Anisotropic Dielectric Constants Research Articles

CIRCULAR POLARIZATION IN A $2\mathrm{D}$ PERIODIC ARTIFICIAL ANISOTROPIC DIELECTRIC MEDIUM

The phenomenon of polarization rotation in dielectric periodic structures is quite interesting. A similar structure was used in a sample with a 2D artificial periodic structure, where the plane wave propagation method was applied. Anisotropic dielectric constants were obtained, which were used for polarization rotation, and polarization rotation in an anisotropic medium was studied based on the dielectric permittivity of the effective medium. Those relationships, quantities, and parameters depending on the dielectric medium, dielectric permittivity, and 2D periodic structure have been studied and analyzed, which provide control of the degree of anisotropy, i.e. provide a distinction between fast and slow modes, which in turn provide the best polarization rotation per unit length.

Strain-induced giant enhancement of anisotropic dielectric constant in layered nitrides SrHfN2 and SrZrN2.

The development of information technology puts forward huge demand for electronic materials with high dielectric constants; first-principles calculations and simulations have been demonstrated as an efficient technique for screening and exploring novel dielectric materials. In the present study, first-principles calculations combined with density functional perturbation theory are employed to study the dielectric properties of two newly discovered layered nitrides SrHfN2 and SrZrN2 under strain. By analyzing the evolution of lattice distortion, dielectric constant, Born effective charge, and phonon modes along with the applied strain, we find that the biaxial strain and isotropic strain can effectively modulate the dielectric constant. The two nitrides SrHfN2 and SrZrN2 are dynamically stable up to biaxial tensile strains of 2.1% and 1.8%, and the dielectric constants have been enlarged to about 500 and 2000. Furthermore, the dielectric constant is dramatically enhanced by 15 (9) times to a maximum value of ∼2600 (2700) under an isotropic tensile strain of 1.2% (0.7%) for SrHfN2 (SrZrN2), which is mainly due to the softening of the lowest-frequency infrared-active phonon mode and the increasing octahedral distortion degree. Particularly, the ionic contribution of the dielectric constant shows very remarkable anisotropy and plays a dominant role in the change of the dielectric constant, whose in-plane components exhibit giant enhancement by 18 (10) times for SrHfN2 (SrZrN2). This work not only sheds light on the experimentally observed high dielectric constants of SrHfN2 and SrZrN2, but also provides an effective route to regulate the anisotropic dielectric constants by applied strain, which indicates promising applications in optical and electronic devices.

A tunable microwave circulator based on a magnetized plasma as an active gyrotropic element

We present a theoretical analysis and experimental demonstration of a microwave circulator that uses a magnetized plasma as the gyrotropic element. Unlike traditional circulators that exploit the anisotropic permeability of ferrite, here, we exploit the anisotropic dielectric constant of a magnetized plasma to achieve non-reciprocity. The advantage of a plasma-loaded circulator is that it allows for tunability, and modulation that is limited in speed by the ionization/recombination time of the plasma. The theoretical analysis treats the problem of electromagnetic scattering from a heterogeneous gyrotropic plasma rod to confirm scattering behavior and to guide in defining the design parameter space covered by more detailed computational simulations. Proof-of-concept experiments are carried out using a custom-fabricated low-frequency gas discharge tube as the plasma element. Here, we confirm the plasma-frequency dependent tunability predicted by the simulations, demonstrating this tunability in the experiments by varying the applied discharge voltage. These initial experiments suggest that isolation in excess of 25 dB is achievable, in reasonable agreement with the theory and simulations for studies in the S and C ranges of the microwave spectrum.

Vortices nucleation by inherent fluctuations in nematic liquid crystal cells

Multistable systems are characterized by exhibiting domain coexistence, where each domain accounts for the different equilibrium states. In case these systems are described by vectorial fields, domains can be connected through topological defects. Vortices are one of the most frequent and studied topological defect points. Optical vortices are equally relevant for their fundamental features as beams with topological features and their applications in image processing, telecommunications, optical tweezers, and quantum information. A natural source of optical vortices is the interaction of light beams with matter vortices in liquid crystal cells. The rhythms that govern the emergence of matter vortices due to fluctuations are not established. Here, we investigate the nucleation mechanisms of the matter vortices in liquid crystal cells and establish statistical laws that govern them. Based on a stochastic amplitude equation, the law for the number of nucleated vortices as a function of anisotropy, voltage, and noise level intensity is set. Experimental observations in a nematic liquid crystal cell with homeotropic anchoring and a negative anisotropic dielectric constant under the influence of a transversal electric field show a qualitative agreement with the theoretical findings.

