Array Of Unit Cells Research Articles

Programmable Acoustic Metasurfaces.

Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two-dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.

Opencage radio frequency coil for magnetic resonance imaging

We present a volume radio frequency coil for Magnetic Resonance Imaging that provides access to the region of interest. While the conventional birdcage coil is composed of a periodic array of similar unit cells making a cylindrical structure, the proposed coil, called “opencage,” is made of an aperiodic array of metamaterial based unit cells presenting different geometries and characteristics. We develop here a dedicated approach based on Bloch impedance matching and phase balance for the design of the opencage coil. We perform full-wave numerical simulations to validate this concept. An experimental demonstration of the opencage coil for small animal imaging at 7 Tesla is presented. The results of the in-vitro, in-vivo imaging and B1+ map reconstruction achieved with a preclinical MRI scanner are presented. We show that B1+ field homogeneity and amplitude generated by the opencage coil are comparable to those of a conventional birdcage coil of the same size.

The Effect of Crystal Contact Forces on the Protein Global Motions

Protein crystals are widely used for structural determination using X-ray crystallography and solid state NMR. X-ray free electron lasers have enabled direct dynamical structural movies of photo or chemically initiated biochemical reactions. However the perturbation of protein dynamics by the crystal contact forces has been persistently under debate. If crystal contact forces are sufficiently strong, the time resolved X-ray diffraction measurements will not accurately reflect the in vivo biological dynamics. Here we probe the effect of crystal contact forces on protein structural dynamics by directly measuring the collective intramolecular vibrations as a function of crystal symmetry for chicken egg white lysozyme (CEWL). These vibrations extend throughout the protein, involving coherent motion of entire domains. While the collective vibrational density of states is a broad continuous spectrum peaked near 100 cm−1, (3 THz), polarization dependent terahertz absorption isolates vibrations with displacements primarily along the polarization direction [1]. Triclinic, monoclinic, tetragonal and orthorhombic CEWL crystals are grown according to previous published protocols. X-ray measurements verify their crystal structure and quality before and after terahertz measurements. The anisotropic terahertz absorbance is measured using our new technique: ideal polarization varying anisotropic THz microscopy (IPV-ATM). This near field THz microscopy method allows rapid determination of the anisotropic THz absorption. By starting from measurements of the lowest symmetry, triclinic crystals, which have a single protein per unit cell and absolute alignment of the protein molecule array, we can predict the spectra for higher symmetry groups, neglecting crystal contact perturbations. We compare our predicted spectra to the measurements of the monoclinic, orthorhombic and tetragonal crystals, attaining a measure of the impact of the intermolecular crystal forces on the intramolecular vibrations. [1]G. Acbas, K. A. Niessen, E. H. Snell, and A. G. Markelz, Nat. Comm.(2014) https://doi.org/10.1038/ncomms4076.

Focusing Ultrasonic Wave using Metamaterial-based Device with Cross Shape

Focusing on the ultrasonic wave became a point of interest for many researchers in recent years. In this paper, we present the design of the acoustic metamaterials device with the broadband gradient index in the water medium, whose effective index of refraction can be controlled. The device consist of an organized array of cross shape unit cells, the focusing effect is obtained by controlling in the geometry parameters of cross shape and the arrangement between each unit cell in the device. The finite element numerical simulations were performed in the frequency domain to study the focusing effect of the structures at designed frequencies on the propagation of the ultrasonic wave and experiments also conducted using the same design with the simulations. The experiment results are in the good agreement with the simulation results which shows the applicability of this device for focusing ultrasonic wave.

Wideband Beam Steering Antenna Array of Printed Cavity-Backed Elements With Integrated EBG Structure

The letter concerns the design of a wideband printed antenna array element based on a cavity-backed patch radiator with an incorporated U-slot. An additional 2 × 3 mushroom-like electromagnetic bandgap structure is introduced in the E-plane in close proximity to the patch element on the same single-layer substrate. The proposed array unit cell design is optimized for operation in the 12% relative bandwidth in C-band and the 80° conical beam steering sector. Both simulated and experimental data demonstrate considerable wideband beam steering performance improvement compared to the basic design (without isolating structure) characteristics, provided by achieved array elements decoupling. Research results predict for the 12% relative bandwidth broadside reflection coefficient magnitude |Γ| ≤ 0.3, and maximum for the 80° conical beam steering sector |Γ| ≤ 0.37.

