Uniaxial Dielectrics Research Articles

Topological Phase Singularities in Light Reflection from Non‐Hermitian Uniaxial Media

AbstractPerfect light transmission into a dielectric at the Brewster angle is one of the simplest effects of macroscopic electromagnetism. The common wisdom states that absorption in the dielectric violates Brewster angle and leads to a non‐vanishing reflection. Yet, incorporating anisotropy may recover perfect transmission of p‐polarized light into the absorbing medium. Unlike the ordinary “lossless” Brewster angle, perfect transmission in this case is accompanied by phase singularities of the reflection amplitude. In this paper, phase singularities and the associated topological charges are theoretically examined as emerging in the wavelength‐incidence angle space upon perfect transmission into absorbing uniaxial dielectrics. The analytical criterion of perfect light transmission into an anisotropic medium is derived, phase singularities are demontrated in these scenarios, and their dynamics are studied as a function of material parameters. Finally, by lowering the symmetry of the problem, this phenomenon is translated into a different parameter space of wave vector components, and illustrate the feasibility of this phenomenon with available optically anisotropic materials. The results may become valuable for the development of novel analog computing schemes and holography approaches.

Directivity Enhancement and Excitation of Space Wave, Leaky Wave, and Creeping Waves for a PEC Cylinder Coated With a Uniaxial Dielectric

Directivity Enhancement and Excitation of Space Wave, Leaky Wave, and Creeping Waves for a PEC Cylinder Coated With a Uniaxial Dielectric

Wideband Wide-Beam Hybrid Dielectric Resonator Antenna Using Uniaxial Material

In this letter, a wideband wide-beam rectangular dielectric resonator antenna (DRA) employing uniaxial anisotropic material is presented. To the best knowledge of the authors, the uniaxial anisotropic dielectric is used in wide-beam DRAs for the first time. By tuning the uniaxial permittivity tensor of the rectangular dielectric, the radiation from the top wall of the DRA is enhanced which improves the gain at low elevation angle in the E-plane. Both the feed slot mode and the fundamental <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{TE}_{{111}}^{\mathrm{y}}$</tex-math></inline-formula> mode are merged to broaden the bandwidth. When a grounded substrate is added and four trapezoidal notches are etched on the ground, the half-power beamwidth (HPBW) in the H-plane is broadened. The measured results achieve an impedance bandwidth of 35.7% over a band of 2.65–3.8 GHz with HPBWs of at least 250° and 162° in the E- and H-planes, respectively.

Characteristics of Space, Leaky, and Creeping Waves in a Uniaxial Dielectric Rod

The excitation of space wave, leaky wave, and creeping wave in a cylindrical rod with uniaxial anisotropic permittivity and permeability is thoroughly characterized. The closed-form expressions of the space, leaky, and creeping waves in the uniaxial rod are derived for all source polarizations through the rigorous formulation, taking into account all higher order azimuthal harmonics and TE-TM mode coupling for the space and creeping waves. Conditions for obtaining a high space- and leaky-wave directivity are thoroughly examined for all source polarizations with variation in the vertical and horizontal permittivities/permeabilities and the rod radius. The leaky-wave phase and attenuation constants with variation in the rod parameters are investigated to identify the leaky-wave propagation regions of the uniaxial rod. Beam-scanning characteristics for the space and leaky waves are examined with variation in the uniaxial rod parameters. The complex pole loci for the azimuthally propagating creeping waves around the uniaxial rod are also investigated to identify the leaky, the creeping, and the trapped-mode regions.

