Wire Grid Research Articles

Ultra‐wideband dual‐polarized transmitarray antenna with Vivaldi elements

The implementation of an ultra-wideband (UWB) dual-polarized transmitarray antenna (TA) is presented in this paper. The proposed TA takes advantages of the UWB performance of Vivaldi array and the true-time-delay (TTD) characteristic of transmission line to overcome the inherent bandwidth limitations. First, a dual-polarized TA element featuring low transmission losses, high polarization purity, and linear TTD phase responses over 5–16 GHz is designed, where an improved high-impedance microstrip phase delay polyline is developed to enhance the transmitting efficiency. Then, in-band resonances caused by wire grid ground in the construction of dual-polarized TA aperture are carefully addressed. As a proof of concept, a prototype including 22 × 23 elements arranged in an egg-crate configuration is manufactured and measured. The experimental results show that the TA achieves stable radiation performance within the operating band.

Polarization modeling and predictions for DKIST, part 8: calibration polarizer spatial variation impacts

Astronomical spectropolarimeters require high accuracy polarizers with large aperture and stringent uniformity requirements. In solar applications, wire grid polarizers are often used as performance is maintained under high heat loads and temperatures over 200°C. DKIST is the NSF’s new 4-m aperture solar telescope designed to deliver accurate spectropolarimetric solar data across a wide wavelength range, covering a large field of view simultaneously using multiple facility instruments. Polarizers at 120 mm diameter are used to calibrate DKIST instruments but vary spatially in transmission, extinction ratio, and orientation of maximum extinction. We combine new spatial and spectral metrology for polarizers and retarders to simulate the accuracy losses with field angle and wavelength caused simultaneously by spatial variation of several optical parameters including beam decenter from misalignments. We also present testing of a new crystal sapphire substrate polarizer designed and fabricated to improve DKIST long wavelength calibrations. We assess spatial thickness variation of sapphire and fused silica wafer substrates using spectral interference fringes.

Wire grid polarizer-based cyclic shearing interferometer with polarization phase shifting

Imaging of transparent phase samples using a wire grid polarizer (WGP)-based shearing interferometer is demonstrated. The proposed setup employs a WGP as a beam splitter in a triangular cyclic interferometer setup, so that the two counterpropagating beams are rendered linearly polarized in orthogonal directions as they emerge from the interferometer. These sheared waved fronts are made to pass through the phase sample. The magnitude of shear is affected by controlled angular shifts of one mirror about the reference X and Y axes. For each of these directional shears, the polarization phase shifting is implemented so as to generate a three-dimensional phase pattern, which is digitally combined and integrated to yield the sample phase. The proposed setup is robust, sufficiently tolerant to ambient vibrations, and shows very encouraging experimental results.

Mineral Fertilizers from Soil Diagnosis Reduce the Decomposition Time of Cocoa Leaf Litter in the South-Western of Cote d'ivoire

A study on soil mineral fertilization was carried out over three years in three cocoa farms in the Nawa region of south-western Côte d'Ivoire. The aim of this study was to evaluate the effects of mineral fertilizers on the decomposition time of cocoa leaf litter. The experimental design was Fisher block with four treatments (T0, T1, T2, T3) repeated three times. The treatments were for T0, the control without fertilizer, for T1, NPK fertilizer 0-23-19, for T2, NPK fertilizer 0-15-15 + 17 CaO + 5 MgO + 1 B2O3 + 0.5 Zn and for T3, NPK fertilizer 4-10-10 + 19 CaO + 4 MgO + 0.8 B2O3 + 0.3 Zn. In each treatment, 3 plots of 1 m2 were delimited with a 1 cm2 mesh wire grid. The rate of decomposition of the cocoa leaf litter was made by measuring twice a year the weight loss of the ground litter contained in the plots. Comparison of the average decomposition rates between treatments showed that the fertilized soils had the best rates during the first 6 and 12 months of the trial. In Soubré, the T2 treatment (24.6±0.6%) in the 6th month and the T1 (72.6±6.6), T2 (82.4±1.6) and T3 (78.8±4.0%) treatments in the 12th month had the best decomposition rates. At Divo, treatment T1 (38.8±0.5%) in month 6 and treatments T1 (57.8±1.7%), T2 (67.9±10%) and T3 (66.9±7.7%) in month 12 were the best. At Méagui, decay rates did not differ between treatments throughout the trial. The application of mineral fertilizers resulting from soil diagnosis reduced the decomposition time of the cocoa leaf litter.

