Isotropic Permittivity Research Articles

3D‐printed heterogeneous isotropic–anisotropic dielectric resonator for singly fed dual‐band circularly polarised antennas

AbstractThe design of three‐dimensional (3D)‐printed heterogeneous ceramics exhibiting isotropic and anisotropic dielectric permittivities at microwave frequencies is studied for dielectric resonator antenna (DRA) applications. The heterogeneous ceramics are engineered by using periodic structures made up of subwavelength unit cells filled with zirconia ceramic and air. Ceramic stereolithography (CSLA) allows the 3D printing of a one‐piece ceramic block mixing isotropic and anisotropic dielectric regions. In order to demonstrate the possibility of exploiting such 3D‐printed ceramics, a singly fed dual‐band circularly polarised DRA is presented. To achieve circular polarisation at both bands, the relative permittivity of the dielectric resonator is locally controlled by introducing anisotropic dielectric regions. The 3D‐printed DRA is finally measured, and its results agree well with the simulations.

Recursive-formula for second-harmonic generation problem in photonic hypercrystal

In this work, we proposed a recursive-formula based on transfer-matrix method to solve problem of second-harmonic generation by obliquely incident fundamental-harmonic wave in a photonic hypercrystal, which is composed of one-dimensional periodically alternating arrangement of nonlinear hyperbolic metamaterial layer with anisotropic permittivities: εaxx, εazz and layer thickness: a and linear dielectric layer with isotropic permittivity: εa and layer thickness: b. This proposed formula was very simple and accurate for calculating conversion efficiency of second-harmonic generation at various incident angles of fundamental-harmonic wave and thicknesses of nonlinear hyperbolic metamaterial layer. The numerical results showed that the maximum conversion efficiency can be achieved by the strong local field confinement of fundamental-harmonic wave in the photonic hypercrystal and phase-matching due to optimal values of hypercrystal layer thickness and incident angle of fundamental-harmonic wave.

Full-silica metamaterial wave plate for high-intensity UV lasers

Bringing light–matter interactions into novel standards of high-energy physics is a major scientific challenge that motivated the funding of ambitious international programs to build high-power laser facilities. The major issue to overcome is to avoid laser intensity heterogeneities over the target that weaken the light–matter interaction strength. Laser beam smoothing aims at homogenizing laser intensities by superimposing on the target laser speckle intensities produced by orthogonal left and right circularly polarized beams. Conventional wave plates based on anisotropic crystals cannot support the laser fluences of such lasers, and the challenge is now to design wave plates exhibiting a high laser induced damage threshold (LIDT). Fused silica exhibits high LIDT, but its isotropic dielectric permittivity prevents effects on polarization retardance. Metamaterials have been widely investigated to tailor the phase and polarization of light but with plasmonic or high-refractive-index materials, and applying this approach with silica is highly challenging due to the weak optical contrast between silica and air or vacuum. Here we design and fabricate a silica-based metasurface acting almost like a quarter-wave plate in the UV spectral range, fulfilling the numerous constraints inherent to high-power laser beamlines, in particular, high LIDT and large sizes. We numerically and experimentally demonstrate that fused silica etched by deep grooves with a period shorter than the wavelength at 351 nm operates the linear-to-quasi circular polarization conversion together with a high transmission efficiency and a high LIDT. The high aspect ratio of the grooves due to the short period imposed by the short wavelength and the deepness of the grooves required to overcome the weak optical contrast between silica and air is experimentally obtained through a CMOS compatible process.

Configurable topological phonon polaritons in twisted hBN metasurfaces.

Phonon polaritons are hybrid excitations that originate from coupling of photons with optical phonons in polar crystals. Hexagonal boron nitride (hBN) is a representative phonon polariton material in mid-infrared that exhibits long lifetimes and ultraslow propagation. However, due to in-plane isotropic permittivities, the dispersion engineering and highly canalized ray-like propagation along the in-plane surface required in photonic and optoelectronic applications cannot be realized in a bare hBN structure. In this paper, we theoretically investigate phonon polaritons in twisted hBN metasurfaces. Due to interactions between different propagating polaritons in the top and bottom metasurfaces, configurable polaritons can be hybridized. Importantly, the hybridized polariton dispersion can be changed from the hyperbolic type to elliptical type via tuning the twisting angle. The demonstrated steerable dispersion evolution and highly canalized propagating polaritons hold promise for nano-optical applications such as in-plane hyperlensing, waveguiding, and focusing.