Infrared-active phonon modes and static dielectric constants in α-(AlxGa1−x)2O3 (0.18 ≤ x ≤ 0.54) alloys

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(AlxGa1−x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

Weakly Anisotropic Dielectric Properties of Water Droplets at the Nanoscale.

The slab-confined water at the nanoscale exhibits anomalous dielectric properties compared to bulk water, for example, significantly low dielectric constant. In this work, we study the dielectric properties of nanoscale water droplets at room temperature using molecular dynamics simulations. We find that the nanoscale water droplets feature weakly anisotropic dielectric constant: the radial component of dielectric constants is distinctly smaller than the tangential component although they both decrease with reducing droplet size in a similar way. Such dielectric behavior is closely related to the orientational preference of water molecules near the convex surface. The molecular dipole prefers to slightly orientate toward the interior of droplets in contrast to the out-of-plane preference for free-standing water films and slab-confined water, which suppresses the fluctuation of dipole moments in the radial direction. Meanwhile, it facilitates the formation of the open hydrogen-bond network in the surface layer and ultimately leads to the relatively weak suppression of tangential fluctuations. The differential suppression is responsible for the anisotropic dielectric constant of water droplets. This anisotropic characteristic is also found in dielectric relaxation: both the radial and the tangential relaxation are consistently slowed down upon approaching surface but the latter is universally slower.

High-frequency and below bandgap anisotropic dielectric constants in α-(AlxGa1−x)2O3 (≤x≤1)

A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum α-(AlxGa1−x)2O3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (ε∞,⊥) and parallel (ε∞,∥) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the high-frequency dielectric constants with b⊥=0.386 and b∥=0.307.

Dielectric properties of the system: 4-n-pentyloxybenzoic acid– N-(4-n-butyloxybenzylidene)-4᾽-methylaniline

Objectives. Our aim was to study the dielectric properties of the 4-n-pentyloxybenzoic acid– N-(4-n-butyloxybenzylidene)-4’-methylaniline system and reveal how different concentrations of N-(4-n-butyloxybenzylidene)-4’-methylaniline additives affect the dielectric properties of 4-n-pentyloxybenzoic acid.Methods. System properties were investigated using polarization thermomicroscopy and dielcometry.Results. We found that dielectric anisotropy changes its sign from positive to negative at the transition temperature of the high-temperature nematic subphase to the low-temperature one. The anisotropy of the dielectric constant of N-4-n-butoxybenzylidene-4’-methylaniline has a positive value and increases as to the system approaches the crystalline phase. The crystal structure of the 4-n-pentyloxybenzoic acid contains dimers formed by two independent molecules due to a pair of hydrogen bonds. The crystal structure of N-(4-n-butoxybenzylidene)-4’-methylaniline contains associates formed by orientational interactions of two independent molecules. 4-n-Pentyloxybenzoic acid dimers (270 nm) and associates of N-4-n-butoxybenzylidene-4’- methylaniline (250 nm) proved to have approximately the identical length. Considering the close length values of the structural units of both compounds and the dielectric anisotropy sign, we assume that the N-4-n-butoxybenzylidene-4’-methylaniline associates are incorporated into the supramolecular structure of the 4-n-pentyloxybenzoic acid. The specific electrical conductivity of the compounds under study lies between 10−7 and 10−12 S∙cm−1. The relationship between the specific electrical conductivity anisotropy and the system composition in the nematic phase at the identical reduced temperature, obtained between 100 and 1000 Hz is symbatic. However, the electrical conductivity anisotropy values of the system obtained at 1000 Hz are lower compared to those obtained at 100 Hz. At N-(4-n-butoxybenzylidene)-4’-methylaniline concentrations between 30 and 60 mol %, the electrical conductivity anisotropy values are higher than those of the individual component.Conclusions. A change in the sign of the dielectric constant anisotropy of the 4-n-pentyloxybenzoic acid during nematic subphase transitions was established. We showed that the system has the highest dielectric constant anisotropy value when components have an equal number of moles. Highest electrical conductivity anisotropy values are observed when the concentration of the N-4-n-butoxybenzylidene-4᾽-methylaniline system lies between 30 and 60 mol %.

Plasmonic-Layered InAs/InGaAs Quantum-Dots-in-a-Well Pixel Detector for Spectral-Shaping and Photocurrent Enhancement.

The algorithmic spectrometry as an alternative to traditional approaches has the potential to become the next generation of infrared (IR) spectral sensing technology, which is free of physical optical filters, and only a very small number of data are required from the IR detector. A key requirement is that the detector spectral responses must be engineered to create an optimal basis that efficiently synthesizes spectral information. Light manipulation through metal perforated with a two-dimensional square array of subwavelength holes provides remarkable opportunities to harness the detector response in a way that is incorporated into the detector. Instead of previous experimental efforts mainly focusing on the change over the resonance wavelength by tuning the geometrical parameters of the plasmonic layer, we experimentally and numerically demonstrate the capability for the control over the shape of bias-tunable response spectra using a fixed plasmonic structure as well as the detector sensitivity improvement, which is enabled by the anisotropic dielectric constants of the quantum dots-in-a-well (DWELL) absorber and the presence of electric field along the growth direction. Our work will pave the way for the development of an intelligent IR detector, which is capable of direct viewing of spectral information without utilizing any intervening the spectral filters.