Comparison of Scattering from Electrically Large PEC and PMC Targets Coated with Uniaxial Dielectric Medium based on Asymptotic Solution in Spectral Domain

In this paper, electromagnetic (EM) scatterings from electrically large Perfect Electric Conductor (PEC) and Perfect Magnetic Conductor (PMC) targets coated with Uniaxial Electric Anisotropic Medium (UEAM) layer are evaluated and compared. The effect of the coating layer’s parameters including eu, evand the thickness (d) are studied. It is shown that radar cross section (RCS) of a PMC with UEAM coating is less than that of PEC with the same coating. In some cases, about 20 dB reduction in RCS hasbeen obtained for UEAM coated PMC plate

Dimensional confinement and waveguide effect of Dyakonov surface waves in twisted confined media

Abstract We theoretically study Dyakonov surface waveguide modes that propagate along the planar strip interfacial waveguide between two uniaxial dielectrics. We demonstrate that owing to the one-dimensional electromagnetic confinement, Dyakonov surface waveguide modes can propagate in the directions that are forbidden for the classical Dyakonov surface waves at the infinite interface. We show that this situation is similar to a waveguide effect and formulate the resonance conditions at which Dyakonov surface waveguide modes exist. We demonstrate that the propagation of such modes without losses is possible. We also consider a case of two-dimensional confinement, where the interface between two anisotropic dielectrics is bounded in both orthogonal directions. We show that such a structure supports Dyakonov surface cavity modes. Analytical results are confirmed by comparing with full-wave solutions of Maxwell’s equations. We believe that our work paves the way toward new insights in the field of surface waves in anisotropic media.

Electromagnetic Confinement via Spin-Orbit Interaction in Anisotropic Dielectrics.

We investigate electromagnetic propagation in uniaxial dielectrics with a transversely varying orientation of the optic axis, the latter staying orthogonal everywhere in the propagation direction. In such a geometry, the field experiences no refractive index gradients, yet it acquires a transversely modulated Pancharatnam–Berry phase, that is, a geometric phase originating from a spin–orbit interaction. We show that the periodic evolution of the geometric phase versus propagation gives rise to a longitudinally invariant effective potential. In certain configurations, this geometric phase can provide transverse confinement and waveguiding. The theoretical findings are tested and validated against numerical simulations of the complete Maxwell’s equations. Our results introduce and illustrate the role of geometric phases on electromagnetic propagation over distances well exceeding the diffraction length, paving the way to a whole new family of guided waves and waveguides that do not rely on refractive index tailoring.

Equivalence principle algorithm using characteristic basis functions with application to finite periodic arrays including uniaxial dielectrics and conducting objects

In this paper, an equivalence principle algorithm combined with the characteristic basis function (CBF) method has been developed to efficiently analyze the finite periodic arrays composed of uniaxial dielectric materials and conducting objects. Three kinds of the CBFs for each equivalent surface and its surrounding object are simultaneously constructed using multiple plane wave illuminations and singular value decomposition technique, and thus memories related to the scattering and translation operators are significantly saved, while retaining good computational accuracy. Numerical results of a frequency selective conducting array, a uniaxial dielectric cube array, and a periodic array of the composite object including a uniaxial rod and a split ring resonator are given to illustrate good performance of proposed method.

On low-temperature properties of uniaxial dielectrics with a soft optic mode

Dielectric properties, thermal expansion, and the rate of spin-lattice relaxation at low temperatures in uniaxial dielectrics with a soft optical mode are discussed.

Exploiting design freedom in biaxial dielectrics to enable spatially overlapping optical instruments

The optical behavior of gradient biaxial dielectrics has not been widely explored in the literature due to their complicated nature, but the extra degrees of freedom in the index tensor have the potential of yielding useful optical instruments which are otherwise unachievable. In this work, a design method is described in detail which allows one to combine the behavior of up to four totally independent isotropic optical instruments in an overlapping region of space. This is non-trivial because of the mixing of the index tensor elements in the Hamiltonians; previously known methods only handled uniaxial dielectrics (where only two independent isotropic optical functions could overlap). The biaxial method introduced also allows three-dimensional multi-faced Janus devices to be designed; these are worked out in an example of what is possible to design with the method.

