Artificial Dielectric Layers Research Articles

Domain patterning in nonpolar cut PMN–PT by focused ion beam

The formation of the ferroelectric domain structure as a result of irradiation by focused ion beam of [100]-cut 0.61Pb(Mg[Formula: see text]Nb[Formula: see text]O3–0.39PbTiO3 (PMN–PT) single crystals covered by surface artificial dielectric layer and with free surface was investigated. The dot irradiation resulted in formation of the wedge-like domains grown along [00[Formula: see text]] direction. For irradiation of the free surface, the domains are mainly located under the surface, while at the irradiated surface with an artificial dielectric layer the domains are located at the surface. It was shown that the subsurface wedge-shaped part of the domain is unstable and completely disappears after a month due to spontaneous backswitching under the action of the residual depolarization field. The revealed nonlinear dose dependence of the domain sizes was attributed to the distribution of the electric field using the point charge model. The domain interaction for the distance between irradiated dots below 30[Formula: see text][Formula: see text]m has been revealed in all samples. It was shown that the decrease of the distance between irradiated dots in the created domain row leads to an increase in the length of the central domains, which is explained by the contribution of all injected charges to the switching field.

Моделирование магнитоэлектрического материала для антенн

The paper is devoted to the modeling of magnetoelectric material for antennas. The effect observed in magnetoelectric gradient structures is discussed. Gradient magnetoelectric structures are a new type of composite structures consisting of a magnetoelectric material and an artificial dielectric. A new principle of controlling the antenna wave properties based on the geometric displacement of the excitation point due to the application of an external electric field is being considered. The use of an artificial dielectric in a multiferroid structure will make it possible to more flexibly control the areas of conversion of a magnetostatic surface wave into an electromagnetic wave, which in practical terms will give new opportunities for controlling the characteristics of antennas. The conclusion about the possibility of controlled conversion of a magnetostatic surface wave into an electromagnetic wave was obtained as a result of calculations based on the developed model by selecting the values of stationary magnetic and electric fields, and the values of the permittivity of the artificial dielectric layer

Magnetoelectric gradient structures: Properties and applications

This paper is devoted to a comprehensive study on a new type of microwave structures named magnetoelectric (ME) gradient structures. These structures are studied in this paper to understand the possibilities and application principles in feasible devices. The structure under study was calculated at different values of the applied electric field and different values of the relative permittivity of the artificial dielectric layer. The layered multiferroic structure in inhomogeneous electric and magnetic fields was calculated on the basis of the previously proposed mathematical model. The eigenwaves spectrum for several considered cases was the result of the performed calculation. The concept of using ME gradient structures in the design of electronically controlled microwave devices is formed on the basis of the results of a numerical experiment. Structures of this type will preferably be used in electronically controlled devices for the directional transmission of microwave signals, as it was shown in the theoretical part of the paper.

Domain switching in KTP crystals induced by electron beam irradiation

1D domain patterning by electron beam in potassium titanyl phosphate crystals covered by an artificial dielectric layer were studied. The polygonal shape of the switched domains elongated along Y-crystallographic direction was obtained. The linear dose dependence of the domain size was attributed to screening of the depolarization field by injected charge. The domain size increase with electron energy is caused by increase in the electron penetration into the resist layer. Formation of an array of nanodomain streamers at the domain wall was observed. Creation of a through 1 D periodical domain pattern with a 10 µm period was demonstrated.

Analysis of Nonsquare Artificial Dielectric Layers and Application to the Design of Anisotropic Slabs

We present a general analysis to describe non-square artificial dielectric layers (ADLs). Closed-form expressions for the equivalent layer impedance are given for generic plane-wave incidence, assuming that the ADLs have different geometrical parameters in the x- and y-directions. The analytical expressions account for the interaction between layers due to higher-order Floquet modes, thus remain valid for arbitrarily small electrical distance between layers. Such non-square geometries allow the design of artificial anisotropic slabs with azimuth-dependent effective refractive index. As an example for an application, the equivalent model is used to design the superstrate of a double slot antenna, with independent pattern shaping in the two main planes.

