Dielectric Elements Research Articles

Symphotic Multiplexing Medium at Microwave Frequencies

As opposed to metasurfaces, which can produce a single output waveform in response to a single input waveform, volumetric metamaterials have the ability to perform independent functions on many distinct input waveforms. Here, we present an experimental demonstration of this multiplexing capability using a volumetric metamaterial designed using the symphotic method, which realizes highly efficient multiplexing structures in the strong scattering limit. In contrast to perturbative design methods such as volume holography that are only applicable in weakly scattering media, we provide a comprehensive approach that takes into consideration design and fabrication constraints and that can be verified in simulations. We then demonstrate an experimental realization of a symphotic device operating at a frequency of 10 GHz, which has been optimized for three distinct input waveforms corresponding to three distinct output waveforms. The device is realized using a low-loss three-dimensionally printed material. The symphotic device consists of a lattice of dielectric cylindrical elements with varying radii, excited in a parallel-plate waveguide to enforce two-dimensional field symmetry. The experimental results show excellent agreement with analytical coupled-dipole-method simulations and finite-element simulations. The experiments further demonstrate the scalability of symphotic metamaterials and their viability for advanced rf and optical devices.

Microwave scattering-absorption properties of a lightweight all-dielectric coding metamaterial based on TiO2/CNTs

The all-dielectric coding metamaterial is obtained by encoding the unit structures constructed of conventional absorbing material. The combined effects of absorption and scattering to electromagnetic waves provide noteworthy application in reducing the radar cross section (RCS). In this Letter, utilizing the T i O 2 / C N T s material, we designed two types of hollow dielectric elements and coded them in a checkerboard format after calculation and analysis. The experimental results manifest that the frequency range where the RCS reduction is less than − 10 d B covers from 5.3 to 18 GHz. The design of the hollow structure and the selection of low-density material make the all-dielectric coding metamaterial a small surface density. Additionally, the design of coding the all-dielectric elements will have potential application in radar stealth.

Laser-induced spatially-selective tailoring of high-index dielectric metasurfaces.

Optically resonant high-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances are usually fabricated by means of planar technologies, which limit the degrees of freedom in tunability and scalability of the fabricated systems. Therefore, we propose a complimentary post-processing technique based on ultrashort (≤ 10ps) laser pulses. The process involves thermal effects: crystallization and reshaping, while the heat is localized by a high-precision positioning of the focused laser beam. Moreover, for the first time, the resonant behavior of dielectric metasurface elements is exploited to engineer a specific absorption profile, which leads to a spatially-selective heating and a customized modification. Such technique has the potential to reduce the complexity in the fabrication of non-uniform metasurface-based optical elements. Two distinct cases, a spatial pixelation of a large-scale metasurface and a height modification of metasurface elements, are explicitly demonstrated.

Method for the diagnosis of high-voltage dielectric elements during operation based on dynamic registration of electromagnetic radiation

Complex remote monitoring and the developed non-contact diagnostic device (NDD) allow monitoring the technical condition and revealing the residual life of high-voltage dielectric elements. It is based on non-contact remote periodic measurement of a set of diagnostic parameters (characteristics of electrical discharges emitted by insulators during operation) necessary for a reliable assessment of the operating state of high-voltage dielectric equipment at the moment of operation and determination of its residual life. NDDs are installed on each phase conductor and used to measure diagnostic parameters, followed by computer processing of the accumulated information signals. An analysis of the set of necessary diagnostic parameters makes it possible to form a conclusion on the degree of performance of dielectric elements. The paper considers a method for detecting defective high-voltage insulators using the developed non-contact diagnostic devices by the radio frequency method and assessing the difference in time of arrival of a partial discharge signal.

3D Printed OAM Reflectarray Using Half-Wavelength Rectangular Dielectric Element

A 3D printed dielectric reflectarray is studied to generate orbital angular momentum (OAM) beam. The array element consists of a half-wavelength rectangular dielectric block and a metallic ground. It is found that the loss tangent and permittivity of the rectangular dielectric element independently affect the reflection magnitude and reflection phase, respectively. Four OAM modes (l = 1, 2, 3, 4) are theoretically studied. A prototype of the mode l = 1 working at Ka band is fabricated and measured. The maximum gain in the radiation direction (θ = 4°) is 20.4 dBi at 30 GHz. A wide OAM bandwidth of 21.7% for the mode weight coefficient larger than 60% is also obtained. The proposed antenna shows its potential applications in OAM communications.