Photorealistic ray tracing of free-space invisibility cloaks made of uniaxial dielectrics

The design rules of transformation optics generally lead to spatially inhomogeneous and anisotropic impedance-matched magneto-dielectric material distributions for, e.g., free-space invisibility cloaks. Recently, simplified anisotropic non-magnetic free-space cloaks made of a locally uniaxial dielectric material (calcite) have been realized experimentally. In a two-dimensional setting and for in-plane polarized light propagating in this plane, the cloaking performance can still be perfect for light rays. However, for general views in three dimensions, various imperfections are expected. In this paper, we study two different purely dielectric uniaxial cylindrical free-space cloaks. For one, the optic axis is along the radial direction, for the other one it is along the azimuthal direction. The azimuthal uniaxial cloak has not been suggested previously to the best of our knowledge. We visualize the cloaking performance of both by calculating photorealistic images rendered by ray tracing. Following and complementing our previous ray-tracing work, we use an equation of motion directly derived from Fermat's principle. The rendered images generally exhibit significant imperfections. This includes the obvious fact that cloaking does not work at all for horizontal or for ordinary linear polarization of light. Moreover, more subtle effects occur such as viewing-angle-dependent aberrations. However, we still find amazingly good cloaking performance for the purely dielectric azimuthal uniaxial cloak.

Quasiguided surface plasmon excitations in anisotropic materials

We present a theoretical study of surface plasmon polaritons in a configuration where a metal layer is bounded by a homogeneous anisotropic dielectric. We consider uniaxial and biaxial dielectrics with the optical axis lying in the plane defined by the metal/dielectric interface. Through a systematic study of the in-plane surface plasmon dispersion relation we confirm the existence of several propagation regimes for different values and signs of the birefringence. We report the discovery of the existence of quasiguided surface plasmon polaritons characterized by non-null radiative components. We identify the conditions for the existence of such modes, and explain their peculiar properties in terms of the hybridization of radiative and guided modes.

Optical Vortices from Liquid Crystal Droplets

We report on the generation of mono- and polychromatic optical phase singularities from micron-sized birefringent droplets. This is done experimentally by using liquid crystal droplets whose three dimensional architecture of the optical axis is controlled within the bulk by surfactant agents. Because of its microscopic size these optical vortex generators are optically trapped and manipulated at will, thus realizing a robust self-aligned micro-optical device for orbital angular momentum conversion. Experimental observations are supported by a simple model of optical spin-orbit coupling in uniaxial dielectrics that emphasizes the prominent role of the transverse optical anisotropy with respect to the beam propagation direction.

A Measurement Process to Characterize Natural and Engineered Low-Loss Uniaxial Dielectric Materials at Microwave Frequencies

We present a measurement method to characterize low-loss engineered materials and natural uniaxial dielectrics. Our approach employs a rectangular cavity coupled with tailored finite-element simulations to accurately determine the permittivity tensors and loss tangents of material assemblies. Although similar approaches for natural crystals have been reported, this is the first time this method is adapted to engineered metamaterials. Loss-tangent measurements with an accuracy of 4-5 significant digits can be achieved by this simple and effective measurement approach. To demonstrate the method, we characterize a layered barium titanate-alumina stack and show that low-loss engineered crystals can be achieved via a proper choice of a bonding agent.

Resonance methods for microwave studies of dielectrics (Review)

The general properties of microwave resonators with dielectric inclusions, which are important in dielectric measurements, are described. The methods for measuring the permittivity and dielectric losses in isotropic and uniaxial dielectrics with the use of cavity, open, and dielectric resonators are considered. Approximations that are often used in the theory and practice of measurements and limit accuracy are analyzed. The formulas used in calculations and results of measurements of the dielectric parameters of various samples, including films, coatings, and substrates, in the range 1.5–75.0 GHz are presented. The possibilities of local measurements and measurements of parameters of media are discussed.

