Anisotropic Dielectrics Research Articles

Resonant Frequencies of TE0mn modes in multilayered resonators containing uniaxial anisotropic dielectrics with complex shapes

Resonant Frequencies of TE0mn modes in multilayered resonators containing uniaxial anisotropic dielectrics with complex shapes

Dyakonov surface waves in a thin interfacial waveguide formed by negatively anisotropic dielectrics.

Dyakonov surface waves (DSWs) are electromagnetic surface waves that exist at the interface of two dissimilar materials, with at least one material being anisotropic. Although there are various types of these waves, they all exist in anisotropic materials with positive anisotropy. The requirement for positive anisotropy limits the choice of materials that can support these waves. In this study, we present a type of Dyakonov surface wave that occurs at the interface of negatively anisotropic materials. Specifically, we demonstrate their existence in a system consisting of two negatively anisotropic slabs confined between two perfect electric conductor (PEC) walls. By assuming a small distance between the walls, we derive analytical expressions for the propagation constant, penetration depth, and field distribution of these surface waves. We numerically demonstrate that these surface waves can also exist in structures beyond the approximations used to develop the theoretical framework. The existence of Dyakonov surface waves in negative crystals broadens the range of materials suitable for their practical implementation.

Nearly Perfect Spin Conversion Based on Topological Singularity in 1D Anisotropic Photonic Crystals

AbstractAlthough the spin‐controlled vortex generation and photonic spin‐Hall effect of spin‐flipped abnormal mode have been widely studied recently, the traditional method based on the metasurface is difficult to fabricate, and the efficiency of the spin‐flipped abnormal mode is rather low due to process errors and intrinsic material loss. Here, a new method is proposed based on the insights into the topological singularity and special Bragger reflections resonant (BRR) mode of one‐dimensional (1D) finite photonic crystals (PhCs) with anisotropic material to realize nearly perfect (100%) spin‐conversion efficiency. For a finite 1D PhC with cell number N, there are 3N complete spin‐conversion (CSC) and complete spin‐maintained (CSM) channels. Two mechanisms of these CSC and CSM channels are revealed. The working bandwidths and the angular ranges of these CSC and CSM are also studied. Based on these theoretical findings, multi‐angles and multi‐frequencies perfect spin‐conversion (‐maintained) devices can be designed. At last, these theoretical results are confirmed by the numerical experiments based on finite‐difference time‐domain (FDTD) methods. This work paves the way to exploring the topological properties and polarization control of PhCs made of anisotropic dielectrics and provides a prospective method for the design of multi‐channels spin optical devices.

Measuring the anisotropic permittivity tensor of DyScO3 to 110 GHz

DyScO3 (DSO) is an attractive substrate on which to grow epitaxial thin films with extraordinary materials physics. However, its highly anisotropic permittivity makes some measurements exceedingly difficult: For instance, its permittivity tensor has not yet been fully characterized at millimeter-wave frequencies. While there are methods to characterize anisotropic permittivity at millimeter-wave frequencies, there are very few methods those are suitable for the small lateral dimensions that DyScO3 can be grown in. To overcome this lack in the material characterization, we tested an on-wafer method based on coplanar waveguides to measure the full anisotropic permittivity tensor from 0.1 to 110 GHz. We characterized two orthogonal sets of coplanar waveguides fabricated on each of two substrates with (001) and (110) crystallographic orientations to resolve the full permittivity tensor. To validate our measurements, we compared our results to data from dc parallel plate capacitors and THz time-domain spectroscopy. Our measurements fill the need for measurements of the permittivity of DyScO3, while the methodology, more generally, enables quantitative characterization of anisotropic dielectrics.

Millikelvin measurements of permittivity and loss tangent of lithium niobate

Lithium niobate is an electro-optic material with many applications in microwave and optical signal processing, communication, quantum sensing, and quantum computing. In this Letter, we present findings on evaluating the complex electromagnetic permittivity of lithium niobate at millikelvin temperatures. Measurements are carried out using a resonant-type method with a superconducting radio-frequency cavity operating at 7 GHz and designed to characterize anisotropic dielectrics. The relative permittivity tensor and loss tangent are measured at 50 mK with unprecedented accuracy.

Electromagnetic angular momentum in anisotropic media and Beth's torque

The interaction of radiation with material media is a very active area of research, in particular the interaction with new exotic materials. However, we find several controversies in this field, like the Abraham-Minkowski controversy, or the attribution of angular momentum to quasi-static fields.In the present work we show that besides the electromagnetic momentum density proportional to Poynting's vector, whose time derivative is force and therefore can produce a torque, there are other momentum densities implied by the macroscopic Maxwell equations transformed into momentum balance equations. These momentum densities are also related to forces and torques. We show in this way that torques like D × E and B × H appear in these balance equations transformed into balance equations of angular momentum. The density D × E was used by Beth [Phys. Rev. 50 115–125 (1936)] to measure the angular momentum of radiation fields in interaction with anisotropic dielectrics. Our analysis show that also static or quasi-static fields carry angular momentum and produce torques when interacting with anisotropic materials.

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.

