Array Of Unit Cells Research Articles

Tunable two dimensional optical beam steering with reconfigurable indium tin oxide plasmonic reflectarray metasurface

In this paper, an electrically tunable reflectarray metasurface for two dimensional (2D) beam steering is designed by integration of a thin layer of indium tin oxide (ITO) material into a metal-insulator-metal (MIM) plasmonic unit-cell. The reflectarray is composed of square-shaped patch nanoantennas placed on a stack of insulator-ITO-metallic ground plane. The resonant characteristic of unit-cell and the accumulation of carrier density at the interface of ITO-insulator play key roles in obtaining over 250° of phase agility at around 218 terahertz (THz), by electrically varying the bias voltage. An array of unit-cells with integrated ITO and 2D voltage biasing distribution (from the side) offer the possibility of designing a reconfigurable antenna in which the main beam can be steered to relatively large angles in both θ- and φ- planes at carefully selected operating frequency. The significant advantage of this design is the dynamically adjustable radiation pattern in both azimuth and elevation planes even after fabrication.

Metasurfaces for perfect transmission of sound from air into water

Three orders of magnitude difference in acoustic impedance between air and water has so far prevented effective sound transmission through the interface. A conventional method for an antireflection coating is a quaterwave plate. Such a coating would be too thick and bulky because acoustic waves in audible range are long. More importantly, one cannot find a suitable coating material among the naturally occurring materials, the acoustic impedance of which is required to be a geometrical mean of those of air and water. Here, we present fabrication of acoustic metasurfaces of thickness much smaller than the wavelength, which allows perfect transmission of sound through the air/water interface. The metasurface is a two dimensional array of unit cells which are consisting of membrane and cavities. Our work demonstrates usage of metamaterials for impedance matching in extreme conditions.

Plasmonic solitons and dromions via plasmon-induced transparency

We propose a method to enhance Kerr nonlinearities and realize low-power gigahertz solitons via plasmon induced transparency (PIT) in a new type of metamaterial, which is constructed by an array of unit cell consisting of a cut-wire and a pair of varactor- loaded split-ring resonators. We show that the PIT in such metamaterial can not only mimic the electromagnetically induced transparency in coherent three-level atomic systems, but also exhibit a crossover from PIT to Autler-Townes splitting. We further show that the system suggested here also possess giant second- and third-order nonlinear optical susceptibilities, which may be used to create plasmon solitons and dromions with extremely low power. Our studies raise the possibility for obtaining strong nonlinear effects of gigahertz radiation at very low intensity based on room temperature metamaterials.

Platform tolerant UWB antenna over multi‐band AMC structure

ABSTRACTA low profile (≈0.09λ2.6 GHz) platform tolerant antenna system having a monopole antenna placed over a multiband artificial magnetic conductor (AMC) is presented for ultrawideband (UWB) applications. The AMC structure is a simple triple band structure having planar array of unit cells. The AMC behavior i.e., zero reflection phase point observed about 4.0, 7.2, and 10.3 GHz, respectively. The simulated and measured results of the antenna when placed in free space, on large metal plate, and over numeric body model/human hand are also compared. The results show that the proposed UWB antenna with AMC structure having platform tolerant properties when placed on large metallic plate and body model for on body applications. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1052–1059, 2016

Multidimensional washboard ratchet potentials for frustrated two-dimensional Josephson junctions arrays on square lattices

In this work, we derive an analytical procedure that allows us to write the multidimensional washboard ratchet potential (MDWBP) Uf for a two-dimensional Josephson junction array. The array has an applied perpendicular magnetic field. The magnetic field is given in units of the quantum flux per plaquette or frustration of the form , where Φ0 is the flux quantum. The derivation is done under the assumption that the checkerboard pattern ground state or unit cell of a two-dimensional Josephson junction array is preserved under current biasing. The resistively and capacitively shunted Josephson junction model with a white noise term describes the dynamics for each junction in the array. The multidimensional potential is the unique expression of the collective effects that emerge from the array in contrast to the single junction. The first step in the procedure is to write the equation for the phases for the unit cell. In doing this, one takes into account the constraints imposed for the gauge invariant phases due to frustration. Second, and the key idea of the procedure, is to perform a variable transformation from the original systems of stochastic equations to a system of variables where the condition for the equality of mixed second partial happens. This is achieved via Poincaré's theorem for differential forms. In this way, we find to a nonlinear matrix equation (equation () in the text), that permits us to find the new coordinate variables xf where the potential exists. The transformation matrix also permits the correct transformation of the original white noise terms of each junction to the intensities in the xf variables. The commensurate symmetries of the ground state pinned vortex lattice leads to discrete symmetries to the part of the washboard potential that does not contain a tilt due to the external bias current (equation () in the text). In this work we apply the procedure for the important cases . For , we show that previously efforts for finding the potential are restricted, leading to a reduced dimension of the potential. The correct potential is given in equation (). We examine this issue in detail. New physics emerge when currents are applied in the x and y directions, in particular, we confirm analytically previous numerical work for , concerning the border of stationary states, a landmark of the potential. For , we give a generalization of previous work, in which we include both the currents in the x and y directions as well the noise terms. We find the MDWBP realizes tilted ratchets analogous to a combustion motor.