Integrating microsystems with metamaterials towards metadevices

Electromagnetic metamaterials, which are a major type of artificially engineered materials, have boosted the development of optical and photonic devices due to their unprecedented and controllable effective properties, including electric permittivity and magnetic permeability. Metamaterials consist of arrays of subwavelength unit cells, which are also known as meta-atoms. Importantly, the effective properties of metamaterials are mainly determined by the geometry of the constituting subwavelength unit cells rather than their chemical composition, enabling versatile designs of their electromagnetic properties. Recent research has mainly focused on reconfigurable, tunable, and nonlinear metamaterials towards the development of metamaterial devices, namely, metadevices, via integrating actuation mechanisms and quantum materials with meta-atoms. Microelectromechanical systems (MEMS), or microsystems, provide powerful platforms for the manipulation of the effective properties of metamaterials and the integration of abundant functions with metamaterials. In this review, we will introduce the fundamentals of metamaterials, approaches to integrate MEMS with metamaterials, functional metadevices from the synergy, and outlooks for metamaterial-enabled photonic devices.

Deployable Linear-to-Circular Polarizer Using PDMS Based on Unloaded and Loaded Circular FSS Arrays for Pico-Satellites

In this paper, flexible and deployable double-sided linear-to-circular polarizers designed on polydimethylsiloxane are proposed for the first time to the best of our knowledge. ShieldIt textile is used as the conducting element of the two designs based on two different unit cell arrays: a loaded circular patch unit cell or an unloaded circular patch unit cell, both backed by a generic rectangular element on its reverse side. This is in contrast to conventional frequency-selective structure-based linear-to-circular polarizers implemented using rigid substrates, which are multi-layered and requires inter-layer physical spacing. This complicates their implementation using flexible substrates and in a deployable format. Upon implementation of this double-sided polarizer, their final performances are evaluated in terms of the phase difference, conversion efficiency, 3-dB axial ratio (AR), and ellipticity bandwidth (from 40° to 45°). Measurements indicated good agreements with simulations, and both structures exhibited more than 90% of conversion efficiency from 2.34 to 3 GHz (for the loaded circular unit cell) and from 2.36 to 3 GHz (for the unloaded circular unit cell). In terms of ellipticity, a bandwidth of 8.67% is observed for the unloaded design and 13.82% for the loaded design. The unloaded structure exhibited a fractional 3-dB AR bandwidth of 36.36% (from 1.98 to 2.86 GHz) in simulations, and 32.64 % (from 2.00 to 2.78 GHz) when evaluated experimentally. Conversely, the loaded design showed only 12.58%. An equivalent circuit model is proposed and validated via a comparison between the circuit and full-wave simulations. Finally, the performances of these polarizers are also assessed under different bending conditions due to the use of flexible materials, prior to the proposal of a suitable deployment mechanism.

Elliptical BandPassThree Dimensional Frequency Selective Surface with Multiple Transmission Zeros

An elliptic band pass response three-dimensional Frequency Selective Surface (3D FSS) is designed from a single unit cell of 2D array of two shielded microstrip lines. The designed FSS provides pseudo-elliptic band-pass frequency response (5.4 – 9.6) GHz with its application in long-distance radio telecommunications and space communications etc. The four transmission zeros at 5.4GHz, 9.6GHz, 12.4GHz and 15GHz provides wide out-of-band frequency rejection. The 3D FSS is independent of the variations in the incident angle of the plane wave up to 60 degree. Each unit cell is a combination of two shielded microstrip lines with one having an air gap and the other one having in between rectangular metallic plate. When a TE polarized plane wave incidents perpendicular to the perfect electric conductor (PEC) boundary walls shielded microstrip lines, it results in two quasi-TEM modes namely air and substrate mode. The 3D FSS consists of multiple resonators with a multimode cavity having number of propagating modes. These resonating modes in phase provide transmission poles and when out of phase give transmission zeros. The 3D FSS structure is simulated using Ansys HFSS software with improved performance over 2DFSS, for many practical applications such as antenna sub-reflector, radomes and spatial filters.