Improving sensitivity to low-mass dark matter in LUX using a novel electrode background mitigation technique

This paper presents a novel technique for mitigating electrode backgrounds that limit the sensitivity of searches for low-mass dark matter (DM) using xenon time projection chambers. In the LUX detector, signatures of low-mass DM interactions would be very low energy ($\sim$keV) scatters in the active target that ionize only a few xenon atoms and seldom produce detectable scintillation signals. In this regime, extra precaution is required to reject a complex set of low-energy electron backgrounds that have long been observed in this class of detector. Noticing backgrounds from the wire grid electrodes near the top and bottom of the active target are particularly pernicious, we develop a machine learning technique based on ionization pulse shape to identify and reject these events. We demonstrate the technique can improve Poisson limits on low-mass DM interactions by a factor of $2$-$7$ with improvement depending heavily on the size of ionization signals. We use the technique on events in an effective $5$ tonne$\cdot$day exposure from LUX's 2013 science operation to place strong limits on low-mass DM particles with masses in the range $m_{\chi}\in0.15$-$10$ GeV. This machine learning technique is expected to be useful for near-future experiments, such as LZ and XENONnT, which hope to perform low-mass DM searches with the stringent background control necessary to make a discovery.

Polarization analysis of random THz lasers

By varying the geometrical filling factor from 8% to 12% and the hole radius from 3 to 5 µm, we investigate the interference pattern of a set of surface emitting, electrically pumped random lasers (RLs) at terahertz frequencies employing a surface pattern of random holes, studying the polarization of the emitted modes, the spectral emission, and the power extraction. By funneling the laser beams onto a wire grid polarizer, we demonstrate spectral reshaping of the RL emission and of the far-field profile, achieving highly collimated emission (5° × 3° angular divergence) and a spectral coverage of 340 GHz with up to 11 random lasing modes. The polarization analysis of the far-field and spectral characteristics of the laser offer an interesting tool to investigate the complex behavior of quantum cascade RLs, providing a route to engineer the emission of RLs in more detail.

Experimental study on the advective heat flux of a heat exchanger for passive cooling of spent fuel pools by temperature anemometry grid sensor

In commercial nuclear power plants spent fuel assemblies are usually stored in actively cooled water pools. The continuous decay heat release represents a potential risk in case of a station black out scenario. Thus two-phase passive heat removal systems are a key technology to enhance the safety of nuclear power plants. Such systems work only by the energy provided from the heat source, e.g. by the maintenance of a natural convection cooling. A heat transfer loop using air as an unlimited heat sink consists of a primary heat exchanger in the spent fuel pool water and a secondary heat exchanger located in ambient air. Thus the measurement of the heat flux, which gets transferred from the pool to the ambient air, is an important task. If one would measure heat flux, flow rates and temperatures in many positions by help of local probes, the natural flow would get strongly disturbed. For that reason we introduce a heat flux measurement around the secondary heat exchanger located in ambient air, which applies temperature and velocity measurement by an anemometric principle.A 6.5m long flow channel with an electrical heated finned tube heat exchanger was set up at the TOPFLOW facility at HZDR. Since the tubes of a heat exchanger would be tilted in a passive heat removal system, i.e. to allow drainage of the condensed heat transfer medium, different tiled angles were adjusted to 0° (horizontal), 20°,30° and 40°. The frontal velocity was varied between 0.5ms and 4ms and three thermocouples were placed up- and downstream of the heat exchanger respectively. A Temperature Anemometry Grind Sensor (TAGS) was located downstream the heat exchanger. It consists of a wire grid with platinum resistance elements, which are placed in the small sub-channels of a flow straightener to generate laminar flow profiles. Two methods were used to calculate the heat flux: arithmetical average and weighting of the flow area. The results of velocity was compared with the average velocity measured by the volume flow control and out of the velocity and temperature the heat flux was calculated and compared with electrical supplied heat flux. The calculated average velocity measured by the TAGS corresponds well with the velocity measured by the volume flow controller up to approximately 3ms with a maximum deviation of ±5%, but underestimates the velocity measured by the volume flow controller at higher velocities. The heat flux was calculated by five methods, 1.) from the three thermocouples up- and downstream of the heat exchanger, 2.) from the average temperatures measured by the TAGS, 3.) from the weighted temperature measured by the TAGS, 4.) from the average temperature and velocity measured by the TAGS and 5.) from the weighted temperature and velocity measured by the TAGS. In this order the accuracy of methods increases compared to the electrical supplied heat flux. For the last method the maximum deviation was 6.5% for all tilt angles. This measurement concept determines the heat flux without disturbing the flow in the loop.