Spin-Dependent Emission from Arrays of Planar Chiral Nanoantennas Due to Lattice and Localized Plasmon Resonances.

Chiral plasmonic nanoantennas manifest a strong asymmetric response to circularly polarized light. Particularly, the geometric handedness of a plasmonic structure can alter the circular polarization state of light emitted from nearby sources, leading to a spin-dependent emission direction. In past experiments, these effects have been attributed entirely to the localized plasmonic resonances of single antennas. In this work, we demonstrate that, when chiral nanoparticles are arranged in diffractive arrays, lattice resonances play a primary role in determining the spin-dependent emission of light. We fabricate 2D diffractive arrays of planar chiral metallic nanoparticles embedded in a light-emitting dye-doped slab. By measuring the polarized photoluminescence enhancement, we show that the geometric chirality of the array's unit cell induces a preferential circular polarization, and that both the localized surface plasmon resonance and the delocalized hybrid plasmonic-photonic mode contribute to this phenomenon. By further mapping the angle-resolved degree of circular polarization, we demonstrate that strong chiral dissymmetries are mainly localized at the narrow emission directions of the surface lattice resonances. We validate these results against a coupled dipole model calculation, which correctly reproduces the main features. Our findings demonstrate that, in diffractive arrays, lattice resonances play a primary role into the light spin-orbit effect, introducing a highly nontrivial behavior in the angular spectra.

Plasmon dromions in a metamaterial via plasmon-induced transparency

We propose a scheme to realize a giant Kerr nonlinearity and create stable high-dimensional nonlinear plasmon polaritons via plasmon-induced transparency (PIT) in a metamaterial, which is constructed by an array of unit cell consisting of a cut-wire and a pair of varactor-loaded split-ring resonators. We show that, due to the PIT effect and the nonlinearity contributed by the varactor, the system may possess very large second-order and third-order nonlinear susceptibilities. We further show that the system supports a resonant interaction between longwave and shortwave and hence effective third-order nonlinear susceptibility can be further enhanced one order of magnitude. Based on these peculiar properties, we derive Davey-Stewartson equations governing the evolution of longwave and shortwave envelope, and demonstrate that it possible to generate plasmon dromions [i.e., (2+1)-dimensional plasmon solitons with coupled longwave and shortwave components] with very low generation power. Our study raises the possibility for obtaining new, giant Kerr effect and stable high-dimensional nonlinear plasmon polaritons at very low radiation intensity by using nonlinear PIT metamaterials.

Dual-Band Radiation Tilting Endfire Antenna for WLAN Applications

A dual-band bow-tie antenna with enhanced tilt angle in the azimuth plane at 2.4 and 5.5 GHz is proposed. The proposed antenna consists of bow-tie radiators with dual-impedance bandwidth operating at 2.3–2.5 and 5.2–5.8 GHz. To enhance the tilt angle in the azimuth plane, an array of H-shaped meander-line unit cells as electric resonators is designed at 2.4 and 5.5 GHz with high refractive index property. Integrating the proposed array in the off-axis of the bow-tie antenna tilts the main beam radiation in the endfire direction by $+ 17^\circ$ and $- 20^\circ $ at 2.4 and 5.5 GHz, respectively. When the beam is tilted by $17^\circ$ , there is a gain enhancement of 2.7 dBi at 5.5 GHz compared to the unloaded case.

무선 LAN용 원형편파 패치안테나에 이중 적층 상부덮개를 적용한 이득 향상

A high-gain circularly-polarized patch antenna with a double-layered superstrate is proposed operating at a wireless LAN frequency. A superstrate has an array of metallic periodic unit cells and is located above the patch antenna with an air-gap. The designed antenna has a high gain of over 9.59dBi, which is the gain enhancement of 6.48dB compared to the patch antenna without superstrate. And it has a low axial ratio of under 3dB, so that it maintains the circular polarization of the patch antenna. The optimum air-gap height at the superstrate of arrays is 25mm, which is equivalent to about at the frequency of 2.45GHz. We confirmed that the double-layered stacking of a superstrate increases the effective aperture size and hence it leads to enhance a gain of the patch antenna.