Two dimensionalization induced enhancing dielectric anisotropy of titania

Abstract Two dimensionalization of dielectric oxides will lead to significant variations of the dielectric properties in comparing with their bulk counterparts. To clarify the longstanding controversy how indeed dielectric properties would change is a critical prerequisite and highly demanded by the correlated scientific community and electronic industry. Here, taking titania as an example, a theoretical study by ab-initio calculations is performed for predicting the dielectric property evolution in combining with the structure transformations. Highly enhanced dielectric constant anisotropy is found that out of plane reduces significantly differed from those enlargement predictions, but electronic screening in plane enhances. While for the breakdown strength, which is always ignored in theoretical investigation, it also demonstrates that permittivity and breakdown strength can be enhanced at the expense of each other during two dimensionalization. The anisotropy between permittivity and breakdown strength can cause a much higher dielectric energy density. Such findings are very helpful for designing future atomic scale electronic and optical devices such as high voltage capacitor, excellent gate dielectrics for high frequency field effect transistor (FET) with two-dimensional oxide dielectrics.

Distinguishing the Structure of High-Pressure Hydrogen with Dielectric Constants.

Identifying the structures of high-pressure hydrogen has been one of the central goals in high-pressure physics; however, it still presents a fundamental challenge because of the lack of an effective measure for distinguishing the structures. Herein, we address this issue by focusing on the potential candidates of phases II and III of high-pressure hydrogen. We find that the anisotropic dielectric constants of the different hydrogen solids and their responses to pressure behave differently depending on the atomic structures, corresponding to the different polarization responses of the structures to the external electric field. These findings are robust regardless of the quantum and thermal motion of hydrogen solids. Therefore, the anisotropic dielectric property can serve as a potential measure for probing the structures of high-pressure hydrogen as well as other high-pressure materials.

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.

Ultrabroad Band Microwave Absorption of Carbonized Waxberry with Hierarchical Structure.

Developing microwave absorption materials with broadband and lightweight characters is of great significance. However, it is still a great challenge for carbonized biomass without loading magnetic particles to cover the broad microwave frequency. Herein, it is proposed to carbonize freeze-dried waxberry to make full use of its natural hierarchical gradient structure to target the ultrabroad band microwave absorption. The carbonized freeze-dried waxberry shows radial-gradient and hierarchical structure. The different components of hierarchical waxberry demonstrate gradient dielectric properties: the outer component shows anisotropic dielectric constants with smaller value, while the inner core shows higher dielectric constants. This gradient dielectric property is beneficial to the impedance matching and strong polarization. As a result, the bandwidth of carbonized waxberry exhibits an ultrabroad band microwave absorption, ranging from 1 to 40 GHz with the reflection loss value below -8 dB. Meanwhile, the bandwidth can cover from 8 to 40 GHz when the reflection loss is below -15 dB. The ultrabroad microwave absorption is attributed to the hierarchical radial-gradient structure of carbonized waxberry, which provides good impedance matching with air media. This achievement paves the way for the exploitation of natural hierarchical biomass as a superlight and broadband high-performance microwave absorption material.

Effects of chemical defects on anisotropic dielectric response of polyethylene

Dielectric polymers such as polyethylene (PE) have a wide range of energy and electronic applications. While recent studies have shown significant effects of chemical defects on the electronic structure of PE, those on the dielectric properties remain elusive. Here, first-principles quantum-mechanical calculations show anisotropic dielectric constants of PE, which are sensitive to the type of defects. Specifically, addition of iodine defects increases the high-frequency dielectric constant. Addition of hydroxyl or carboxyl group, on the other hand, causes noticeable anisotropic changes in the static dielectric constant, which is well elucidated through the rotation and concerted motions of chemical groups. The sensitivity of these defects may be exploited to rationally alter the behavior of PE.

Ellipsometry of anisotropic graphene-like two-dimensional materials on transparent substrates

New possibilities for determining anisotropic properties of the dielectric constants of two-dimensional materials by ellipsometry are developed. Graphene-like 2D materials are considered within the framework of macroscopic electrodynamics as ultrathin absorbing anisotropic films where the optical axis is perpendicular to the film surface. The ellipsometric inversion problem is resolved analytically. The resulting inversion formulas are very fast because they allow you to directly calculate the complex anisotropic dielectric constants without the use of sophisticated regression analysis or iterative root-finding procedures. In particular, the method offers an interest in graphene and related 2D materials because the anisotropic properties of such materials have not been studied to date.