Can Isotropic Negative Permittivity ε and Permeability μ Metamaterials Exist?

An attempt is made to answer the question posed in the title. Periodic metamaterials, namely, cubic photonic crystals with dielectric inclusions (including those with metallic properties), methods for introducing material equations (homogenization), and the possibility of describing metamaterials by scalar permeability and permittivity (or two scalar parameters) including negative values of their real parts, are considered. For photonic crystals, the Landau conclusion about the absence of high-frequency (optical) magnetic properties is confirmed. In particular, negative values of magnetic permeability or any of its components are impossible. Configurations of metamaterials with almost isotropic properties are presented without regard for spatial dispersion.

Light scattering by a spherical particle with a time-periodic refractive index

A rigorous time Floquet method for the calculation of scattering of electromagnetic waves by a homogeneous spherical object, characterized by a periodically varying-in-time isotropic permittivity, is presented. The method is applied to the study of Mie scattering by such a modulated dielectric particle. Our results are in excellent agreement with the quasistatic adiabatic approximation in the slow-modulation limit. At higher modulation frequencies, a remarkable spectral response, including resonant inelastic scattering and frequency conversion as well as energy transfer between the dynamic sphere and the electromagnetic field, is revealed and consistently explained.

Optical properties of tungsten trioxide, palladium, and platinum thin films for functional nanostructures engineering.

In recent years, we have been witnessing the intensive development of optical gas sensors. Thin palladium and platinum films as well as tungsten trioxide films with palladium or platinum catalysts are widely used for hydrogen detection, and the optical constants of these materials are required for sensor development. We report the optical parameters retrieved from a set of ellipsometric and transmission spectra for electron-beam evaporated palladium, platinum, and tungsten trioxide films. The tungsten trioxide films were 81 nm, 162 nm, and 515 nm thick and the metal films were as thin as 5-7 nm. Ultrathin palladium and platinum films were shown to be successfully described by local and isotropic permittivity, which is quite different from known bulk values. However, this permittivity showed a strong dependence on adjacent materials, thus illustrating that the ultrathin metallic films can be considered composites characterized by effective permittivity. With the obtained refractive indices and permittivities, the optical spectra of fabricated WO3/Pd and WO3/Pt nanostructures incorporating 1D grating of Al2O3 were in an excellent agreement with the calculated ones without requiring any additional fitting procedures or inclusion of surface roughness layers in numerical models.

A Backward Zenneck Wave Propagating along a Plane Interface between Media

We consider a surface Zenneck wave that propagates along a plane interface between a vacuum and a medium that is described by an isotropic dielectric permittivity and a surface Zenneck wave that propagates along a plane interface between a vacuum and a medium that is described by isotropic dielectric permittivity and magnetic permeability. In the latter case, we also consider an H wave and the possibility that the real parts of the permittivity and permeability are negative. Forward and backward waves at the boundary of a material with inhomogeneous profiles of the dielectric permittivity and magnetic permeability are considered. The conditions of the existence of forward and backward surface and volume waves, as well as of fast and slow surface waves are investigated.

Operator Pencil Approach in an Electromagnetic Problem of Symmetric Wave Propagation in a Plane Shielded Waveguide

The paper focuses on the problem of electromagnetic wave propagation in plane shielded dielectric waveguide. The waveguide is characterized by an isotropic inhomogeneous permittivity and constant permeability. We study symmetric guided waves that characterised by a pair of (coupled) propagation constants. Using an operator pencil approach, we prove a discreteness property for the searched-for propagation constants.