Tunable, Homoepitaxial Hyperbolic Metamaterials Enabled by High Mobility CdO

AbstractPropagating light exhibits hyperbolicity in strongly anisotropic materials where the principal components of the dielectric tensor are opposite in sign. While hyperbolicity occurs naturally in anisotropic polar dielectrics, wherein optical phonons along orthogonal crystal axes are nondegenerate, such optical anisotropy can also be engineered in hyperbolic metamaterials (HMMs): thin film superlattices of alternating dielectric and metallic layers. Contrasted with the severely limited tunability of natural hyperbolic materials, the hyperbolic behavior of HMMs can be tailored significantly both through superlattice design and material selection. However, so far HMMs have suffered from high optical losses, hindering their performance. In this report, broadly tunable (λ = 2–5 µm) Type I and II hyperbolic modes with low losses (quality (Q)‐factors up to 19.7) are observed through attenuated total reflectance measurements of monolithic, homoepitaxial superlattices of high‐ and low‐doped cadmium oxide (CdO). Further, the low losses offered by CdO enable the first demonstration of real‐space imaging of hyperbolic plasmon polaritons in nanoresonators by scattering‐type scanning near‐field optical microscopy—previously only possible for hyperbolic phonon polariton materials. Tunable, low‐loss CdO HMMs promise designability for applications such as on‐chip photonics, super‐resolution imaging (hyperlensing), enhanced emission, novel emitter designs, and possibly quantum nanophotonic and time variant metasurfaces.

Photonic Bandgaps of One-Dimensional Photonic Crystals Containing Anisotropic Chiral Metamaterials

Conventional photonic bandgaps (PBGs) for linear polarization waves strongly depend on the incident angle. Usually, PBGs will shift toward short wavelengths (i.e., blue-shifted gaps) as the incident angle increases, which limits their applications. In some practices, the manipulation of PBGs for circular polarization waves is also important. Here, the manipulation of PBGs for circular polarization waves is theoretically investigated. We propose one-dimensional photonic crystals (1DPCs) containing anisotropic chiral metamaterials which exhibit hyperbolic dispersion for left circular polarization (LCP) wave and elliptical dispersion for right circular polarization (RCP) wave. Based on the phase variation compensation effect between anisotropic chiral metamaterials and dielectrics, we can design arbitrary PBGs including zero-shifted and red-shifted PBGs for LCP wave. However, the PBGs remain blue-shifted for RCP wave. Therefore, we can design a high-efficiency wide-angle polarization selector based on the chiral PBGs. Our work extends the manipulation of PBGs for circular polarization waves, which has a broad range of potential applications, including omnidirectional reflection, splitting wave and enhancing photonic spin Hall effect.

Measurement of Uniaxially Anisotropic Dielectrics With a Fabry–Perot Open Resonator in the 20–50 GHz Range

Measurement results of uniaxially anisotropic materials performed with the aid of a plano-concave Fabry–Perot open resonator in the 20–50 GHz frequency range are presented and discussed in this letter. It is shown that a linear polarization of Gaussian modes applied in the characterization allows extracting in-plane complex permittivity of such materials as crystals and foils in a single measurement cycle. It is also shown that dielectric losses exhibit anisotropy, which can be quite significant, like in the sapphire, or relatively small, like in polymer foils.

Surface plasmon polaritons in metal films on anisotropic and bianisotropic substrates

The existence of nonradiative surface plasmon polaritons (SPPs) is theoretically studied in an isotropic metal film surrounded by optically anisotropic half-infinite dielectrics. By using general properties of the impedance matrices of half-infinite dielectrics and metal films the maximum number of SPPs has been established at a given value of the tangential wave number. It has been proved that, if the dielectrics are nonbianisotropic and magneto-optically inactive but uniaxial or biaxial and oriented arbitrarily, then at most two SPPs can exist independently of the frequency dispersion of the dielectric permittivity and magnetic permeability. At most four SPPs emerge when both the dielectric media are bianisotropic and/or magneto-optically active but the maximum number of SPPs reduces to 3 provided that one of the dielectrics does not exhibit either bianisotropy or magneto-optical activity. A metal film inserted into an infinite bianisotropic and/or magneto-optically active medium can also guide at most three SPPs.

The cylindrical column lenses and reflectors made of transverse an-isotropic plasma and dielectrics and their response to presence of plane electromagnetic waves

In this paper, a long column lens with circular cross section made of two transversely non-isotropic parts (dielectric and plasma ) covered by isotropic dielectric layer, is studied. The response of the mentioned object to presence of plane monochromatic electromagnetic waves will be investigated. The finite element method (FEM) have been used to calculate the field equations and the amplitudes of scattered wave in this research. This structure can be purposefully used as a device to focus electromagnetic wave energy in a specific area that its operation will be discussed by scattering theory. The electromagnetically responses of this structure have been simulated for two modes TE, TM of the incident wave, separately. Since in this lens a plasma region exist, therefore the plasma can be considered as transparent or non-transparent states, with variations of plasma frequency in a fixed incident wave frequency. Here we consider the mentioned states too.

