Ordinary Dielectrics Research Articles

Compositional heterogeneity enhancing the flexoelectric response of BaTiO3 -based ceramics

Flexoelectric effect possesses potential applications in electromechanical conversion, and the flexoelectric coefficient of ferroelectrics is greatly larger than that of ordinary dielectrics. Among the ferroelectrics, BaTiO3 (BT) ceramics are lead-free materials, also receive a high flexoelectric response. However, it is still inferior to the piezoelectric response of lead-based piezoelectric materials. Herein, we inhomogeneously added x wt. % ZrO2 into BT ceramics (x wt. % ZrO2/BT ceramics, and x = 0.05, 0.5, 1, 2) to enhance the flexoelectric response of BaTiO3-based ceramics. The largest effective flexoelectric coefficients (μρ) of ∼800 μC/m at room temperature (RT) and ∼1500 μC/m near Curie temperature (TC ∼ 127 °C) were obtained in 0.5 wt % ZrO2/BT ceramics, which is ∼6.7 times and ∼3.9 times larger than that of the as-prepared BT ceramics at RT and near TC (∼131.7 °C here) respectively. Although TC of the 0.5 wt % ZrO2/BT ceramics decreases compared with that of BT ceramics, its higher μρ than that of the as-prepared BT ceramics maintains a wide temperature range, extending to 160 °C. The enhancement of the μρ of the 0.5 wt % ZrO2/BT ceramics is largely attributed to the compositional heterogeneity between grain boundaries gathered with irregular paraelectric and orthorhombic Ba(Zr, Ti)O3, and grains which are tetragonal BaTiO3. The compositional heterogeneity is easily shaped into a strain gradient (namely a flexoelectric polarization), which plays a promoting effect on the flexoelectric coefficients.

Giant flexoelectric coefficients at critical ferroelectric transition

Flexoelectric materials, which could generate polarization in response to a strain gradient, could be utilized in a wide range of devices such as actuators and sensors. However, the flexoelectric coefficients of most dielectrics are too small to be utilized. One of the most promising flexoelectric materials is ferroelectrics whose flexoelectric coefficients are orders of magnitude higher than those of ordinary dielectrics. Despite of intensive research in the past twenty years, the record-high flexoelectric coefficient (μ1122=100μC/m) of Ba0.67Sr0.33TiO3 ferroelectric has not been surpassed. Here we show that by tuning the first-order paraelectric-ferroelectric phase transition of BaTiO3 to near-critical through doping BaZrO3, a drastic enhancement of flexoelectric coefficient (μ1122=140μC/m) occurs, which is ascribed to the enhancement of both intrinsic and extrinsic flexoelectricity resulting from the strong lattice instability at the critical transition. This study demonstrates a general design method to achieve high flexoelectric coefficients in a wide range of ferroelectric systems through searching for critical ferroelectric transition points.

A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis

In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high refractive index silicon layer enhances the electric field on the surface of the sensor, so that the sensor has a better overall performance in terms of sensitivity and figure of merit (FOM). Compared with ordinary dielectrics, 2D materials have excellent photoelectric properties, such as larger specific surface area, higher carrier density and stronger adsorption capacity, which improve the detection ability of the sensor. The sensitivity of the optimized sensor achieves 297.2°/RIU, 274°/RIU and 246°/RIU when the silicon layer is covered with graphene, MXene (Ti3T2Cx) and MoS2, respectively. Compared with the traditional SPR sensor, the sensitivity of the structure has been significantly improved, and its excellent performance has broad application prospects in biosensing and other fields.

Second and third harmonic generation from gold nanolayers: experiment versus theory

The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. Understanding accurately how light interacts with these materials at the nanoscale is crucial if one is to properly engineer nano-devices. When the nanoscale is reached, light-matter interactions display new phenomena where conventional approximations may not always be applicable and they should be either revised or voided. In this work, we measure the efficiency of second and third harmonic generation from gold nanolayers. The experimental results are compared with numerical simulations based on a detailed microscopic hydrodynamic model that considers different effects playing a role in the nonlinear response, not usually considered by more generic models. The agreement between experimental and theoretical results proves the importance of all these contributions.