Multiparametric Electromagnetic Inversion of 3-D Biaxial Anisotropic Objects Embedded in Layered Uniaxial Media Using VBIM Enhanced by Structural Consistency Constraint

This article presents the 3-D inversion of biaxial anisotropic objects embedded in layered uniaxial media by the variational Born iterative method (VBIM). Although previously only the isotropic complex permittivity profile was reconstructed, here we retrieve all the six diagonal anisotropic permittivity and conductivity parameters simultaneously. In the forward model, the stabilized biconjugate gradient fast Fourier transform (BCGS-FFT) method is used to solve the volume electric field integral equation (EFIE). In the inversion model, VBIM is employed to minimize the cost function. In order to improve the inversion results, we propose a novel structural consistency constraint (SCC) applied to VBIM. The SCC is based on the Monte Carlo method and reduces the computational domain in the Born iteration. The major new contribution of this work is to combine the deterministic inversion algorithm VBIM with the stochastic Monte Carlo method to reconstruct all the diagonal anisotropic parameters of the scatterers. Several typical numerical models are simulated, and the results validate the efficiency, antinoise ability as well as the accuracy of VBIM-SCC for reconstructing 3-D biaxial anisotropic objects embedded in multilayered uniaxial media.

Assessing the Role of a Semiconductor’s Anisotropic Permittivity in Hafnium Disulfide Monolayer Field-Effect Transistors

2-D semiconductors show great promise to serve as channel materials in next-generation field-effect transistors (FETs). The permittivity of many 2-D semiconductors is anisotropic, though many recent simulation works studying 2-D FETs have treated these materials as though they have isotropic permittivities. Because there have been no works that investigate the role of each element of a semiconductor’s anisotropic permittivity on a device’s performance, the impact that this isotropic approximation has on a simulation’s accuracy is unknown. Furthermore, the impact of a semiconductor’s anisotropic permittivity on a device’s performance cannot be explained using existing theory. In this simulation study, we investigate, for the first time, the impact of a semiconductor’s anisotropic permittivity on the performance of FETs. Our main findings are that the isotropic approximation becomes inaccurate as the channel lengths of FETs are scaled down and that short-channel effects become less significant when the semiconductor’s in-plane permittivity decreases or its out-of-plane permittivity increases. We also find that the capacitance of the semiconductor in the out-of-plane direction (i.e., the capacitance associated with the out-of-plane permittivity) more significantly influences a device’s gate capacitance when equivalent oxide thickness (EOT) decreases. Therefore, EOT alone cannot be used to assess total gate control in aggressively scaled 2-D devices.

A conformal transformation approach to wide-angle illusion device and absorber

Abstract We theoretically investigate the illusion device designed by a conformal transformation that can render an elliptic defect behave like a flat mirror. Different from illusion devices consisting of the complementary medium and anti-object with negative permittivity and permeability, our proposed illusion device requires only isotropic positive permittivity medium. It offers a possible route to eliminate the lateral shift in the conventional quasi-conformal carpet cloak. Interestingly, with the same conformal transformation, we can achieve an impedance-matched flat absorber by simply varying the shape and the refractive index of the defects. Both the illumination device and the absorber in our design have broad bandwidth, wide-illumination range and polarization-insensitive performance.

Обратная волна Ценнека вдоль плоской границы раздела сред

We consider the surface Zennek wave along the plane boundaries of the vacuum-medium interface with isotropic permittivity, and the vacuum-medium described by isotropic dielectric and magnetic permittivity. In the latter case, we also consider the H-wave and the possibility of negative real parts of the permeability. Forward and backward waves at the boundary of a material with an inhomogeneous permittivity profile are considered. The conditions for the existence of forward and backward surface and volume waves, as well as fast and slow surface waves, are studied.

On the derivation of a general thermodynamic expression for the reaction rate constant for cosolvent reaction systems

AbstractThis article presents the derivation of the thermodynamic expressions for the activation free energy and reaction rate constant for cosolvent reaction systems. These expressions account for the factors that are specific to solution‐phase reactions, which include isotropic electrostatic effects and close‐range solvent−solute interactions. This article discusses the idea that electrostatic effects can be correlated with the isotropic relative permittivity, and solvent−solute interactions can be correlated with the cosolvent mole fraction. This article also shows that this type of thermodynamic analysis is necessary for understanding certain nuances of solution‐phase reaction processes not tractable by other types of analyses.