A finite element model of terahertz substrate-based wire-grid polarizer

Using the finite element approximation of electromagnetic wave propagation problem in frequency domain analysis, we have developed and implemented a finite element model of a terahertz substrate-based wire grid polarizer in a form of unit cell. The proposed polarization unit cell is considered as a waveguide with a specific boundary condition. The model is implemented by the COMSOL software system as a ready to use tool for terahertz polarizers design or refinement. Simulation results show an excellent agreement with known experimental and simulation data.

Tunable Frequency Selective Surface With Angular Stability

In this letter, a paradigm for designing tunable frequency selective surfaces (FSSs) with angular stability is presented. The tunable FSS is implemented by nonresonant subwavelength unit cells loaded with varactor diodes, which can be applied in a dynamic and efficient compensation mechanism. The red shifts introduced by varactor diodes are used to compensate the deviation under oblique incident angles. The proposed tunable FSS is a two-layer structure composed of an array of wire grid and an array of split square ring loaded two varactor diodes. The periodicity of one split square ring is same with that of 2 × 2 wire grids. The equivalent circuit based synthesis method is analyzed to investigate the underlying principle of the proposed FSS. The experimental measurement is carried out in C-band frequency, which demonstrates the feasibility of this method. The measured results are in good agreement with the simulations, manifesting the angular stability at 4.25 GHz.

An Asymptotic Approach for the Scan Impedance in Infinite Phased Arrays of Dipoles

This work presents an analytic model to derive the scan impedance of planar infinite phased arrays of dipoles at a dielectric interface when only the (0, 0) Floquet mode propagates. The proposed model builds on the boundary conditions met by the fundamental Floquet mode at the interface to provide a novel derivation of the equivalent circuit for the scan impedance. The analysis is also extended to include the effect of a sufficiently thick grounded substrate that does not affect the elements’ current distribution and does not interact with the evanescent fields. Next, formulas for the ratio of intensity radiated toward each half-space for an interfacial array are provided. In a consecutive step, an asymptotic approximation is derived for the current distribution in arrays of arbitrary loaded dipoles, and the reactance of a dipole in the array environment is related to the inductance of a grid of wires. This model constitutes a useful tool to clearly identify the role of the different array variables (dipole dimensions, relative permittivity of the substrate, periodicity, end load, and scan angle in the principal planes) on the scan impedance by simple expressions and equivalent circuits. The model predictions are in good agreement with full-wave simulations and with previously published works.

Dark Mode Excitation in Three-Dimensional Interlaced Metallic Meshes.

Interlaced metallic meshes form a class of three-dimensional metamaterials that exhibit nondispersive, broadband modes at low frequencies, without the low frequency cutoff typical of generic wire grid geometries. However, the experimental observation of these modes has remained an open challenge, both due to the difficulties in fabricating such complex structures and also because the broadband mode is longitudinal and does not couple to free-space radiation (dark mode). Here we report the first experimental observation of the low frequency modes in a block of interlaced meshes fabricated through 3D printing. We demonstrate how the addition of monopole antennas to opposing faces of one of the meshes enables coupling of a plane wave to the low frequency “dark mode” and use this to obtain the dispersion of the mode. In addition, we utilize orthogonal antennas on opposite faces to achieve polarization rotation as well as phase shifting of radiation passing through the structure. Our work paves the way toward further experimental study into interlaced meshes and other complex 3D metamaterials.