Exact equations for averaged electromagnetic field and its fluctuations at wave multiple scattering by plane periodic array of magnetic microelements

Electromagnetic wave (EM) multiple scattering by a plane periodic array of magnetic microelements in free space is considered analytically by natural subdividing of the EM wave into the averaged and fluctuation components. Each magnetic element is characterized by magnetic susceptibility tensor and shape. An exact Dyson integral equation is derived for the magnetic field Floquet–Bloch amplitude in-plane averaged over an array unit cell. The mass operator of the Dyson equation is expressed via the T-scattering operator of the array unit cell that satisfies a type of the Lippmann–Schwinger equation. We showed that magnetic field fluctuations are generated by the Bragg–Laue diffraction of an averaged magnetic field on the periodic array and are described inside the array as waves propagating with the Laue wave vectors equal to the difference between the in-plane wave vector of the incident magnetic field and the reciprocal lattice wave vector. We derived, for the first time, an exact quadrature to calculate magnetic field fluctuations from their averaged value. These general results are illustrated by a simple Born approximation. In particular, we revealed a mechanism of discrete waveguide excitation by an incident plane EM wave via the averaged EM wave Brag–Laue diffraction on the magnetic microelement array in the quasi-static approach when the wavelength of incident EM is much larger than the sizes of magnetic elements and periods of the array. The mode energy excitation coefficient at normal incidence of the plane EM wave on the array is evaluated.

A Reconfigurable Dual-Polarization Slot-Ring Antenna Element With Wide Bandwidth for Array Applications

A reconfigurable dual-polarized slot-ring antenna/array, switching between S- and C-band with wide bandwidth in each operating state is presented. By simultaneously switching eight p-i-n diode switches, a $2 \times 2$ C-band antenna array can be reconfigured to an S-band antenna unit cell, and vice versa. Fractal shapes are incorporated into the antenna structure to significantly enhance bandwidth. In the S-/C-band operating state, this antenna shows 69.1%/58.3% fractional bandwidth with a measured maximum realized gain of 2.4/3.1 dBi. For the $2 \times 2$ C-band array, the spacing between the elements is less than $0.5\lambda _{{0}}$ over the frequency band which enables grating-lobe-free beam scanning. A unidirectional version of this antenna/array with more than 12/16 dB front-to-back ratio at 3.5/8 GHz is also demonstrated. This wideband antenna/array can be used as a unit cell of a larger array with element spacing of half wavelength at the center frequency of both frequency bands.

Wearable Electromagnetic Head Imaging System Using Flexible Wideband Antenna Array Based on Polymer Technology for Brain Stroke Diagnosis.

Given the increased interest in a fast, portable, and on-spot medical diagnostic tool that enables early diagnosis for patients with brain stroke, a new approach of a wearable electromagnetic head imaging system based on the polymer material is proposed. A flexible low-profile, wideband, and unidirectional antenna array with electromagnetic band gap (EBG) and metamaterial (MTM) unit cells reflector is utilized. The designed antenna consists of a 4 × 4 radiating patch loaded with symmetrical extended open-ended U-slots and fed by combination of series and corporate transmission lines. A mushroom-like 10-EBG unit cell arrays are arranged around the feeding network to reduce surface waves, whereas 4 × 4 MTM unit cells are placed on the back-side of the antenna to enable unidirectional radiation. The antenna is designed and embedded on a multilayer low cost, low loss, transparent, and robust polymer poly-di-methyl-siloxane (PDMS) substrate and optimized to operate in contact with the human head. The simulated and measured results show that the antenna has a fractional bandwidth of 53.8% (1.16-1.94GHz), more than 80% of radiation efficiency, and satisfactory field penetration in the head tissues with a safe specific absorption rate. An eight-element array is then configured on 300 × 360 × 4.1 mm3 PDMS material covering an average human head size and used as a worn part of the imaging system. A realistic-shaped 3-D specific anthropomorphic mannequin (SAM) head phantom is used to verify the performance of the designed array. The imaging results indicate the possibility of using the designed conformal array to detect a bleeding inside the brain using a confocal image algorithm.

Compact wide band directional antenna using cross-slot artificial magnetic conductor (CSAMC)

In this article a novel wide-band artificial magnetic conductor (AMC) based wideband directional antenna is presented for ultra-wideband (UWB) applications. The proposed novel cross-slot AMC (CSAMC) achieves wide ±90° reflection phase bandwidth of 4.07 GHz (44.69%) and is used as a reflector. The overall antenna structure is designed with 4 × 4 CSAMC unit cell array and has very compact size of (0.584λ0 × 0.584λ0). The proposed structure improves the radiation properties and exhibits 91.5% (3.13-8.41 GHz) impedance bandwidth (VSWR ≤2). Additionally, it results in significant improvement in gain and front to back ratio. The proposed antenna is fabricated and its measured performance is in good agreement with simulation results.