Electromagnetic fields induced by an electric charge near a Weyl semimetal

Weyl semimetals (WSM) are a new class of topological materials that exhibit a bulk Hall effect due to time-reversal symmetry breaking, as well as a chiral magnetic effect due to inversion symmetry breaking. These unusual electromagnetic responses can be characterized by an axion term $\theta \textbf{E} \cdot \textbf{B}$ with space and time dependent axion angle $\theta (\textbf{r} ,t)$. In this paper we compute the electromagnetic fields produced by an electric charge near to a topological Weyl semimetal with two Weyl nodes in the bulk Brillouin zone. We find that, as in ordinary metals and dielectrics, outside the WSM the electric field is mainly determined by the optical properties of the material. The magnetic field is, on the contrary, of topological origin in nature due to the magnetoelectric effect of topological phases. We show that the magnetic field exhibits a particularly interesting behavior above the WSM: the field lines begin at the surface and then end at the surface (but not at the same point). This behavior is quite different from that produced by an electric charge near the surface of a topological insulator, where the magnetic field above the surface is generated by an image magnetic monopole beneath the surface, in which case, the magnetic field lines are straight rays. The unconventional behavior of the magnetic field is an experimentally observable signature of the anomalous Hall effect in the bulk of the WSM. We discuss a simple candidate material for testing our predictions, as well as two experimental setups which must be sensitive to the effects of the induced magnetic field.

Influence of Quantum Effects on the Magnetic Field Behavior in Overdense Plasma

We study the conditions for the anomalous transmission of electromagnetic waves through quantum overdense plasma. We show that this anomalous transmission is triggered due to the excitation of surface waves, as was observed in the classical overdense plasma. The conditions for the excitation of surface waves are obtained by studying the dispersion relation within the framework of quantum hydrodynamics. The corresponding consequences at the classical limits are consistent with the previous studies. In comparison with the classical regimes, the quantum dispersion curve exhibits an asymptotic behavior which indicates significant effects, in particular, at large wavelengths. Herein, to create the required evanescent waves, we consider the quantum plasma to be placed between two ordinary prisms and dielectrics. The effects of the main parameters, such as the permittivity of the prisms and dielectrics and the Fermi velocity, on the rate of the transmission and the magnetic field propagation are also evaluated.

Double-Channel Filter Based on the Structure of Two Symmetric Layers with Defects in One-Dimensional Photonic Crystals

Using 2×2 transfer matrix method, we numerically investigate a kind of structure in which two symmetric layers with defects are sandwiched in one-dimensional photonic crystals (PCs). The PCs are made of ordinary dielectrics and placed in the air. When a light beam is incident into PCs, the two resonant peaks can be achieved which constitute a couple of photonic channels. The transmittance of the resonant peaks can nearly reach up to 1. Furthermore, the tunability of the resonant peaks is discussed in detail, the results shows that the position of the resonant peak depending on the value of the incident angle. These properties can provide a theoretical basis for design of a new type of tunable double-channel photonic crystal filter.

Casimir force between hyperbolic metamaterials

The Casimir force between two hyperbolic metamaterials (HMMs) constructed by alternative metal-dielectric layers is investigated. Due to the existence of the hyperbolic dispersion, the electromagnetic response of HMMs becomes extremely dramatic, which is embodied by the nearly total reflection in such frequency region. As a result, the Casimir force between HMMs is much greater than that between ordinary dielectrics. In addition, it is shown that the Casimir force is proportional to the bandwidth of this hyperbolic dispersion, which is dependent on the filling factor as well as the characteristic frequencies of ingredient materials. Therefore, the relations between the force and these parameters are discussed. We show that the Casimir force can be controlled by tuning the bandwidth possessing hyperbolic dispersion of the structures. This work provides promising applications of HMMs on microelectromechanical systems and nanoelectromechanical systems.

Optomechanics of Soft Materials

Some molecules change shape upon receiving photons of certain frequencies, but here we study light-induced deformation in ordinary dielectrics with no special optical effects. All dielectrics deform in response to light of all frequencies. We derive a dimensionless number to estimate when light can induce large deformation. For a structure made of soft dielectrics, with feature size comparable to the wavelength of light, the structure shapes the light, and the light deforms the structure. We study this two-way interaction between light and structure by combining the electrodynamics of light and the nonlinear mechanics of elasticity. We show that optical forces vary nonlinearly with deformation and readily cause optomechanical snap-through instability. These theoretical ideas may help to create optomechanical devices of soft materials, complex shapes, and small features.