A 2.45 GHz Phased Array Antenna Unit Cell Fabricated Using 3-D Multi-Layer Direct Digital Manufacturing

This paper reports on the design, fabrication and characterization of a 3-D printed RF front end for a 2.45 GHz phased array unit cell. The printed unit cell, which includes a circularly-polarized dipole antenna, a miniaturized capacitive-loaded open-loop resonator filter and a 4-bit phase shifter, is fabricated using a direct digital manufacturing (DDM) approach that integrates fused deposition of thermoplastic substrates with micro-dispensing for deposition of conductive traces. The individual components are combined in a passive phased array antenna unit cell comprised of seven stacked substrate layers with seven conductor layers. The measured return loss of the unit cell is ${>}12$ dB across the 2.45 GHz ISM band and the measured gain is ${-}11$ dBi including all components. Experimental and simulation-based characterization is performed to investigate electrical properties of as-printed materials, in particular the inhomogeneity of printed thick-film conductors and substrate surface roughness. The results demonstrate the strong potential for fully-printed RF front ends for light weight, low cost, conformal and readily customized applications.

Micrometer-scale fabrication of complex three dimensional lattice + basis structures in silicon

A complementary metal oxide semiconductor (CMOS) compatible version of membrane projection lithography (MPL) for fabrication of micrometer-scale three-dimensional structures is presented. The approach uses all inorganic materials and standard CMOS processing equipment. In a single layer, MPL is capable of creating all 5 2D-Bravais lattices. Furthermore, standard semiconductor processing steps can be used in a layer-by-layer approach to create fully three dimensional structures with any of the 14 3D-Bravais lattices. The unit cell basis is determined by the projection of the membrane pattern, with many degrees of freedom for defining functional inclusions. Here we demonstrate several unique structural motifs, and characterize 2D arrays of unit cells with split ring resonators in a silicon matrix. The structures exhibit strong polarization dependent resonances and, for properly oriented split ring resonators (SRRs), coupling to the magnetic field of a normally incident transverse electromagnetic wave, a response unique to 3D inclusions.

Giant Kerr nonlinearity and low-power gigahertz solitons via plasmon-induced transparency

We propose a method to enhance Kerr nonlinearity and realize low-power gigahertz solitons via plasmon-induced transparency (PIT) in a new type of metamaterial, which is constructed by an array of unit cell consisting of a cut-wire and a pair of varactor-loaded split-ring resonators. We show that the PIT in such metamaterial can not only mimic the electromagnetically induced transparency in coherent three-level atomic systems, but also exhibit a crossover from PIT to Autler-Townes splitting. We further show that the system suggested here also possess a giant third-order nonlinear susceptibility and may be used to create solitons with extremely low generation power. Our study raises the possibility for obtaining strong nonlinear effect of gigahertz radiation at very low intensity based on room-temperature metamaterials.

A Double-Negative Metamaterial-Inspired Mobile Wireless Antenna for Electromagnetic Absorption Reduction.

A double-negative metamaterial-inspired antenna is presented for mobile wireless applications. The antenna consists of a semi-circular radiating patch and a 3 × 4 hexagonal shaped metamaterial unit cell array in the ground plane. The antenna is fed with a 50 Ω microstrip feed line. The electric dimensions of the proposed antenna are 0.20λ × 0.26λ × 0.004λ, at the low-end frequency. The proposed antenna achieves a −10 dB impedance with a bandwidth of 2.29 GHz at the lower band and 1.28 GHz at the upper band and can operate for most of the mobile applications such as upper GSM bands, WiMAX, Bluetooth, and wireless local area network (WLAN) frequency bands. The focused novelties of the proposed antenna are its small size, multi-standard operating bands, and electromagnetic absorption reduction at all the operating frequencies using the double-negative metamaterial ground plane.

Vortex detection and quantum transport in mesoscopic graphene Josephson-junction arrays

We investigate mesoscopic Josephson junction arrays created by patterning superconducting disks on monolayer graphene, concentrating on the high-$T/T_c$ regime of these devices and the phenomena which contribute to the superconducting glass state in diffusive arrays. We observe features in the magnetoconductance at rational fractions of flux quanta per array unit cell, which we attribute to the formation of flux-quantized vortices. The applied fields at which the features occur are well described by Ginzburg-Landau simulations that take into account the number of unit cells in the array. We find that the mean conductance and universal conductance fluctuations are both enhanced below the critical temperature and field of the superconductor, with greater enhancement away from the graphene Dirac point.

Tunable circular polarization selective surfaces for low-THz applications using patterned graphene.