Pixelated Metasurface for Dual-Band and Multi-Polarization Electromagnetic Energy Harvesting

A dual-band and polarization-independent electromagnetic energy harvester composed of an array of pixelated unit cells is proposed. The pixelated unit cell is basically a dual-band resonator loaded with two resistors which model the input impedance of a power combining circuit in a complete harvesting system. To design the unit cell, a topology optimization approach based on pixelization of the surface of the unit cell and application of a binary optimization algorithm is used. The optimization goal is set to maximize harvesting efficiency at 2.45 GHz and 6 GHz. In our design, full symmetry of the unit cell is considered to achieve insensitivity to the polarization of the incident wave. Once, the unit cell is designed, as a proof of the concept, a metasurface harvester composed of 9 × 9 pixelated cells is designed. The full-wave electromagnetic simulation results demonstrate that the proposed metasurface absorbs the incident electromagnetic wave energy with nearly unity efficiency at both frequencies of interest and irrespective the polarization of the incident field while simultaneously delivering the absorbed power to the loads. To validate the simulations, the metasurface harvester is fabricated and tested in an anechoic chamber. A strong agreement between the simulation results and measurements is observed.

Design of triple-band-pass frequency selective structure based on spoof surface plasmon polariton

In this paper, a triple-band-pass frequency selective structure is designed by combining traditional frequency selective surfaces (FSSs) with the spoof surface plasmon polariton (SSPP) guiding structure array. The FSSs consist of Jerusalem cross unit cell array with dual-band-pass property. And the SSPP guiding structure array is composed of vertical metallic blade structure array with high-efficiency transmission by engineering the dispersion of SSPP. By arranging the FSSs above the SSPP guiding structure array, a null located between the two passbands is introduced, resulting in a triple-band-pass characteristic. Furthermore, the triple-band-pass frequency selective structure is simulated, fabricated and measured. Both the simulated and measured results agree well, and verify that the transmissivities are all higher than -0.5dB in three frequency ranges: 2.42-2.90GHz, 5.57-5.85GHz and 9.48-9.85GHz. And the transmissivities of stopbands are lower than -10dB in three frequency ranges: 3.12-5.46GHz, 6.65-7.89GHz and 10.32-15.20GHz.

High-gain bow-tie antenna using array of two-sided planar metamaterial loading for H -band applications

A method to significantly increase the gain of a bow-tie antenna based on metamaterial concept is presented. The μ-negative feature of the proposed two-sided planar metamaterial (TSPM) structure is investigated by retrieving its constitutive parameters from the S-parameters. The proposed structure consists of six slabs that are loaded with a 3 × 3 array of TSPM unit cells, which are embedded in azimuth plane of a modified bow-tie antenna vertically. The TSPM unit cells provide a medium of low effective permeability. The dimensions of the slabs are 30.6 × 20.6 mm2 at 7 GHz. To show the adaptability of the proposed high-gain antenna, the structure was designed to cover H-band applications (6-8 GHz). Consequently, a small antenna is designed in comparison with initial bow-tie structures. The corresponding return loss of the antenna is better than 10 dB over 6 to 8 GHz. The proposed metamaterial antenna was manufactured and its characteristics measured to verify the proposed idea. Good agreement between the measured and simulated data is obtained. Maximum gain measured of the antenna is 10.21 dB at 7.5 GHz, constituting a maximum gain enhancement of 7 dB for H-band in comparison with a bow-tie antenna without any metamaterial structures.

Gain enhanced circularly polarized antenna integrated with artificial magnetic conductor for S-band pico-satellites

A circularly polarized (CP) antenna integrated with an artificial magnetic conductor (AMC) plane is presented. The AMC plane is aimed at enhancing the gain of the CP antenna, besides reduced back-radiation, and avoiding radiation toward other high-frequency components on the CubeSat. It consists of a 3 × 3 array of diamond-shaped unit cells placed behind the complementary dipole-based antenna. To enable deployability and compact deployment size, the AMC structure is designed on a thin flexible Kapton film as substrate, whereas its metallic components are formed using Nickel conductive paint. Meanwhile, the CP antenna is designed on a thin Rogers RO4003C substrate and located on top of the AMC plane at a 20 mm distance. The combined structure operates at 2.4 GHz with satisfactory reflection and radiation performance. It is sized at 99 mm × 99 mm × 21.485 mm3, with 450 MHz (18%) of 10 dB impedance bandwidth and 250 MHz (10%) of 3 dB axial ratio bandwidth. The AMC plane also improved its gain from 3.05 dBi up to 5.35 dBi.

