Enhanced diffraction efficiency of gratings in multilayers

Computations with the rigorous differential method show that single gratings made by ion implantation have a diffraction efficiency in the +1 transmitted order under TE illumination of only 0.78%. The insertion of such gratings into multilayer dielectric Fabry-Perot cavities leads to an enhancement of the free-space diffraction efficiency. Different designs for the multilayer are considered. An 18.8% efficiency is reached with 11-layer mirrors. This result is obtained by optimization of the thickness of the spacer of the Fabry-Perot cavity that contains the grating and centering of the wavelength of the mirrors. The dependence of optical properties of the structure on the various optogeometrical parameters of the structure is discussed.     

Residual polarization of non-coherently backscattered linearly polarized light: the influence of the anisotropy parameter of the scattering medium

The effect of preservation of the residual polarization of backscattered light in the case of multiply scattered disordered media illumination by a linearly polarized plane wave is examined using the path-integral approach and Monte Carlo simulation. Disordered ensembles of non-interacting dielectric particles are considered as the model of scattering media. The influence of the anisotropy parameter of the scattering system on the degree of residual polarization is analysed. Experimental results obtained for various scattering systems at different wavelengths of illuminating light are in satisfactory agreement with the results of theoretical analysis and Monte Carlo simulation. The dependence of statistical properties of the polarization states of backscattered field partial components, such as probability distributions of ellipticity, on the anisotropy parameter is studied.     

Effect of illumination on a dielectric nonlinearity in Cr-doped SBN-75 single crystal

The dielectric nonlinearity in an SBN-75 single crystal doped with 0.01 at % Cr was investigated in the vicinity of a diffuse phase transition by studying the reverse dielectric dependences ε′(E ₌). The influence of illumination and exposure (aging) on the behavior of ε′(E ₌) was demonstrated. A reduction in the dielectric memory effect after sample illumination was detected.     

紫外辐照对聚醚酰亚胺薄膜介电性能的影响北大核心CSCD

研究了不同紫外辐照时间对聚醚酰亚胺(PEI)薄膜介电性能的影响。采用FT-IR和SEM表征了PEI薄膜的分子结构和微观形貌。结果表明,紫外辐照后PEI薄膜在1742cm^(-1)处的吸收峰比原薄膜增大,说明PEI分子链中的C=O基团随辐照时间的增加而增加,并在薄膜表面产生了微裂纹。对PEI薄膜的介电性能进行的研究结果表明,随着紫外辐照时间的增加,PEI薄膜的介电常数和介电损耗增大,而表面电阻率下降,体积电阻率基本不变。并随紫外辐照时间的增加,直流击穿强度呈先增加后降低的趋势,一定辐照剂量可使薄膜发生交联反应,使击穿场强较原薄膜提高20%以上。     

Comparison of photonic nanojets key parameters produced by nonspherical microparticles

Photonic nanojet (PNJ) phenomenon arising near transparent dielectric microparticles subject to plane wave illumination in the visible is considered. The near-field light scattering patterns produced by shaped wavelength-sized particles (hexahedron, cuboid, sphere, hemisphere, axicon, assembled particles) are numerically simulated and key PNJ parameters are analyzed. Particle shape influence on the peak intensity and spatial resolution of produced PNJ is investigated. We demonstrate that due to the reciprocal action of spherical-type and conical-type focusing of the special type of composite particles constituted of a hemisphere and an axicon can produce highly localized PNJ with peak intensity considerable higher than that for isolated regular particle (sphere, microaxicon, hemisphere).     

Optical properties of nonequilibrium low-dimensional systems

The optical properties of low-dimensional carrier systems ("quantum wire" type) driven away from equilibrium are studied. The frequency and wave-vector-dependent dielectric function of a quasi-one-dimensional electron system under the action of an exciting external pumping source is derived. The optical responses of the system are obtained in terms of its nonequilibrium thermodynamic state, the latter characterized resorting to a nonequilibrium statistical ensemble formalism.     