Magnetoelectric gradient structures

This work is a comprehensive theoretical study of a new class of structures – magnetoelectric gradient structures, which are a product of the integration of magnetoelectric mutliferroic layered structures and an artificial dielectric. In this paper, a mathematical model of the behaviour of a layered multiferroic structure in inhomogeneous electric and magnetic fields is built on the basis of solving Maxwell’s equations, taken in the magnetostatic approximation, and the equation of motion of the magnetization. Based on the constructed mathematical model, the influence on the spectrum of eigenwaves of stationary magnetic and electric fields, as well as the values of the dielectric constant of an artificial dielectric layer, are analysed. The results of the study confirm the possibility of designing a new class of electronically controlled microwave devices with a triple control mechanism: magnetic field – electric field – artificial distribution of dielectric constant.

Design of Broadband Wide-Angle Scatterer Surface Using Artificial Dielectric Layers

In this letter, the reflection elements based on artificial dielectric layers (ADLs) are proposed for broadband wide-angle bistatic radar cross section (RCS) reduction. ADLs are introduced to improve the interference cancellation bandwidth of the reflection elements, and avoid the dilemma of mixing multiple dielectric mediums. The reflection elements achieve an approximate 180° phase difference over the frequency range of 4.8–17.5 GHz (113.9%) under normal incidence, and the bandwidth under TE 40° incidence and TM 55° is 113.9% and 100%, respectively. To validate this design, a coding scatterer surface is simulated and fabricated. Measured results show that the proposed coding scatterer surface achieves a 10 dB bistatic RCS reduction from 5 to 17.5 GHz under normal incidence for both polarizations. Bistatic RCS reduction under oblique incidence is analyzed and presented as well.

Domain splitting in lithium niobate with surface dielectric layer

We present the results of approaching domain walls merging in lithium niobate single crystals with artificial surface dielectric layer produced by proton exchange. It was shown that existence of dielectric layer led to formation of quasi-regular chain of residual rounded domains near Z + polar surface. The residual comb-like domain was formed near Z-polar surface. The tilt angle of charged domain wall was 5°. The formation of chain of isolated rounded domains was attributed to splitting instability of residual stripe domain. The formation of comb-like domain was related to the cyclic formation and termination of growing dents.

Frequency Agile Microstrip Patch Antenna Using an Anisotropic Artificial Dielectric Layer (AADL): Modeling and Design

This paper presents the modeling and design of a frequency agile antenna using an anistropic artificial dielectric layer (AADL) for multi-band phased array radar applications. The proposed AADL material is placed underneath the patch radiator and is designed using periodically arranged metallic cylinders in which varying height, diameter, and the distance used between them allows the control of the effective permittivity of the patch antenna. Closed form design expressions are formulated to synthesize the dielectric properties of the AADL as a function of the cylindrical unit cell dimensions. Design trade-offs based on the proposed formulation and numerical simulations show the overall performance of the AADL on microstrip patch (MS) antennas. To validate the proposed concept, five individual AADL MS patch antennas in C-band were designed, fabricated, and tested. Simulated and measured results ( ${s}$ -parameters and radiation patterns) are in good agreement with the results obtained from the theoretical model. The proposed AADL concept has the potential to be used in the development of future reconfigurable tunable multiband antennas that use liquid metal to dynamically change the heights of the cylinders.

Domain structure formation by electron beam irradiation in lithium niobate crystals at elevated temperatures

We have studied experimentally the domain formation by electron beam irradiation in congruent lithium niobate crystals covered by a photoresist layer at the elevated temperatures up to 200 °C. The measured linear dose dependence of the switched domain area created by isolated dot irradiation has been attributed to external screening of the depolarization field by charges produced by incoming electrons. The significant decrease in the threshold field at the elevated temperatures has led to an increase in the domain area. The observed qualitative change of the domain shape with the temperature increase has been attributed to the highly nonequilibrium switching conditions caused by the existence of the artificial dielectric layer and domination of the stochastic nucleation. The optimal values of temperature and dose have allowed obtaining almost flat boundary of the switched area for line irradiation. The periodically poled structures of the through domains with periods from 10 to 40 μm have been created.

Dissipation Losses in Artificial Dielectric Layers

Closed-form expressions to describe artificial dielectric layers with finite conductivity are presented. The propagation of a generic plane wave within the artificial material is described by means of transmission line models, where each layer is represented as an equivalent shunt impedance. The given analytical formulas for the shunt impedance are derived assuming finite conductivity of the metal, and thus, also an accurate estimation of the losses within the artificial dielectric is obtained from the equivalent circuit. The expressions account for the reactive coupling between the layers due to higher order Floquet modes, thus remain valid even for extremely small electrical distance between layers.