Broadband phase-shifting mirrors for ultrafast lasers.

Metal-dielectric phase-shifting multilayer optical elements have been developed, providing broadband, virtually dispersion-free polarization manipulation down to the few-cycle level. These optical elements are Ag/Al2O3 mirrors that operate in the spectral range from 500 to 100 nm, exhibiting reflectance higher than 95%, and a differential phase shift between the s- and p-polarization of about 90° distributed over four bounces. The mirrors have been designed, produced, and reliably characterized based on spectral photometric and ellipsometric data using a non-parametric approach as well as a multi-oscillator model. The optical elements were implemented into a few-cycle laser system, where they transformed linearly polarized few-cycle light pulses to circular polarization.

Using contactless methods of diagnostics of high electric fields

In the electric power industry of Russia and abroad, special attention is paid to the problem of an increase in the number of accidents caused by damage to high-voltage insulators in high-voltage equipment. An analysis of emergencies at substations and open switchgears [1- 2, 4] was carried out, which showed that in most cases the causes of damage to high-voltage insulators are natural aging in an applied electric field, overvoltage, and the presence of initial defects in the manufacture of high-voltage insulators. Based on this fact, we developed various methods of non-contact diagnostics of high-voltage insulators in order to identify defects at an early stage of their development. Particular attention was paid to the method of partial discharges because the characteristics of partial discharges provide information on the parameters of defects. The article describes a set of non-contact methods for remote diagnostics of high-voltage insulators; a two-channel method for remote diagnostics of the operating state of high-voltage insulators, based on the registration of partial discharges by electromagnetic and acoustic sensors; a device that allows visual inspection and the search for faulty high-voltage equipment; remote non-contact method for recording electric fields of high tension of industrial frequency, as well as their spatial orientation based on the electro-optical effect. We developed a mock-up of a portable diagnostic device for implementing research methods for high-voltage dielectric elements to diagnose their technical condition using the described complex of non-contact methods. The measuring device as part of a portable diagnostic device consists of a set of sensors for collecting diagnostic information detected by electromagnetic, acoustic and electro-optical sensors and a voltage phase signal applied to the studied highvoltage insulator. The simultaneous use of several sensors at once made it possible to increase the accuracy of localization of partial discharges in high-voltage insulators. Visualization of diagnostic results is possible at the control room in the form of amplitude-phase, frequencyphase and amplitude-frequency diagrams of the distribution of characteristics of partial discharges and on a portable device in the form of radiation intensity from the selected sensor. A portable diagnostic device made it possible in laboratory conditions to study electrophysical processes in various dielectric materials and products under the influence of strong alternating electric fields. A study was made of the features of defects on the rod and the terminal-terminal contact, a diagram of the electrophysical processes accompanying the emission of partial discharges was constructed, and the causes of their occurrence were established.

Static and Dynamic Evaluation of a Winding Deformation FBG Sensor for Power Transformer Applications.

Power transformer is the most important and expensive equipment used in the electric power industry. Fiber Bragg grating (FBG) sensors has stood out as a flexible and particularly suitable tool for power transformer monitoring being a passive and dielectric sensor element. In this work we evaluated the performance of FBG pressure sensors developed to monitor the static and dynamic pressure in high voltage winding transformers during events such as short-circuit and inrush current. Two types of sensors packaging materials were evaluated in laboratory: polyether ether ketone (PEEK) and transformerboard (TB). The sensors have been tested for high intensity and short duration impacts similar to those occurring in short circuits. In addition, we evaluated the time response of sensors using an interrogation system with a 5 kHz sweep in order to analyze the short circuit response time properly. The results pointed that FBG pressure sensors using PEEK and TB are suitable for transformer winding monitoring. The static sensitivity obtained to PEEK based sensors was 0.911 pm/N, in the range of 800 N to 1500 N. This sensitivity is 4.47 higher than TB based sensors sensitivity. Dynamical tests performance showed an excellent repeatability for both sensors, in agreement with static observation.