A mathematical model of the electric field perturbation about an array of conductive wires and its impact on the passing charged particle distribution

An analytic model of the electric field perturbation about an array of wires, such as a grid or a mesh, is presented to assess the impact of using such arrays in charged particle analyzers. The analysis begins with an exact solution for the electric field about a grid of flat wires. The result is connected to the Fourier analysis of a grid with high transparency, which is extended to give a solution for the field perturbation about a mesh of wires. The Fourier decomposition of the electric field profile is utilized to calculate the effective barrier that a grid or a mesh provides to charged particles and to calculate the diffusion of the particle energies that pass through such arrays of wires. The results are applied to the measurements of the temperature of a Maxwellian particle distribution by these arrays and by a collector placed downstream.

Комбінований числово-аналітичний метод параметричного проєктування тера¬герцового підкладкового поляризатора на підставі дротяних сіток

Розглянуто комбінований числово-аналітичний метод параметричного проєктування підкладкового поляризатора терагерцового електромагнітного випромінювання, побудованого на підставі дротяних сіток. Здійснено літературний огляд порівняльних характеристик можливих конструкцій поляризаторів різних типів та описано перевагу конструкцій на підставі дротяних сіток. Наведено огляд матеріалів, що використовують для виготовлення відповідних поляризаторів для різної довжини хвилі випромінювання. Проаналізовано методи, що застосовують для розрахунку характеристик і параметрів поляризаторів на підставі дротяних сіток. Розроблений числово-аналітичний метод параметричного проєктування базується на аналітичному розрахунку ефективних антивідбивних характеристик конструкції елементарної комірки поляризатора. У межах методу розроблено скінченно-елементну модель поляризатора, реалізовану за допомогою програмного забезпечення COMSOL Multiphysics, що дає змогу уточнити характеристики конструкції елементарної комірки і можливості для їх розширення за допомогою числового моделювання методом скінченних елементів задачі поширення електромагнітних хвиль у цій конструкції в частотній області зі специфічними граничними умовами, що сукупно дає наукову новизну. Отримані результати моделювання добре узгоджуються з відомими експериментальними результатами та тісно корелюють з результатами моделювання, що отримали інші наукові групи, що свідчить про адекватність та практичну цінність методу для завдань проєктування поляризаторів терагерцового випромінювання з антивідбивними, високопропускними, низьковтратними та надійними характеристиками. Окрім цього, завдяки використанню числової скінченно-елементної складової, запропонований метод досить просто розширити способом зміни геометрії елементарної комірки для дослідження таких перспективних об'єктів, як багатошарові поляризатори чи поляризатори на підставі складніших мета- чи композиційних матеріалів.

HOMOGENIZATION OF METAL GRID REINFORCED COMPOSITES FOR NEAR-FIELD LOW FREQUENCY MAGNETIC SHIELDING

The purpose of this paper is to provide simple analytical homogenization methods for composite materials containing a metallic wire grid. Estimating their effective electrical properties facilitates the numerical simulation of composite structures for shielding applications in the automotive industry. The presented methods are based on surface impedance approaches and effective media theory. The obtained results show that the shielding properties of the described wire grid composites can be accurately estimated and bounded, using the proposed theories in the low frequency range. The frequency limits vary according to the studied sample. For the presented materials, the validity of the results is shown to be up to a few megahertz. The experimental validation is done by measuring the shielding effectiveness of composite samples using a near-field test bench.

Development of Infrared Wide Band Polarizing Elements with Subwavelength Metal Wire Grids

Development of Infrared Wide Band Polarizing Elements with Subwavelength Metal Wire Grids

A Tri-band Second-order Frequency Selective Surface Designing and Analysis

In this paper, inspired from the technique of patch–aperture–patch, a novel tri-band frequency selective surface with second-order band-pass response in each operation band is presented. The design is implemented by cascading a two-dimensional periodic array of three square loops and an array of wire grids. The proposed structure composed of three metal and two dielectric layers acts as a spatial dual band microwave filter with large band separation. The predicted FSS has the merits of broadband response, excellent stability for different incident angles, and sharp roll-off at X-, Ku- and K-band, respectively. The simulation and measurement are carried out and discussed. The measured results agree well with the simulated ones.