Theoretical investigations of material modification using temporally shaped femtosecond laser pulses

We present a two-dimensional model, based on a drift–diffusion approach, developed to describe the dynamics of electronic excitation and lattice heating in several dielectric materials with different electron–phonon coupling properties (e.g. fused silica and sapphire) under the action of femtosecond near-infrared laser pulse trains with variable separation time between pulses. The modeling approach was aimed to describe the mechanisms that enable the spatial modulation of the structures induced by temporally modulated laser excitation and ablation of wide-band-gap dielectric materials. The possible geometric contours of the laser-induced craters on the target surfaces are discussed on the basis of the lattice-temperature profiles obtained by modeling. It was found that the observed difference in the crater shapes generated in fused silica and sapphire is conditioned by the difference in dynamics of electron excitation and recombination channels characteristic of these two materials. This effect can be used to convert a given temporal pulse modulation into spatial modulation, opening up new perspectives for material processing in order to obtain desired structure profiles. PACS 79.20.Ds; 42.62.-b     

Photoinduced permittivity in lead molybdate

The effect of illumination on the dielectric properties of lead molybdate (PbMoO₄) single crystals was studied. Illumination with intrinsic light gives rise to a substantial enhancement of low-frequency permittivity (the photodielectric effect, PDE). The PDE is dependent on temperature and reaches a maximum near 400 K. The permittivity reaches the steady state a long time after the light is turned on (off). The relaxation time and activation energy of the process were determined. The relation of the PDE to the photoconductivity of the crystal is discussed. A possible mechanism of the PDE in PbMoO₄ crystals is proposed.     

Organic electro-optic thin films by simultaneous vacuum deposition and laser-assisted poling

Organic materials with a high second-order optical nonlinearity have an important application for the next generation of computing, telecommunications, and other industries. Because of a high electro-optic coefficient and low dielectric constant, N-benzyl-2-methyl-4-nitroaniline (BNA) single crystals have been grown and their optical properties have been extensively studied. In this Letter, a poled BNA thin film material was prepared through simultaneous vacuum evaporation and laser-assisted electrical poling. The linear electro-optic coefficients of BNA were measured by a Young's two-slit interference electro-optic measurement method. The dependence of the resulting electro-optic properties on optical power was explored. Enhancement induced by laser illumination was demonstrated. The BNA thin film can exhibit an r(33) value comparable to that of BNA single crystals.     

Characteristics of Ba0.5Sr0.5TiO3 thin films grown by ultraviolet-assisted pulsed laser deposition

The microstructure and properties of barium strontium titanate (BST) thin films grown by an in situ ultraviolet-assisted (UV-assisted) pulsed laser deposition (UVPLD) technique are reported in this paper. In comparison with BST films grown by conventional pulsed laser deposition (PLD) under similar conditions, but without UV illumination, the UVPLD-grown films exhibited improved structural, electrical, and optical properties. X-ray photoelectron spectroscopy showed that when exposed to atmosphere, Ba atoms from the outermost layers formed a thin layer of barium carbonate, which negatively affects the film electrical characteristics. UVPLD-grown films exhibited a smaller amount of Ba atoms within the carbonate layer, resulting in better electrical characteristics. The dielectric constant of 40-nm-thick films deposited at 650 °C by UVPLD and PLD were determined to be 281 and 172, respectively. The leakage current density of the UVPLD-grown films was in the mid-10^-8 A/cm² range, a factor of 2 lower than that obtained from PLD-grown films.     