Reconfigurable Transparent All-Dielectric Water-Based Metamaterial for Microstrip Patch Antenna Gain Enhancement

This paper present transparent, all dielectric water-based metamaterial (MM) superstrates with reconfigurable characteristics is employed for gain and bandwidth enhancement of a water-based microstrip patch antenna. The water-based microstrip patch antenna is fed by an L-shape probe. All dielectric water-based MM unit-cell element consists of dielectric cubic boxes filled with water is designed and analyzed. The reconfigurable electric properties are achieved via changing the water height in the MM unit-cell element. Different arrangements of the MM array with water height tapering are optimized and designed for microstrip patch antenna gain enhancement. The MM array is used as a single layer superstrate placed normal to the microstrip patch. A water-based MM lens consists of three layers is designed to collimate the radiation from the microstrip patch antenna. The phase compensation in the MM lens is achieved via changing the water height in the MM unit-cell elements of the lens. A reconfigurable beam in different directions from −30° to +30° is steered by changing the water level distribution over the MM lens unit-cell elements. A full-wave analysis using the finite integration technique is used for the design and analysis of the water-based MM lens arrangements.

Q -factor optimization in dielectric oligomers

In this paper we propose an optimization procedure for quality-factor (Q-factor) enhancement in dielectric oligomers. The oligomers are introduced as linear periodic chains of dielectric elements the individual Q factors of which are optimized via multipolar conversion in an avoided crossing of two resonant eigenfrequencies. It is demonstrated that when such dielectric elements are assembled into an infinite periodic array the coupling between individual resonances gives rise to an in-$\mathrm{\ensuremath{\Gamma}}$ optical bound state in the continuum (BIC) unprotected by symmetry. By setting up the oligomers of finite numbers of dielectric elements we observe high quality resonances which occur as traces of the infinite array band structure. The resulting high quality resonances exhibit light localization spots of a few squared wavelengths with Q factors $Qg{10}^{4}$. The localization is provided by similarity of the field pattern to that of the BIC with the losses at the edges of the array suppressed due to an almost flat BIC host band. The scaling law of the Q factor against the dielectric permittivity is derived.

A low-cost and highly integrated sensing insole for plantar pressure measurement

Plantar pressure measurement plays an important role in monitoring health and sports performance. Here, instead of embedding force sensitive resistors in the insole, we developed a simple and low-cost, highly integrated, textile-only sensing insole for the measurement of plantar pressures. These sensors were composed entirely of fabric, and utilized a capacitive mechanism, with silver cloth as electrodes and cotton cloth as the dielectric element. The similarity of elasticity and flexibility between sensor and insole enabled them to be closely integrated, and avoided negative impact on comfort. Additionally, the silver cloth electrodes contributed good antibacterial properties. The sensors exhibited fast response, negligible hysteresis, high sensitivity (0.947 MPa−1), and good stability. Based upon capacitive responses to plantar pressures from the instrumented insole, gait phases (events) in different patterns were detected, giving the potential to monitor the health of the foot.

Design of Defected Ground Structure Band Stop/Band Pass Filters Using Dielectric Resonator

A square dielectric resonator element (SDR) with a defected ground structure (DGS) is investigated. The proposed DGS is composed of two rectangular slots connected by two transverse slots and is placed in the ground plane. It is fed by a strip line through the substrate layer. The objective of this structure is to design dielectric resonator band- stop filter (DRF) and enhance the performance in terms of better insertion loss and increased bandwidth. The DRF has been fabricated and measurements are taken. The results for band- stop filter show a cut-off frequency of 2.25 GHz, transmission loss of 2 dB and the 3-dB bandwidth ranges from 1.4 to 2.64 GHz. The effect of the transverse slot width on the filter response curve is studied. The same structure is modulated to be frequency reconfigurable DRF for achieving frequency agility by using ideal metallic switches. The cut-off frequency is shifted by 1 GHz, the 3-dB bandwidth by 1.5 GHz, while the transmission loss is decreased by 0.75 dB. Finally, the effect of loading SDR with metal plate is investigated. This structure combines the dielectric resonator antenna and the DRF to propose dielectric resonator antenna filter (DRAF), this structure is used to miniaturize the global-positioning-system receivers that contain both the antenna and filter. The DRAF has been fabricated and measured, it has 3-dB pass bandwidth of about 1 GHz. Factors such as return loss, insertion loss, radiation pattern and mutual coupling of DRAF are calculated using Finite element method. Comparison of calculated and measured factors of DRAF shows a good agreement.