Effect of sample anisotropy on scanning near-field optical microscope images

Scattering-type scanning near-field optical microscopy (s-SNOM) has been widely used to characterize strongly correlated electronic, two dimensional, and plasmonic materials, and it has enormous potential for biological applications. Many of these materials exhibit anisotropic responses that complicate the extraction of dielectric constants from s-SNOM measurements. Here, we generalize our recently developed approach for retrieving the near-field scattering signal from isotropic systems and apply it to anisotropic dielectrics. Specifically, we compare our theoretical results with experimental measurements on modestly anisotropic sapphire that exhibit strong resonances at the infrared frequency range. Good agreement with the experimental result is found. Our result is important for understanding the near-field response of low damping, anisotropic polaritonic states in dielectric media.

Nonlinear anisotropic dielectrics for photonic state engineering

We investigate generating and tailoring photon states based on controllable light-matter interaction in nonlinear anisotropic dielectrics. Through structure design, the optical properties of corresponding media are effectively tunable, and nonlinear optical processes can be modulated. Such a system provides a multi-functional platform, which is available to tailor the wavelength, polarization and spatial properties of photon states. These functions could be valuable for the practical applications of quantum information processing.

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.

Theory and experimental observation of hyperbolic media based on structural dispersions

Hyperbolic media usually possess large dissipation induced by the inherent imaginary part in the material dispersion of their metallic components. Here, we theoretically and experimentally demonstrate a route toward hyperbolic media with low dissipation: using structural dispersions. In a planar microwave waveguide filled with anisotropic dielectrics, we insert an array of metal wires and transform the original hybrid modes into pure transverse-electric modes. Interestingly, by engineering the structural dispersions such that waves are evanescent in one direction and propagating in the orthogonal direction, hyperbolic media with negligible loss can be achieved. Conical radiation pattern and negative refraction, which are characteristic phenomena of the hyperbolic media, have been numerically or experimentally demonstrated in microwaves. Our work opens an approach to utilize structural dispersions instead of material dispersions in realizing hyperbolic media with negligible loss.

Hidden-symmetry-enforced nexus points of nodal lines in layer-stacked dielectric photonic crystals

It was recently demonstrated that the connectivities of bands emerging from zero frequency in dielectric photonic crystals are distinct from their electronic counterparts with the same space groups. We discover that in an AB-layer-stacked photonic crystal composed of anisotropic dielectrics, the unique photonic band connectivity leads to a new kind of symmetry-enforced triply degenerate points at the nexuses of two nodal rings and a Kramers-like nodal line. The emergence and intersection of the line nodes are guaranteed by a generalized 1/4-period screw rotation symmetry of Maxwell’s equations. The bands with a constant kz and iso-frequency surfaces near a nexus point both disperse as a spin-1 Dirac-like cone, giving rise to exotic transport features of light at the nexus point. We show that spin-1 conical diffraction occurs at the nexus point, which can be used to manipulate the charges of optical vortices. Our work reveals that Maxwell’s equations can have hidden symmetries induced by the fractional periodicity of the material tensor components and hence paves the way to finding novel topological nodal structures unique to photonic systems.

Systematic Design of Transmission-Type Polarization Converters Comprising Multilayered Anisotropic Metasurfaces

A simple but efficient approach for the synthesis of transmission-type wideband polarization converters is presented. The proposed configuration comprises multilayer metasurfaces including resonant particles which are progressively rotated layer by layer. The progressive rotation of the particles allows for a polarization conversion over a large frequency band. The polarizing structure is efficiently designed and optimized through a transmission line model approach handling the cascade of anisotropic impedance layers and dielectrics. An optimized 8-layers design based on gradually rotated dipole resonators is presented as a proof of concept. The results obtained through the efficient transmission line model are compared with full-wave simulations once that the structure was optimized showing satisfactory agreement. A prototype of the wideband polarization converter has been fabricated and measured.

Dielectric Anisotropy Sensor Using Coupled Resonators

In this article, a new approach is proposed for the measurement of the uniaxial anisotropic dielectric constant of different planar samples by means of a single sensor. The sensor is based on a couple of straight-line coupled resonators in microstrip technology that can be excited in odd and even propagation modes. This sensor is designed on an isotropic substrate at the design frequency. Due to the electric field configuration specific for each mode, it is possible to relate these modes to the dielectric constant in two different directions (parallel and perpendicular) of a dielectric material placed on top of the sensor. This technique is used for the successful characterization of the dielectric constant anisotropic of anisotropic dielectrics (FR4, Rogers 4350B, and Arlon Diclad 880), and the isotropic material PTFE.

Multilayered Transmission Lines on Quasi-planar Substrates With Anisotropic Medium

Abstract. This paper exhibits the extension of the discrete mode matching (DMM) method to analyze conformal structures with anisotropy. It represents a simple formalism as a basis to analyze multilayered structures with quasi-planar anisotropic dielectric layers. The dyadic Green's function is then calculated using a full-wave equivalent circuit (FWEC) of the structure, where each layer is represented with the hybrid block consisting of the tangential field components. The application is demonstrated by computing propagation constants for partially filled quasi-planar waveguides and microstrip lines with isotropic, uniaxial and biaxial anisotropic dielectrics.