Anomalies of polarization processes in relaxor ferroelectrics

The anomalous polarization of relaxor ferroelectrics (relaxors), such as solid solutions of barium-strontium niobate (SBN) and lead magnesium niobate (PMN), is studied in ac low-frequency (up to 10^?4 Hz) and dc electric fields. The results of studying dielectric hysteresis loops, polarization relaxation, the coercive-field distribution spectrum, nonlinear polarization for various scales of inhomogeneities, and polarization in a photosensitive SBN relaxor subjected to illumination are presented. All of the anomalous polarization properties of relaxors substantially distinguish them from ordinary homogeneous ferroelectrics and serve as signs and a measure of structural disorder. This article presents a brief review of our recent studies and some new results obtained as a result of their analysis.     

电流变液的流变学响应与其非线性介电性质的关系

对两类电流变液的介电性质的测量结果表明，强交流电场下电流变液经历介面极化；而且是一种非线性极化．根据海藻酸钠和NaY沸石电流变液的动态剪切应力与强电场介电谱相似这一事实，确定了这两种样品的流变学响应时间下限，它们分别是1ms和0.7ms．还发现NaY电流变液的动态切应力与非线性三次谐波的强度有关；一种高分子半导体电流变液的流变学响应基本不受样品含水量的影响．对上述实验结果进行了解释 关键词：     

Effect of external fields on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe<Subscript>2</Subscript>

The effect of external fields (dc electric field, light illumination) on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe₂ is studied using the measured dielectric constant. The results obtained are discussed. It is shown for the first time that the effect of external fields on the anomaly related to the memory effect in TlGaSe₂ can be reduced to the following universal empirical rule: when a sample is held for many hours at a constant temperature T ₀ in the temperature range of the incommensurate phase in a dc electric field, the deflection amplitude in the low-temperature part of the anomaly in the temperature dependence of the relative change in the dielectric constant Δ?/? increases (the deflection in the high-temperature part of the Δ?/? anomaly disappears) as compared to this segment in the dependence obtained during isothermal annealing of this sample at the same temperature without an electric field. The crystal remembers its thermal history at a temperature that is several kelvins higher than T ₀. Light illumination increases the deflection amplitude in the high-temperature part of the Δ?/?(T) anomaly and shifts the temperature at which the crystal remembers its thermal history toward lower temperatures with respect to T ₀.     

Equations of electrodynamics in a rotating solid dielectric

The equations of electrodynamics in a rotating isotropic homogeneous dielectric are obtained in a covariant form in coordinates of a reference frame that accompanies the rotation of the dielectric. It is found from these equations, which have variable coefficients, that the medium of the rotating dielectric is anisotropic and inhomogeneous. To derive tensors of the electromagnetic field in a rotating reference frame (RRF), the fields and inductions of a virtual inertial reference frame (IRF) that instantaneously accompanies the motion of one of the points of the dielectric are used twice. Initially, using instantaneous local relations, they are expressed in terms of real fields and inductions of the rotating medium, and then they are transformed into fields and inductions of a stationary IRF, in which they are used as components of the tensors of the electromagnetic field. Thus, the electromagnetic field tensors in the IRF are determined taking into account a priori unknown real inhomogeneous permittivity \(\bar \varepsilon \) and permeability \(\bar \varepsilon \) of the rotating medium. At the final stage, the tensors in the RRF are obtained by transformation rules for covariant and contravariant tensor components in accordance with known analytical relationships of fixed and rotating coordinates. The properties of modes of a rotating ring resonator in the form of homogeneous TE waves that travel along and against the direction of rotation and, in particular, their normal frequencies are examined. The contribution of inhomogeneous properties of the medium of a rotating dielectric to the difference between the normal frequencies of the counterpropagating waves (to the Sagnac effect) is determined. In a solid material with known elastic and striction characteristics, its density and dielectric permittivity depend on the radial coordinate. These dependences are caused by the action of the centrifugal force and changes in the polarization and magnetization of the medium because of the rotational motion of charged particles. The coordinate dependences of permittivity \(\bar \varepsilon \) and permeability \(\bar \varepsilon \) make additional contributions to the inhomogeneous properties of the medium of the rotating dielectric and to the Sagnac effect.     