All‐Dielectric Silicon Metasurface with Strong Subterahertz Toroidal Dipole Resonance

AbstractA single‐layer, all‐dielectric metasurface exhibiting a strong toroidal resonance in the low‐atmospheric loss radio window of the subterahertz W‐band is theoretically proposed and experimentally demonstrated. The metasurface is fabricated on a high‐resistivity floating‐zone silicon wafer by means of a single‐process, wet anisotropic etching technique. The properties of the toroidal mode of both the constituent dielectric elements and the metasurface are rigorously investigated by means of the multipole decomposition technique and full‐wave simulations. The experimental demonstration of such a compact, all‐silicon metasurface opens new venues of research in the investigation of toroidal modes and the engineering of functional millimeter‐wave components, which can be scaled to terahertz and higher frequencies of the electromagnetic spectrum.

Planar Porous Components for Low‐Loss Terahertz Optics

AbstractThere is a strong interest in using the terahertz (THz) frequency band for applications in sensing, imaging, and wireless communications. To enable many of these applications, compact low‐loss components for beamforming are required. Typically, such components are made using solid dielectric elements with spatially variable thickness, for example, planoconvex lenses or spiral phase plates. However, as losses in dielectrics typically greatly increase with THz frequency, so do the losses of the solid components. This work demonstrates that when introducing low‐refractive index, low‐loss subwavelength inclusions (air holes) into a solid material matrix, the loss of porous components can be greatly reduced compared to the loss of solid components with otherwise identical optical properties, thus opening a way to create efficient optical components even with nominally high‐loss materials. Additionally, porous optical components can be created completely flat as spatially dependent optical path difference is achieved by varying the local porosity rather than the component thickness. This offers additional advantages for free‐space alignment and integration of such components into optical systems. As an example, the design, fabrication, and experimental characterization of planar lenses and planar orbital angular momentum phase plates are carried out. It is then demonstrated how these porous components outperform their all‐solid counterparts.

Determination of refractive index and dielectric constant in the terahertz and microwave ranges

The results of measurements of the refractive index and dielectric constant of several samples of porous materials in terahertz (THz) and microwave ranges are presented in the work. Such measurements are relevant, since porous materials have a wide variation of parameters and reference data do not reflect all their diversity.The permittivity of the materials was measured in the microwave range. The cylindrical waveguide-dielectric resonator on an evanescent waveguide was used. Measurements were made by two methods at frequencies of 10 ÷ 12 GHz. In one method, the investigated material was used as the dielectric element of the resonator. In another method, the dielectric element was made of fluoroplastic and the investigated material filled the evanescent parts of the waveguide. The accuracy of measurements was compared and the advantages and disadvantages of these methods were analyzed.The measurements were carried out in the THz range by a resonance method at a frequency of 890 GHz. A gas-discharge submillimeter laser with a wavelength of 337 μm was used for this purpose. The measuring device was a quasi-optical resonator with a smooth length adjustment. One of the mirrors of the quasi-optical resonator was simultaneously the output mirror of a submillimeter laser. The test material was placed in a quasi-optical resonator. The refractive index was determined from the change in the optical length of the resonator, when the investigated material was introduced into it.The difference in the measured parameters for the different materials studied correlates with the specific gravity of the materials. Significant changes in the parameters when changing the frequency have not been detected.The results of measurements can be used in the development of various devices with THz and microwave ranges, in particular, in the design of antenna fairings, dielectric waveguides, quasioptical elements, etc.

EHF Band Linear Antenna Array Based on Surface Wave Transformation

The experience of using of experimental samples of diffraction radiation antennas shows the need to expand the list of options and modifications of the technical solutions for such antennas. One of variants is the modification of a linear dielectric waveguide for rigidly fixation of the dielectric rod. H-shaped surface-wave waveguides in the form of combination of dielectric rod and rigid metal elements have been proposed for a basic element of diffraction radiation antennas designed for operation in conditions of increased mechanical loads and vibrations at frequencies above 80 GHz. The results of experimental studies of the near-field distribution, radiation patterns, gain and energy losses of the antenna are presented. The obtained results show the effectiveness of the implemented constructive approach. The configuration of the linear antenna grating of diffraction radiation with a modified dielectric waveguide is proposed, which can be applied for the development of scanning antennas in the 80–100 GHz frequency range.