Electronically controlled liquid crystal terahertz polarization manipulation under static magnetic field pre-anchoring

In this work, a new strategy was proposed for active control of mm-thick liquid crystals (LC) cell to realize the polarization manipulation in terahertz (THz) regime, which through the electric field control and static magnetic field pre-anchoring. The LC cell was fabricated by a nematic 5CB LC and two silica substrates that were coated with the graphite layer as the transparent electrode. Under the pre-anchoring of the static magnetic field, the optical axis of LC can be precisely controlled by the variable electric field. By using a THz-TDS with a wire grid polarizer, the output THz polarization from the LC cell can be deduced from the amplitude and phase shift of ±45° components. Here, we systematically analyzed three different outfield configurations. Only if the ±45° components that output from the polarizer have phase shifts, can the polarization state conversion be realized. The results show that the linear to circular or the cross-polarization conversion were realized under the specific electric field. This work provides a new approach for the thick-LC layer anchoring and orientation control, and also the tunable polarization manipulation of THz LC devices.

Improved selectivity for the determination of trinitrotoluene through reactive stage tandem ion mobility spectrometry and a quantitative measure of source-based suppression of ionization

A tandem ion mobility spectrometer was used to mobility isolate ions at the drift time for trinitrotoluene (TNT) in a first mobility stage, remove an interfering compound by ion decomposition in a middle reactive stage, and mobility characterize the remaining TNT ions in a second mobility stage. This sequential processing of ions provided decisive detection of TNT in the presence of an interfering peak differing from TNT in reduced mobility coefficient (Ko) by only 0.02 cm2/V. Even though ions of TNT (as M − 1)- and the interfering compound were more than 90% convolved, TNT could be selectively detected with more than 95% decomposition of the interferent at 123 Td to an ion now separated by ΔKo of 0.2 cm2/V from TNT. Ions for TNT were not decomposed in these electric fields though transmission efficiency was decreased by 20% through a wire grid assembly (the reactive stage). Although tandem ion mobility spectrometry with a reactive stage improves selectivity of measurement in the drift time dimension, the chemistry of ion formation in the ion source is affected still by ion suppression. Response to 1 ng TNT was decreased as much as 30% from 200 ng of interferent deposited on sample trap.

Measurements for verification and validation of thermal-hydraulic computer code used for thermal-hydraulic analysis of VVER440 typical fuel assembly

The paper contains experimental data and analysis of the velocity distribution and pressure drop of turbulent flow through a fuel bundle of 19 rods as a partial model of the VVER-440 reactor core. Also, results from the code calculations at the outlet of the test assembly at different flow rates and comparison with experimental data are presented. A pitot tube was used to measure the velocity and pressure drop. Detailed measurements of the velocity distribution in a fuel bundle are done in a fully developed turbulent flow region. Moreover, detailed measurements of the pressure drop of wire and spacer grids typical for PWRs are discussed together with the total pressure drops performance. The experimental result was compared with the predictions by our code and good agreement was found. The calibration protocol and uncertainties of the measurements are presented.

Nanowire Grid Reflecting Polarizers for Ultraviolet Applications

Polarimetry in the far ultraviolet (FUV) is a powerful tool in many applications such as UV/EUV ellipsometry, characterization and control of the beam polarization status in large scale facilities and solar physics. FUV polarizers are among the most difficult components to manufacture, mainly due to the lack of dichroic and transparent materials in this spectral range. Although many different solutions for their fabrication have been investigated in the last decades, surprisingly, the use of Wire Grid Polarizers (WGPs) is still poorly investigated in this spectral region. In this work, two different concepts of WGPs have been designed and optimized for the FUV range: one is based on absorptive nano-wires on top of an highly reflective substrate, and the second one is based on highly reflective nano-wires on top of an absorptive substrate. Different wires’ shapes have been considered and relative structures optimized at a target wavelength of 121.6 nm. Two very promising solutions have been selected, which exhibit a polarization degree over 99.9% and a TE-reflectance over 0.2. Their sensitivity to the wires’ dimension parameters have been investigated to assess their feasibility.