Preliminary design and comparative study of thermal control in a nanosatellite through smart variable emissivity surfaces

The thermal radiation that is rejected or absorbed into deep space is highly variable. Ultralight smart surfaces with arrays of unit cells can be designed to change their effective emissivity and absorptivity without energy consumption, actuators, and controllers, and can be used for the temperature control of satellites. The smart surfaces work in a similar manner to thermal louvers but they are hingeless, lighter, and their activation depends on their anisotropic mechanical properties and multilayer structure. The generated thermal stresses between layers that have a high mismatch in the coefficient of thermal expansion cause large deformations and rotations within small temperature changes. The arrays of the surface open or close, and transform their geometry as a function of temperature; therefore, coatings of different thermo-optical properties are revealed or concealed, thus creating variable emissivity surfaces. The emissivity and absorptivity curves of the smart surfaces can be entirely designed as a function of temperature. Theoretically, an emissivity change equal to Δε = 0.8 can be achieved. The small thermal capacitance renders nanosatellites very susceptible to temperature fluctuations. In this study, different emissivity curves were generated to re-calculate the worst cold and hot cases, and to redesign the thermal control system of a certain nanosatellite. We studied a plethora of design cases based on the energy balance equation in steady state while considering the nanosatellite as one-node geometry. In two ideal designs, the temperature deviation of the nanosatellite in the worst cold and hot cases is limited to Δ Τ = 37 ℃ or 43 ℃ without the use of heaters. Moreover, with a power equal to 0.7 W the temperature deviation is limited to Δ Τ = 20 ℃. Consequently, the thermal fatigue is minimized and the energy consumption during the eclipse phase is reduced.

Gain Enhancement of Monopole Antenna using AMC Surface

A CPW rectangular-ring antenna over an Artificial Magnetic Conductor (AMC) is presented in this work. The AMC is a designed as a dual-band structure having an array of unit cells and operates at 2.45GHz and 5.20 GHz. A CPW antenna uses this dual-band AMC structures as a back-plane. Performance comparison is carried out with and without incorporation of AMC. The simulated and measured results show that the combination of the AMC reflector and the antenna provide directional properties at both frequency bands. It has been found that the antenna gain increases by about 5 dB.

High-order virtual element method for the homogenization of long fiber nonlinear composites

A high-order virtual element method (VEM) for homogenization of long fiber reinforced composites is presented. In particular, periodic composites are considered studying square or rectangular unit cell arrays and circular inclusions. A suitable displacement representation form is adopted reducing the three-dimensional problem to an equivalent two-dimensional one. Material nonlinearity is taken into account for the matrix which can be plastic or visco-plastic. The formulation is proposed for linear and high-order virtual elements. Numerical applications are performed to assess the accuracy of the VEM formulation in comparison with the classical finite element approach. In particular, convergence investigations on the overall elastic moduli and on the Mises equivalent stress are performed. Elasto-plastic and visco-plastic analyses are carried out exploiting the local mesh refinement features typical of VEM showing efficiency of polygonal discretizations.

A dual-band multiple-input multiple-output microstrip antenna with metamaterial structure for LTE and WLAN applications

A method to enhance the gain of microstrip dual-band multiple-input multiple-output (MIMO) antenna using partially reflective surface (PRS) layer is introduced and investigated in this paper. The proposed antenna consists of two FR4 substrates. The lower substrate has two radiating patches with parasitic elements that are supplied independently and create the MIMO property of the antenna. The upper substrate which is known as superstrate is arrays of PRS unit cells. The PRS layer printed on either side of a dielectric substrate and causes the antenna gain to increase in both frequency bands. The proposed antenna is appropriate for LTE (2.4–3.1 GHz) and WLAN (5.1–5.8 GHz) applications. The measured values of S11 and S22 parameters of the antenna are less than −10 dB and its FBR and gain are 12.5 dB and 5dBi, respectively. The average half power beam-width (HPBW) is roughly 108○.