Graphene Oxide as a Dielectric and Charge Trap Element in Pentacene-Based Organic Thin-Film Transistors for Nonvolatile Memory.

In this report, the dielectric nature of graphene oxide (GO) was exploited for the successful implementation of low-power pentacene thin-film transistors suitable for nonvolatile memory applications. Two different types of devices were fabricated on indium tin oxide-coated glass substrates with two different metals, viz., gold and aluminum, as the source and drain contacts. The performance of the devices was analyzed from their field-effect characteristics. Both the devices showed dominant p-type charge transport behavior. The breakdown electric field was determined to be 1.02 × 108 V/m. The current transport mechanism was explained from the output characteristics using the Fowler–Nordheim tunneling theory. Capacitance–voltage (C–V) measurements have been employed to determine the value of the oxide capacitance and to examine the memory effect. The hysteresis behavior observed from the C–V characteristics show the suitability of the device for memory applications with a low operating voltage of 3 V. The charge trapping behavior of GO was explained by the energy band diagram. Frequency-dependent C–V measurements in the range 100 kHz to 1 MHz were also performed to account for the memory window obtained in the devices. The charge retention and endurance characteristics were evaluated under a constant voltage stress to check the reliability of device operation.

Ultra-Light Reduced Graphene Oxide Based Aerogel/Foam Absorber of Microwave Radiation.

We present the polarization-dependent highly absorptive in Ka-band composition of conventional polyurethane foam filled with in situ synthesized aerogel coated by reduced graphene oxide (rGO). The rGO-based aerogel was in situ prepared into the open-cell polyurethane foam (PUF) skeleton through a bidirectional freeze-drying process. The aerogel is composed of the flat lamellas stacks, possessing the anisotropic structure and unique electromagnetic properties. Further improvement of the electromagnetic shielding ability was possible by the rGO coating introduction as a coupling layer between PUF and rGO-based aerogel. This enhances the overall conductivity of the resulting composites: 1.41 + 3.33i S/m vs. 0.9 + 2.45i S/m for PUF loaded with in situ synthesized aerogel without rGO coating.With this mechanically robust plane easy to process coating one could achieve −20 dB by power with the record light structure (0.0462 g/cm2). That could compete in view of the weight per cm2 even with graphene-based absorbers comprising either dielectric matching elements or back metal reflectors, or both.

Metasurface fan-out diffractive optical elements

Here we report the design and realization of dielectric fan-out diffractive optical elements (DOEs) operating in visible range and with nanobrick depth-width-ratio as low as 1.5. The metasurface-based fan-out DOEs realized here, when illuminated by an incident beam with 633 nm wavelength, the uniform 4×4 spot arrays with an extending angle of 32°×32° can be obtained in the far field. Experimental results agree well with the theoretical analysis and numerical simulation conclusions. Because of these advantages of the single-step nonstructural fabrication procedures, continuous and accurate phase manipulation, and strong polarization conversion, the metasurface-based fan-out DOEs could be utilized for various applications such as optical sensing, lidar, laser machining and so on.

Additively Manufactured Perforated Superstrate to Improve Directive Radiation Characteristics of Electromagnetic Source

Additively manufactured perforated superstrate (AMPS) is presented to realize directive radio frequency (RF) front-end antennas. The superstrate comprises spatially distributed dielectric unit-cell elements with square perforations, which creates a pre-defined transmission phase delay pattern in the propagating electric field. The proposed square perforation has superior transmission phase characteristics compared to traditionally machined circular perforations and full-wave simulations based parametric analysis has been performed to highlight this supremacy. The AMPS is used with a classical electromagnetic-bandgap resonator antenna (ERA) to improve its directive radiation characteristics. A prototype is developed using the most common, low-cost and easily accessible Acrylonitrile Butadiene Styrene (ABS) filament. The prototype was rapidly fabricated in less than five hours and weighs 139.3 g., which corresponds to the material cost of only 2.1 USD. The AMPS has remarkably improved the radiation performance of ERA by increasing its far-field directivity from 12.67 dB to 21.12 dB and reducing side-lobe level from -7.3 dB to -17.2 dB.