric sensors may show promise if certain strategies are employed to improve their performance, which could thus bridge the gap between the two systems.

Sodium nickel titanate (Na2/3Ni1/3Ti2/3O2), a potential electrode system with low ohmic loss: Insights using opto-impedance probe

In the present work, with the goal of finding a viable alternative to Li-ion based electrode system, sodium nickel titanate system (NNT) has been synthesized and its impedance characteristics compared in relation to the established LNMC (LiNi1/3Mn1/3Co1/3O2) electrode system using opto-impedance technique. Thus, obtained opto-impedance results suggest a clear dominant electronic character in the form of charge build-up leading to high capacitance for the former. Also, the significant drop in ohmic resistance observed for the NNT system upon UV irradiation, at elevated temperature (at 55 °C) indicates an enhanced thermal safety for the new Na2/3Ni1/3Ti2/3O2 electrode system having potential for electrode applications.

High-power nitrided TiO2 carbon felt as the negative electrode for all-vanadium redox flow batteries

This work describes the design of an electrode with enhanced performance applied to all-vanadium redox flow batteries (VRFBs). This new electrode consists of a structural porous carbon felt decorated with TiO2 rutile nanoparticles, which has been nitrided using ammonolysis at 900 °C. An outstanding charge and mass transfer over the electrode-electrolyte interface was observed as a consequence of the synergetic effect of N- and O-functionalization over carbon felt (CF) and the partial formation of TiN (metallic conductor) phase. Moreover, this material has not only improved in terms of catalysis towards the V3+/V2+ redox reaction (k0 = 1.6 × 10−3 cm s−1), but also inhibited the hydrogen evolution reaction (HER), which is one of the main causes of imbalances that lead to battery failure. This led to an impressive high-power peak output value up to 700 mW cm−2, as well as work at high current density in galvanostatic conditions (i.e. 150 mA cm−2), exhibiting low ohmic losses (overpotential) and great redox single cell reversibility, with a superior energy efficiency of 71%. An inexpensive, earth abundant and scalable synthesis method to boost VRFBs technology based on nitrided CF@TiO2 is presented, being able to overcome certain constrains, and therefore to achieve high energy and power densities.

Realization of Both High Absorption of Active Materials and Low Ohmic Loss in Plasmonic Cavities

AbstractEnhancing active material absorption and suppressing ohmic loss is desired by a lot of applications based on plasmonic structures. In this work, an effective photonic approach is presented to achieve this purpose in plasmonic cavities. Enhancing the absorption ratio of the active material to the metal (by properly increasing the volume of the active region), and meanwhile keeping the system close to a critical coupling status for perfect light trapping (by switching the resonance to a higher‐order one or making real facets as the cavity boundaries) is proposed. As a photonic approach, this is generally applicable to different plasmonic materials. With the help of this approach, the absorption of the quantum wells in a plasmonic‐cavity‐integrated quantum well infrared photodetector operating in the longwave infrared range can be enhanced to 82% of the incident power and the ohmic loss can be suppressed to 18%. For plasmonic‐cavity‐integrated GaAs devices operating in the near infrared range, the approach can enhance the active material absorption to 78% of the incident power and suppress the ohmic loss to 20% at the wavelengths close to the bandgap. Also, it would improve the performance at a more extended wavelength.

Ultracompact AZO-based TE-pass and TM-pass hybrid plasmonic polarizers

Ultracompact transverse electric (TE)- and transverse magnetic (TM)-pass polarizers based on a hybrid plasmonic silicon-on-insulator platform are reported and studied. The aluminum-doped zinc oxide is used as an alternative plasmonic material in the proposed structures due to its attractive advantages, including high optical tunability, high thermal stability, and low ohmic loss. The numerical results show that the TE-pass polarizer of a compact device length of 3 μm has 20.6 dB extinction ratio (ER) and 0.21 dB insertion loss (IL) at an operating wavelength of 1.55 μm. However, the TM-pass polarizer achieves 22 dB ER and 0.11 dB IL at only 1 μm device length. Further, the fabrication tolerance of the proposed polarizers is studied, which ensures the high robustness of the suggested designs to the fabrication errors.

Circular Corrugated Miter Bend and Gap Losses for Broadband Frequency Applications

Circular corrugated waveguides are often used in fusion applications at single, multiple, or broadband millimeter frequencies due to their low ohmic loss, expected large frequency bandwidth, and direct coupling to free-space Gaussian modes. For single-frequency corrugated waveguide systems, transmission line components can be optimized to the desired frequency. For broadband or multiple-frequency applications, this is not possible. The goal of this paper is to demonstrate that the frequency bandwidth of circular corrugated waveguides can be compromised by diffraction losses to miter bends and gaps. It is shown that if the corrugation depth differs significantly from $\lambda $ /4, a theory can substantially underpredict the gap and miter bend losses. The simulations are also shown to compare favorably to experimental measurements. To improve the transmission line performance in large frequency bandwidth systems, such as 33–165 GHz reflectometry, reducing the number of miter bends may, therefore, be necessary. For the improvement of performance in narrower frequency bandwidth or multiple-frequency systems, especially high-powered systems, wavelength-dependent techniques may be applicable. One such application is shown for a Bragg reflection technique to reduce the thermal load to insulating ceramic rings in a dc break component for multiple-frequency ITER electron cyclotron heating transmission lines.