Topology‐Optimized 3D Photonic Structures with Maximal Omnidirectional Bandgaps

Three-dimensional photonic crystals with large omnidirectional bandgaps are critical for diverse device applications, but have been challenging to design due to their structural complexity. A topology optimization algorithm based on the evolutionary topology optimization method to design omnidirectional bandgaps on demand is proposed. Considering an asymmetric simple cubic lattice, large bandgaps between any two neighbor bands are successfully realized and novel 3D photonic structures obtained. The topology-optimized designs have a uniform bicontinuous character, which originates from the orthogonal distribution of electric fields at the bounds of the bandgaps. The distribution of the dielectric material follows the directions of the wave vectors of the maximum frequencies at the lower bounds. Consequently, the macroscopic effective properties, such as permittivity, exhibit corresponding directionality. The isotropy of effective permittivity indicates the flatness of the lower bound and photonic crystals with large bandgaps tend to have isotropic effective permittivity.

Constitutive Equations of Piezoelectric Layered Beams With Interdigitated Electrodes

This paper establishes the constitutive equations, or linear two-port models, of piezoelectric layered beams with interdigital electrodes (IDEs). The effect of the nontrivial field on the transduction is analyzed. Based on conformal mapping techniques, we derive new analytic expressions for the capacitance, the electric field, and the electromechanical coupling factor of an anisotropic dielectric with the IDE configuration on top. The IDE capacitance with an anisotropic permittivity can be treated as the one with an isotropic permittivity. The complex expression for the nonuniform field is simplified to a quadratic form. A correction is required for the transducer's coupling constant. All modifications are expressed analytically. The analytic models are verified against the finite element method. Finally, the two-port models help to compare the devices with other electrode configurations, such as beams with a top and bottom electrode.

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

ABSTRACTThis study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested.

Analysis of Multilayered Gyroelectric Spherical Cavities by Weak Form VIE Formulation

In this paper, the volume integral equation (VIE) is employed to rigorously formulate the electromagnetic theory inside spherical metallic cavities loaded by highly multilayered isotropic, uniaxial, and gyroelectric-type permittivities. First, the VIE is constructed inside the cavity. However, a modified dyadic Green’s function (DGF) must also be constructed, which takes into account the electric boundary condition on cavity’s perfect electric conducting surface. This modified DGF is not used to solve the VIE. However, its procedure of construction lets to construct a new tensorial expansion for Green’s function. Employing the weak form VIE formulation allows to evaluate the integrals analytically and reduce the VIE to an algebraic form. Rigorous analysis reveals that the weak form formulation arrives exactly to the solution governed by separation of variables method. Among other validations presented, the theory is also validated by HFSS. The eigenfrequencies of highly multilayered cavities and results showing the very different effects in the characteristic modes due to change of the permittivity type and inhomogeneity are discussed.

Q-tensor model for electrokinetics in nematic liquid crystals

We use a variational principle to derive a mathematical model for a nematic electrolyte in which the liquid crystalline component is described in terms of a second-rank order tensor. The model extends the previously developed director-based theory and accounts for presence of disclinations and possible biaxiality. We verify the model by considering a simple but illustrative example of liquid crystal-enabled electro-osmotic flow (LCEO) around a stationary dielectric spherical particle placed at the center of a large cylindrical container filled with a nematic electrolyte. Assuming homeotropic anchoring of the nematic on the surface of the particle and uniform distribution of the director on the surface of the container, we consider two configurations with a disclination equatorial ring and with a hyperbolic hedgehog, respectively. The computed electro-osmotic flows show a strong dependence on the director configurations and on the anisotropies of dielectric permittivity and electric conductivity of the nematic characteristic of liquid crystal-enabled electrokinetics. Further, the simulations demonstrate space charge separation around the dielectric sphere, even in the case of isotropic permittivity and conductivity. This is in agreement with the induced-charge electro-osmotic effect described for isotropic electrolytes surrounding dielectric spheres.

Acousto-optical phonon excitation in piezoelectric wurtzite slabs and crystal growth orientation effects

This paper presents a theoretical investigation of phonon dispersion in piezoelectric slabs of hexagonal crystal symmetry (wurtzite). Specifically we solve the fully coupled dispersion relations in a GaN free standing quantum well by varying the crystal growth direction from the [001] axis to the [010] axis. Accounting for the Drude model in solving the fully-coupled dispersion relations, phonon modes will generate an additional phonon band, with a high local density of phonon states, close to the plasma frequency. As opposed to cubic crystals with isotropic permittivity, the location of this band varies with crystal orientation. We also find that the phonon mode dependence on the crystal orientation is more pronounced for small in-plane wavenumbers.