Analytical Formulas for Artificial Dielectrics With Nonaligned Layers

In this paper, we present analytical models to describe artificial dielectric layers (ADLs), when a lateral shift between layers is present. The alternate lateral displacement between layers is an important parameter to engineer the desired effective electromagnetic properties of the equivalent homogeneous material realized with the ADLs. More specifically, the equivalent dielectric constants that can be realized by alternatively shifting the layers are higher compared with the aligned case. Closed-form expressions are derived for the equivalent layer reactance that includes the higher order interaction between shifted layers. The given analytical formulas can be used to derive an equivalent circuit model that describes the scattering parameters of a plane wave impinging on a slab composed by an arbitrary finite number of metal layers. To aid the design of artificial dielectric slabs, the effective permittivity and permeability tensors are also retrieved from the scattering parameters.

Formation of self-assembled micro- and nano-domain structures in uniaxial ferroelectrics

We present the experimental study of appearance of domain wall shape instabilities and self-assembled domain structures in uniaxial ferroelectrics lithium niobate and lithium tantalate covered by artificial dielectric layer. The domain structure evolution has been considered as a manifestation of nucleation processes similar to the first order phase transformation. The necessary conditions for formation of self-assembled domain structures including the highly non-equilibrium switching conditions and stability of concave angles were proposed. The formation of the self-assembled domain shape instabilities under application of the uniform external electric field during switching (domain growth) and backswitching (domain shrinkage) has been studied experimentally with these conditions fulfilled. The formation of the bumps at the vortexes of hexagon domain for diameter above 3 - 5 μm with subsequent oriented growth was obtained in stoichiometric lithium tantalate. The formation of quasi-regular fish-bone domain structure has been revealed during spontaneous backswitching in Mg doped lithium niobate. The resulted structure consisted of the narrow residual domains with width from 200 to 500 nm appeared as a result of finger growth to the center of hexagon domain. The obtained decreasing of the number of the residual domains during growth has been attributed to strong electrostatic interaction of domain walls.

Influence of the artificial surface dielectric layer on domain patterning by ion beam in MgO-doped lithium niobate single crystals

We experimentally compared the shapes and sizes of isolated domains created by the focused ion beam irradiation in the lithium niobate crystals doped by MgO with polar surface covered by dielectric (resist) layer and those with free surface. We attributed the larger sizes of isolated domains in the samples covered by the resist layer to ion localization in the resist. We revealed a change in the domain shape as a function of increasing dose and explained it in terms of kinetic approach taking into account the modification of the surface layers by ion irradiation. We applied the obtained knowledge for 1D and 2D periodical poling using ion beam. Finally, we created the 2D square 1-μm-period array of isolated domains with radius about 300 nm and maximal depth of all domains in the array up to 100 μm as well as the 1D pattern of through stripe domains with 2-μm-period in 1-mm-thick wafer.

Formation of self-assembled domain structures in single crystals of lithium tantalate with artificial dielectric layer

ABSTRACTThe polarization reversal was studied in single crystals of lithium tantalate with intermediate deviation from stoichiometric composition (([Li]/([Li]+[Ta]) = 48.9%) with and without surface photoresist layer. The switching current was analyzed according to the stages of domain structure evolution revealed during in situ visualization of domain kinetics. The characteristic times of the switching process were extracted by fitting of the experimental data using modified Kolmogorov-Avrami model. The formation of the self-assembled domain chains was observed during polarization reversal with photoresist dielectric layer on one polar surface.