Comprehensive Understanding of Host- and Guest-Dependent Helix Inversion in Chiral Nematic Liquid Crystals: Experimental and Molecular Dynamics Simulation Study.

Nematic liquid crystals (LCs) are known to transform into chiral nematic LCs with helical structures upon doping with enantiomeric compounds (called chiral dopants or guests). Here, we investigated the mechanism of host LC- and guest-dependent helix inversion by using octahedral metal complexes as guests. The helical twisting powers (HTPs/μm-1) of eight metal complexes with Δ, Λ chirality were examined in five nematic LC hosts. For example, Δ-[Ru(acac)2(trop)] (Ru-trop, Hacac = 2,4-pentanedione, Htrop = tropolone) induces a left-handed ( M) helix upon doping with N-(4-methoxybenzylidene)-4-butylaniline (MBBA), whereas it induces an opposite right-handed ( P) helix in 4-cyano-4'-pentylbiphenyl (5CB). The monobrominated complex of Ru-trop, Δ-[Ru(acac)2(Brtrop)] (Ru-Br1trop, HBr1trop = 5-bromotropolone), induces P helices in both MBBA and 5CB, whereas the dibrominated complex, Δ-[Ru(acac)2(Br2trop)] (Ru-Br2trop, HBr2trop = 3,7-dibromotroplone) induces M helices in both the media. The molecular dynamics simulation performed in parallel confirmed the drastic effect of the bromo groups on the microscopic ordering direction of guests in nematics. Further, HTPs of all Δ isomers of metal complexes investigated were found to shift in the positive direction as the dielectric constant anisotropy (Δε) of the host LCs increases (and vice versa for Λ isomers). These experimental and calculated results highlight the interplay of steric (excluded volume effect) and electrostatic (dipole-dielectric body) interactions between the host LCs and guest metal complexes.

Probing optical anisotropy of nanometer-thin van der waals microcrystals by near-field imaging

Most van der Waals crystals present highly anisotropic optical responses due to their strong in-plane covalent bonding and weak out-of-plane interactions. However, the determination of the polarization-dependent dielectric constants of van der Waals crystals remains a nontrivial task, since the size and dimension of the samples are often below or close to the diffraction limit of the probe light. In this work, we apply an optical nano-imaging technique to determine the anisotropic dielectric constants in representative van der Waals crystals. Through the study of both ordinary and extraordinary waveguide modes in real space, we are able to quantitatively determine the full dielectric tensors of nanometer-thin molybdenum disulfide and hexagonal boron nitride microcrystals, the most-promising van der Waals semiconductor and dielectric. Unlike traditional reflection-based methods, our measurements are reliable below the length scale of the free-space wavelength and reveal a universal route for characterizing low-dimensional crystals with high anisotropies.

Reflectance calculations of anisotropic dielectric constants of graphene-like two-dimensional materials.

A new optical method for determining anisotropic dielectric constants of graphene-like two-dimensional materials on semiconductor or metal substrates is developed. The method is based on the surface differential reflectance measurements at three different incident angles. The inversion problem is resolved analytically in a long-wavelength approximation, where graphene is discussed within the framework of macroscopic electrodynamics as a uniaxially anisotropic film with the optical axis perpendicular to the film surface. The method is fast and has no need for the initial guesses of the desired parameters. It also offers an effective technique to get good start points for iterative numerical methods to improve the preliminary results. The method is unique in its simplicity and, therefore, important for graphene research.

Enhancement of electrostatic field by a metamaterial electrostatic concentrator

In this work, we propose an electrostatic field concentrator using electrostatic metamaterial. Specifically, a shell with anisotropic dielectric constant is employed to concentrate the electrostatic field into core region to achieve an enhanced electrostatic field (192% of the applied electrostatic field intensity) and keep the external field undistorted simultaneously. As such, a fan-shaped structure consisting of alternating wedges is designed, fabricated and tested. The good agreement between the experimental and simulated results certifies the feasibility of our scheme. Our study suggests that the proposed device has potential applications in noninvasive detection, energy transfer and other electrostatic related fields.

(Invited) Electrical Integrity and Anisotropy in Dielectric Breakdown of Layered h-BN Insulator

We report the complete set of dielectric breakdown strength (E BD) for h-BN, that is, ~12 MV/cm for out-of-plane E BD is four times higher than that for in-plane E BD and it is larger than that for diamond. The large anisotropy in dielectric constant (i.e., related with band gap) due to the layered structure causes this anisotropy.