Symmetric Absorbers Realized as Gratings of PEC Cylinders Covered by Ordinary Dielectrics

Nearly all known electrically thin perfect absorbers are asymmetric with respect to their two sides and show perfect (rather, nearly perfect) absorption only from one side of the absorbing layer. Moreover, the vast majority of the designs contain a ground plane, which makes the transmission coefficient zero at all relevant frequencies. Here, we introduce a simple symmetric configuration of a single infinite grating of perfectly conducting rods coated by ordinary dielectrics as a symmetric single-layer perfect absorber not containing any impenetrable layers. With the goal to examine its capacity to absorb electromagnetic radiation, we replace each cylinder by a pair of dipole lines and derive the conditions for the perfect absorption (zero reflection and zero transmission) in terms of their dipole moments. It is a general approach that is not limited to circular cylinders or homogeneous textures; in this way, the generic concept of carrying over properties from simpler to more complicated structures, is realized. A metric of fulfilling the aforementioned two requirements for perfect absorption is defined and evaluated for a number of different cases. A realistic example design demonstrating 96%-97% absorption is presented. The performance can be increased further to reach 98% absorption in case of permitting arbitrary permittivities.

Wave propagation in chiral media: composite Fresnel equations

In this paper, the author studies the features of wave propagation in chiral media. A general form of wave equations in biisotropic media is employed to derive concise formulas for the reflection and transmission coefficients. These coefficients are represented as a composite form of Fresnel equations for ordinary dielectrics, which reveal the circularly polarized nature of chiral media. The important features of negative refraction and a backward wave associated with left-handed waves are analyzed.

Goos–Hänchen and Imbert–Fedorov shifts at the interface of ordinary dielectric and topological insulator

Using Yasumoto and Oishi’s energy flux method, we evaluated the Goos–Hanchen (GH) and Imbert–Fedorov (IF) shifts of beam incident from the ordinary dielectric upon the topological insulator (TI) with totally internal reflection. Comparing with the case of two ordinary isotropic dielectrics, it is found that the topological parameter Θ of TI can affect two shifts. More important, IF shift appears even for a linear polarized TE or TM beam and achieves the maximum with elliptical polarization, which completely originates from the TI’s intrinsic magnetoelectric coupling effect. This observation provides an optical experimental approach to determine the topological parameters Θ and provide a new way to control the GH shift and IF shift.

A route for efficient non-resonance cloaking by using multilayer dielectric coating

An approach for designing transmission cloaks by using ordinary dielectrics instead of meta- and plasmonic materials is proposed and demonstrated by the development of a multi-layer cloak for hiding cylindrical objects larger than the wavelengths of incident radiation. The parameters of the cloak layers were found by using the Genetic Algorithm-based optimization procedure, which employed the reciprocal of total scattering cross width of the cloaked target, derived from the solution of the Helmholtz equation, as the fitness function. The proposed cloak demonstrated better cloaking efficiency than did a similarly sized metamaterial cloak designed by using the transformation optics relations.

Zone-center dynamical matrix in magnetoelectrics

In ordinary dielectrics the dynamical matrix at the zone center in general is a nonanalytic function of degree zero in the wavevector q. Its expression (for a crystal of arbitrary symmetry) is well known and is routinely implemented in first principle calculations. The nonanalytic behavior occurs in polar crystals and owes to the coupling of the macroscopic electric field E to the lattice. In magnetoelectric crystals both electric and magnetic fields, E and H, are coupled to the lattice, formally on equal footing. We provide the general expression for the zone center dynamical matrix in a magnetoelectric, where the E and H couplings are accounted for in a symmetric way. As in the ordinary case, the dynamical matrix is a nonanalytic function of degree zero in q, and is exact in the harmonic approximation. For the sake of completeness, we address other issues, and in particular we solve a problem which might arise in first-principle implementations, where-differently than here-the basic fields are E and B (not H).

Gain and plasmon dynamics in active negative-index metamaterials

Photonic metamaterials allow for a range of exciting applications unattainable with ordinary dielectrics. However, the metallic nature of their meta-atoms may result in increased optical losses. Gain-enhanced metamaterials are a potential solution to this problem, but the conception of realistic, three-dimensional designs is a challenging task. Starting from fundamental electrodynamic and quantum mechanical equations, we establish and deploy a rigorous theoretical model for the spatial and temporal interaction of lightwaves with free and bound electrons inside and around metallic (nano-) structures and gain media. The derived numerical framework allows us to self-consistently study the dynamics and impact of the coherent plasmon-gain interaction, nonlinear saturation, field enhancement, radiative damping and spatial dispersion. Using numerical pump-probe experiments on a double-fishnet metamaterial structure with dye molecule inclusions, we investigate the build-up of the inversion profile and the formation of the plasmonic modes in a low-Q cavity. We find that full loss compensation occurs in a regime where the real part of the effective refractive index of the metamaterial becomes more negative compared to the passive case. Our results provide a deep insight into how internal processes affect the overall optical properties of active photonic metamaterials fostering new approaches to the design of practical, loss-compensated plasmonic nanostructures.