Wideband, Wide-Scan Planar Array of Connected Slots Loaded With Artificial Dielectric Superstrates

Microwave broadband wide-scan antenna arrays are typically implemented resorting to vertical arrangements of printed circuit boards (PCBs). Here, we propose a planar solution realized with a single multi-layer PCB, with consequent reduction in cost and complexity of the array. It consists of an array of connected slots backed by a metallic reflector and loaded with superstrates. Artificial dielectric layers (ADLs) are used in place of real dielectrics to realize the superstrates, as they are characterized by very low surface-wave losses. For the unit-cell design, we developed an analysis tool based on closed-form expressions and thus requiring minimal computational resources. Finite-array simulations are also performed by generalizing the analysis method to account for the truncation effects. The presence of the ADL superstrate allows reducing the distance between the array plane and the backing reflector while maintaining good matching performance. A realistic feed structure is also proposed, which consists of a microstrip line connected to a coaxial feed. Such a solution does not require balanced-to-unbalanced transitions, which often limit the achievable bandwidth. The proposed structure achieves in simulations more than an octave bandwidth (6.5–14.5 GHz), within a scanning range of $\pm 50$ ° in all azimuth planes.

Enhancement of Tilted Beam in Elevation Plane for Planar End-Fire Antennas Using Artificial Dielectric Medium

This communication describes a simple and effective beam-tilting technique for planar end-fire antennas using an artificial dielectric layer. The proposed approach is based on the phase differential resulting from a high refractive-index medium that is achieved using double G-shaped resonators (DGRs) in a ${5} \times {4}$ array. The array is oriented normal to the direction of the main beam emanating from the antenna. To demonstrate the principle, the technique is applied to a bow-tie antenna, which is designed at the WiMAX frequency band (3.4–3.6 GHz). The antenna performance was verified practically, and the measured results confirm that the direction of antenna’s maximum beam can be refracted by 35° in the H-plane, which is larger than conventional techniques. In addition, a maximum gain enhancement of 5 dB is achieved when the beam is tilted. Reflection-coefficient of the proposed structure is maintained better than $- {10}\;\text{dB}$ across its operational band.

Design, Fabrication, and Measurements of a 0.3 THz On-Chip Double Slot Antenna Enhanced by Artificial Dielectrics

In this paper, we demonstrate, at 300 GHz and with integrated technology, the effectiveness of artificial dielectric layers to enhance the front-to-back ratio of printed antennas. This concept was previously proposed at microwave frequencies and using printed circuit board technology. The artificial material is now realized by introducing non-resonant metallic inclusions in a silicon dioxide host material. This allows to enhance the permittivity of the host medium and renders it anisotropic. By loading an electrically thin dielectric with these metallic inclusions, an engineered slab with effectively quarter wavelength thickness has been realized. Despite the large effective height and density of the artificial dielectric, the surface wave efficiency of the antenna is 99%. This is entirely due to the anisotropic properties of the material. A prototype antenna was built using an in-house CMOS back-end compatible integrated circuits (IC) process. Measured results from the antenna are presented and show a good agreement with the expected results.

Closed-Form Analysis of Artificial Dielectric Layers—Part II: Extension to Multiple Layers and Arbitrary Illumination

In this second part of a two-paper sequence, we present an analytical formulation to model artificial dielectric layers (ADLs) of finite height. We extend the method developed in Part I (IEEE Trans. Antennas Propag., vol. 62, no. 12, pp. 6256-6264, 2014) describing the single layer to the multilayer case, by including the higher-order interaction between parallel sheets in analytical form. From the equivalent impedance of the layers, we can construct a transmission line model that provides the spectral Green's function of ADLs in closed form. This allows to characterize the propagation through finite ADLs and to study the dispersion characteristics from the investigation of the Green's function singularities. Finally, by integrating the responses of a continuous set of plane waves, sources at finite distances from the ADL slab are treated. The method is validated by comparison with commercial electromagnetic solvers and experimental results obtained from a X-band prototype demonstrator.

Closed-Form Analysis of Artificial Dielectric Layers—Part I: Properties of a Single Layer Under Plane-Wave Incidence

This two-part sequence of papers deals with the analysis of artificial dielectric layers (ADLs). ADLs consist of a cascade of planar periodic surfaces designed to realize dielectric slabs with a desired equivalent permittivity. A single layer is typically composed of an array of electrically small patches periodically arranged in a rectangular lattice. In this paper we present an analytical formulation to model a single layer for arbitrary plane-wave incidence, highlighting the characteristic properties and their frequency range of validity. We derive a closed-form solution for the magnetic current distribution, including the reactive near field on the crossing slots, by expanding the total current with ad-hoc entire-domain basis functions. Simple analytical expressions are also derived for the equivalent sheet impedance of the layer. The second part of the paper extends the analysis to finite ADL slabs and nonplane wave sources.