Backward waves and negative refraction in uniaxial dielectrics with negative dielectric permittivity along the anisotropy axis

AbstractVeselago medium (also called double negative material, backward medium, left‐handed medium) is a medium with both negative isotropic dielectric permittivity and permeability. It has the effect of backward waves with negative (anomalous) refraction, in contrast to the usual forward waves with normal refraction in normal isotropic dielectrics and/or magnetics. In this paper, examples of the existence of backward waves (with respect to the interface) without negative refraction, and negative refraction without backward waves in uniaxial dielectrics with negative permittivity along the anisotropy axis, are presented. Considering these examples increases the possibility of designing backward wave materials and negative refraction materials without using magnetic media. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 259–263, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10887

A Wiener-Hopf-type analysis of uniaxial substrate-superstrate microstrip structures

A Wiener-Hopf-type technique in conjunction with the hybrid-mode analysis and a space-domain Fourier transform pair was employed for the solution of the canonical problem of a TEM wave obliquely incident upon the edge and defined by a semi-infinite plate conductor lying at the interface of two uniaxial dielectrics, forming an otherwise grounded double-layer geometry. The single-cover layer and double-layer surface-wave modes' characteristic equations are examined and their cutoff conditions, along with safe conditions avoiding longitudinal-section magnetic (LSM) modes are given. The scattered field components and the TEM-wave reflection coefficient are given analytically in the form of Sommerfeld-type integrals. A thin layers approximation and a numerical integration scheme were adopted for the evaluation of the reflection coefficient. Its expression can be directly used for the analysis of wide microstrip lines and patch antennas printed in a substrate/superstrate geometry. The whole analysis gives a clear physical insight into the problem. Furthermore, a twofold theoretical verification was adopted by either forcing the absence of the superstrate or considering both layers to be isotropic. Numerical parametric Investigations show the effects of either the presence of the superstrate or both layers' dielectric anisotropy.

Artificial uniaxial and biaxial dielectrics with the use of photoinduced gratings

We investigate the form birefringence due to photoinduced one- and two-dimensional volume index gratings in DuPont photopolymers. The effective medium theory has been applied to the continuous volume subwavelength gratings. With the presence of a one-dimensional volume subwavelength grating, the originally isotropic photopolymer exhibits a negative form birefringence and behaves like a uniaxial dielectric. A form birefringence of −1.2×10−3 at λ=632.8 nm is measured at a grating period of Λ=447 nm. Experimental results on the dispersion of the form birefringence versus the illumination wavelength and the grating period are presented. With the presence of a two-dimensional grating, in general, the originally isotropic photopolymer exhibits a biaxial form birefringence and behaves like a biaxial dielectric. Positive form birefringence can also be obtained by recording two volume gratings with different grating wave vectors. By recording two photoinduced volume gratings we can synthesize biaxial dielectrics with the principal axes along any desired directions. We also report the measured principal refractive indices of the medium due to the presence of two-dimensional volume gratings in DuPont photopolymer.

A Wiener-Hopf-type analysis of microstrips printed on uniaxial substrates: effects of the substrate thickness

A Wiener-Hopf-type analysis of the canonical problem of a TEM wave obliquely incident at the edge of the truncated upper conductor of a parallel plate waveguide loaded with a uniaxial anisotropic dielectric is presented. A numerical integration scheme as well as a thin substrate approximation for the reflection coefficient is given. The influence of the dielectric anisotropy and the slab thickness on the reflection coefficient and the edge admittance are investigated. Numerical results show the importance of the dielectric anisotropy and the expected effects in microstrip applications. >

Artificial uniaxial and biaxial dielectrics with use of two-dimensional subwavelength binary gratings

Two-dimensional symmetric and asymmetric subwavelength binary gratings are investigated. A method for determining the three effective indices of a two-dimensional (2-D) subwavelength grating is presented, as well as a theoretical formalization for the effective index parallel with the normal to the surface. It is shown that a 2-D asymmetric binary grating on the surface of a dielectric substrate is analogous to a biaxial thin film. If the grating is symmetric, then the two effective indices perpendicular to the normal are equal, and the grating is analogous to a uniaxial thin film. Using these effective indices and the quarter-wave Tschebyscheff synthesis technique, we designed two- and three-level binary gratings to suppress reflections over a broad band. It is shown that for a substrate index of ns = 3.0 a three-level 2-D binary grating reduced reflections below 0.1% from 8 μm to 12 μm.