This paper demonstrates a new type of frequency tunable polarization selective surface operating at low THz, which is devised by utilizing the unique features of graphene. The device is comprised of an infinite array of identical unit cells in three layers. Multiple graphene dipoles are placed on the top and bottom layers to form the vertical and horizontal electric field filters. Using this new configuration, the proposed device exhibits reflection for the incident Left-Hand-Circular-Polarization (LHCP) waves and becomes transparent to the incoming Right-Hand-Circular-Polarization (RHCP) waves. The excited localized surface plasmonic resonance mode on the graphene based unit cells significantly reduces the physical dimension of the device. The unit cell dimension of the proposed design is in the order of 0.18 wavelengths in comparison to conventional metallic structures, where it is of order a half a wavelength. In the full wave analysis, the graphene based polarization selective surfaces exhibit an isolation of 21 dB for LHCP waves and a transmission loss of around 5.1 dB for waves with RHCP characteristics. The performance has also been examined under oblique incidence. The results fully verify that the proposed planar device operates properly for incident angles up to 40°. The tuning effect of the described device is investigated by varying the chemical potentials of graphene. Significant frequency reconfiguration capability is achieved in the isolation of LHCP incident waves, and meanwhile, for RHCP incidence, the transmission rate remains reasonably high.

Validating a Central Approximation in Theories of Regular Electrode Electrochemical Arrays of Various Common Geometries

AbstractThirty years ago one of us introduced a central approximation according to which the regular arrays behaviors could be estimated based on those of arrays of cylindrical unit cells with the same relative surface area of their central disks for the insulating section. This work examines and validates this approximation on the basis of 3D‐simulations of Brownian particles motion for inlaid or recessed electroactive disks as well as for protruding cylindrical electrodes. The approximation is perfectly adequate for predicting the behavior of realistic experimental arrays and is valid within a few percent even in extreme situations when adjacent electroactive sections are almost touching.

Evolution of the ferromagnetic resonance spectrum of a hexagonal antidot lattice with film thickness: Experiment and numerical simulations

Experimental and theoretical studies of the evolution of the ferromagnetic resonance spectrum with film thickness for hexagonal lattice of antidots have been reported. We found three different quasi-uniform modes in the resonance spectrum. Calculations of the local dynamic susceptibility tensor show that these modes are related to the resonances from quasiuniformly magnetized regions of the hexagonal antidot array unit cell. The increase of the film thickness results in the broadening of the resonance lines as well as the appearance of additional resonance lines. Good qualitative correlation of the theory and experiment was obtained. Additional relaxation mechanisms due to magnetic inhomogeneities are also discussed.

Ultra-dark graphene stack metamaterials

We present a fabrication method to achieve a graphene stack metamaterial, a periodic array of unit cells composed of graphene and a thin insulating spacer, that allows accumulation of the strong absorption from individual graphene sheets and low reflectivity from the stack. The complex sheet conductivity of graphene from experimental data models the measured power transmitted as a function of wavelength and number of periods in the stack. Simulated results based on the extracted graphene complex sheet conductivity for thicker stacks suggest that the graphene stack reflectivity and the per-unit-length absorption can be controlled to exceed the performance of competing light absorbers. Furthermore, the electrical properties of graphene coupled with the stack absorption characteristics provide for applications in optoelectronic devices.

Molecular Insights into the Signaling Mechanism of the Histidine Kinase CheA within an Intact Bacterial Chemosensory Array

The chemotactic response in bacteria relies on the formation of large, highly ordered complexes of sensory proteins, known as chemosensory arrays, which mediate the transduction and regulation of signals that ultimately control cellular motility. Although, a coarse representation of the array's extended organization has recently been sketched out, an outstanding problem concerns the detailed description of the molecular events occurring within the array during signaling. Progress in this area necessitates a high-resolution understanding of the intact chemosensory array structure. Utilizing cryo-electron tomography and 3D subvolume averaging, we have derived density maps of a resolution sufficient for the construction and refinement of an atomic model of the chemosensory array's core structure. Large-scale, all-atom molecular dynamics (MD) simulations of an atomic model of the array unit cell, 1.2 million atoms in size, have revealed the molecular details of the interaction interfaces between the receptor, CheA, and CheW proteins, as well as a distinctive conformational change in CheA kinase domain. Mutagenesis and chemical cross-linking studies have further confirmed the important roles of specific residues at these critical interaction interfaces.

Interference-induced angle-independent acoustical transparency

It is revealed that the Fano-like interference leads to the extraordinary acoustic transmission through a slab metamaterial of thickness much smaller than the wavelength, with each unit cell consisting of a Helmholtz resonator and a narrow subwavelength slit. More importantly, both the theoretical analysis and experimental measurement show that the angle-independent acoustical transparency can be realized by grafting a Helmholtz resonator and a quarter-wave resonator to the wall of a narrow subwavelength slit in each unit cell of a slit array. The observed phenomenon results from the interferences between the waves propagating in the slit, those re-radiated by the Helmholtz resonator, and those re-radiated by the quarter-wave resonator. The proposed design may find its applications in designing angle-independent acoustical filters and controlling the phase of the transmitted waves.