Flexoelectric properties of ferroelectrics and the nanoindentation size-effect

Recent works have established the critical role of flexoelectricity in a variety of size-dependent physical phenomena related to ferroelectrics including giant piezoelectricity at the nanoscale, dead-layer effect in nanocapacitors, dielectric properties of nanostructures among others. Flexoelectricity couples strain gradients to polarization in both ordinary and piezoelectric dielectrics. Relatively few experimental works exist that have determined flexoelectric properties and they all generally involve some sort of bending tests on micro-specimens. In this work, we present a straightforward method based on nanoindentation that allows the evaluation of flexoelectric properties in a facile manner. The key contribution is the development of an analytical model that, in conjunction with indentation load–displacement data, allows an estimate of the flexoelectric constants. In particular, we confirm the experimental results of other groups on BaTiO 3 which differ by three orders of magnitude from atomistic predictions. Our analytical model predicts (duly confirmed by our experiments) a strong indentation size-effect due to flexoelectricity.

An optical reflectarray nanoantenna: The concept and design

This paper presents the concept and design of a reflectarray nanoantenna at optical frequencies whose elements are nano-sized concentric spherical particles with the core made of ordinary dielectrics and the shell made of a plasmonic material. Modeling approaches based on finite difference time domain (FDTD) numerical method and dipole-modes scattering theory are used to characterize and tune the reflectarray design. A 6x6 elements reflectarray nanoantenna operating at wavelength 357.1nm with narrow beamwidth is presented, and its scanned radiation characteristics for 15 degrees and 30 degrees are demonstrated.

The origins of electromechanical indentation size effect in ferroelectrics

Metals exhibit a size-dependent hardening when subject to indentation. Mechanisms for this phenomenon have been intensely researched in recent times. Does such a size effect also exist in the electromechanical behavior of ferroelectrics?—if yes, what are the operative mechanisms? Our experiments on BaTiO3 indeed suggest an elastic electromechanical size effect. We argue, through theoretical calculations and differential experiments on another nonferroelectric piezoelectric (quartz), that the phenomenon of flexoelectricity (as opposed to dislocation activity) is most likely responsible for our observations. Flexoelectricity is the coupling of strain gradients to polarization and exists in both ordinary and piezoelectric dielectrics. In particular, ferroelectrics exhibit an unusually large flexoelectric response.

Magnetic and dielectric properties of YbFe 2−xMn xO 4 (0⩽ x⩽1)

We have investigated the magnetic and dielectric properties of YbFe 2− x Mn x O 4 (0⩽ x⩽1), which is an Fe-site-substituted system of new multiferroic oxides RFe 2O 4 ( R=Y, Ho–Lu). X-ray diffraction measurements show that a solid solution is formed between YbFe 2O 4 ( x=0) and YbFeMnO 4 ( x=1) for 0⩽ x⩽1, whereas only compounds with x=1 (i.e., RM 1 M 2O 4; M 1 and M 2=trivalent and divalent cations, respectively) have been known for the Fe-site substitution in RFe 2O 4. The valence of the Mn ion is determined to be ∼2+, consistently with the suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature ( T N) and the dielectric constant ( ε′) decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe 2O 4 are governed by the electron transfer between Fe-site ions, unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed as well.

Energy functions and basic equations for ferroelectrics

Energy functions (or characteristic functions) and basic equations for ferroelectrics in use today are given by those for ordinary dielectrics in the physical and mechanical communications. Based on these basic equations and energy functions, the finite element computation of the nonlinear behavior of the ferroelectrics has been carried out by several research groups. However, it is difficult to process the finite element computation further after domain switching, and the computation results are remarkably deviating from the experimental results. For the crack problem, the iterative solution of the finite element calculation could not converge and the solutions for fields near the crack tip oscillate. In order to finish the calculation smoothly, the finite element formulation should be modified to neglect the equivalent nodal load produced by spontaneous polarization gradient. Meanwhile, certain energy functions for ferroelectrics in use today are not compatible with the constitutive equations of ferroelectrics and need to be modified. This paper proposes a set of new formulae of the energy functions for ferroelectrics. With regard to the new formulae of the energy functions, the new basic equations for ferroelectrics are derived and can reasonably explain the question in the current finite element analysis for ferroelectrics.