A novel method to measure dielectric properties of materials in terahertz spectroscopy

We present a simple method to obtain the optical and dielectric properties of samples without reference measurement in the reflection-type terahertz time-domain spectroscopy. The dielectric properties of the samples of silicon and gallium arsenide were examined. The optical and dielectric properties of the samples were measured through only simple configuration, without the misplacement error. The obtained dielectric functions of the samples in reflection geometry are in good agreement with that predicated by the theory. The main advantage of this method over other methods is its simplicity and accuracy and ease for application of the reflection systems with different incident angle.     

Dielectric properties of antiferroelectric liquid crystals composed of banana-like molecules

The dielectric properties of an antiferroelectric B₂ phase representing a homolog (n=14) from a series of 4-chloro-1,3-phenylene-bis[4-(4-n-alkylphenyl)aminobenzoates] with banana-like molecules were studied. The temperature dependence of the dielectric relaxation time was measured, and the corresponding activation energy was determined. The kinetics of the dielectric permittivity was studied in the course of the polarization vector variation under the action of a low-slope triangular bias voltage in the entire temperature range of existence of the B₂ phase. Dependence of the dielectric permittivity on the electric field strength (bias voltage) is determined by the presence of two polar subsystems forming the antiferroelectric B₂ phase.     

Multiferroic TbMnO3 nanoparticles

We report on the synthesis of TbMnO₃ nanoparticles by chemical co-precipitation route and their structural, chemical bonding, magnetic and dielectric properties. It is shown that the interesting multiferroic properties of this system as reflected by the concurrent occurrence of magnetic and dielectric transitions are retained in the nanoparticles (size∼40 nm). However, the nanoparticle constitution and properties are seen to depend significantly on the calcination temperature. While the nanoparticles obtained by calcination at 800 °C correspond very well with the reported properties of single phase TbMnO₃ (all the key magnetic and dielectric features near 7, 27 and 41 K, albeit with reduced dielectric constant) the nanoparticles obtained by calcination at 900 °C develop a Tb deficient skin which softens the transitions, reducing the dielectric constant further.     

Abnormal switching behavior of nanoparticle composite systems

We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 kHz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells.     

A spectral invariant representation of spectral reflectance

Spectral image acquisition as well as color image is affected by several illumination factors such as shading, gloss, and specular highlight. Spectral invariant representations for these factors were proposed for the standard dichromatic reflection model of inhomogeneous dielectric materials. However, these representations are inadequate for other characteristic materials like metal. This paper proposes a more general spectral invariant representation for obtaining reliable spectral reflectance images. Our invariant representation is derived from the standard dichromatic reflection model for dielectric materials and the extended dichromatic reflection model for metals. We proof that the invariant formulas for spectral images of natural objects preserve spectral information and are invariant to highlights, shading, surface geometry, and illumination intensity. It is proved that the conventional spectral invariant technique can be applied to metals in addition to dielectric objects. Experimental results show that the proposed spectral invariant representation is effective for image segmentation.     

The electrostriction of dielectrics

The deformation of bodies in an electric field was first observed by Fontana [1] and Volpicelli [2], and Outer [3] analyzed the phenomenon quantitatively. Lippman predicted the phenomenon of electrostriction from general thermodynamic principles after the discovery of piezoelectricity by the Curie brothers [4].We may presume that any dielectric will exhibit electrostrictive properties to some extent. In fact, Zheludev [5] has shown that the oscillation of a dielectric pendulum is closely connected to, and dependent on, the polarization of the material. Since the polarization effect is inherent in all dielectrics, electrostriction is also a basic property of any dielectric.Both polarization and electrostriction appear in a dielectric under the influence of an electric field. Clearly, these effects will cause a change in the capacitance of condensers containing the experimental samples under the action of the electric field. The goal of the present work is to verify this reasoning and to determine which of the factors under consideration plays the dominating role.