On the Bergman-Milton bounds for the homogenization of dielectric composite materials

The Bergman-Milton bounds provide limits on the effective permittivity of a composite material comprising two isotropic dielectric materials. These provide tight bounds for composites arising from many conventional materials. We reconsider the Bergman-Milton bounds in light of the recent emergence of metamaterials, in which unconventional parameter regimes for relative permittivities are encountered. Specifically, it is demonstrated that: (a) for nondissipative materials the bounds may be unlimited if the constituent materials have relative permittivities of opposite signs; (b) for weakly dissipative materials characterized by relative permittivities with real parts of opposite signs, the bounds may be exceedingly large.     

Percolation thresholds in the homogenization of spheroidal particles oriented in two directions

We consider percolation thresholds which arise in the homogenization of composite mediums based on three different types of component particles. An extension of the standard Bruggeman homogenization formalism is implemented in order to take account of the sizes, shapes, and orientations of the component particles. The relationships between the geometric attributes of the component particles and the constitutive parameters of the homogenized composite mediums are investigated. In particular, percolation thresholds arising in the homogenization of conducting component particles oriented in two directions and nonconducting component particles are explored via representative numerical examples. Anisotropies in these percolation thresholds are highlighted.     

Application of Bruggeman and Maxwell Garnett homogenization formalisms to random composite materials containing dimers

The homogenization of a composite material comprising three isotropic dielectric materials was investigated. The component materials were randomly distributed as spherical particles, with the particles of two of the component materials being coupled to form dimers. The Bruggeman and Maxwell Garnett formalisms were developed to estimate the permittivity dyadic of the homogenized composite material (HCM), under the quasi-electrostatic approximation. Both randomly oriented and identically oriented dimers were accommodated; in the former case the HCM is isotropic, whereas in the latter case the HCM is uniaxial. Representative numerical results for composite materials containing dielectric–dielectric dimers demonstrate close agreement between the estimates delivered by the Bruggeman and Maxwell Garnett formalisms. For composite materials containing metal–dielectric dimers and metal–metal dimers with moderate degrees of dissipation, the estimates of the two formalisms are in broad agreement, provided that the dimer volume fractions are relatively low. In general, the effects of intradimer coupling on the estimates of the HCM’s permittivity are relatively modest but not insignificant, these effects being pronounced by anisotropy when all dimers are identically oriented.     

On the characterization of sculptured columnar Mn thin films; correlation between nanostructure and the optical and electrical properties using experimental and simulation results

Mn columnar (nanorod shaped) thin films of 40 and 150?nm thicknesses were deposited on glass substrates at different deposition angles, using oblique angle deposition technique. The concept of local homogenization proposed by Bruggeman is used for optical analyses of these samples. Results of this simulation method are compared with the experimental data and values for void fraction and film thickness are predicted. In addition, using the perturbation simulation method, variation of relative resistance of these thin films was obtained as a function of material inclusion. It is observed that both simulation and experimental results show that the thinnest samples with the least amount of material inclusion have the highest electrical resistance. Results showed a rise at about 30° deposition angle, which may be attributed to the surface diffusion effect or thermal vibration causing rearrangement of atoms as well as crystallographic available sites for relaxation of adatoms deposited at this certain direction, though this requires a thorough investigation.     

The maximal advance path constraint for the homogenization of materials with random network microstructure

This paper presents a non-affine homogenization scheme for materials with a random network microstructure. It is based on a newly developed kinematic constraint that links the microscopic deformation of the network to the macroscopic strain of the material. This relation accounts for the network functionality and is established by means of maximal advance paths that are long enough to reach the macroscopic scales of the continuous body and deform accordingly but are also composed of the microscopic fibres that follow the network deformation. The exact distribution of the variable fibre stretch is determined by the principle of minimum averaged free energy, which ultimately allows one to derive the homogenized elastic response of the network at equilibrium. Besides the general formulation, the model is presented in detail for the case of tetrafunctional networks, for which the micro–macro relation and the expression for the homogenized elastic stress are derived in a compact and interpretable tensorial form. The performance of the model as well as the convexity and stability of the obtained homogenized response of the material is examined for networks composed of two different types of fibres, namely flexible chains and stiff filaments. The qualitative behaviour of the networks predicted for the two considered cases agrees with experimentally observed phenomena for soft materials. This includes a consistent explanation for the difference in the stiffness of elastomers at uniaxial and equibiaxial extension as well as a validation of recent experimental investigations of atypical normal stress amplitudes in biopolymer gels under shear loading.     

Investigation of the relation between photoluminescence intensity and decay time reduction with plasmon effect in InGaAs quantum dots

Plasmonic effects on photoluminescence are investigated via time-integrated and resolved photoluminescence (PL) in epitaxially grown InGaAs quantum dots (QDs). The decay time and PL intensities are compared as a function of the density of Ag nanoplates. Optimal conditions for both reduction lifetime and enhanced PL intensity were found to be a 1:15 ratio of Ag nanoplates to water. Both less and greater than that ratio 1:15, the lifetime increased and the enhancement factor of PL intensity decreased. In addition, the plasmon effect was investigated via resonance wavelength and temperature-dependent PL measurements. At 150K near the resonance conditions between PL from InGaAs QDs and Ag nanoplates, both the lifetime reduction and enhancement factor are maximized. Intensity enhancement is correlated to lifetime reduction for various conditions to identify a condition for maximized enhancement of radiative recombination for designing future ultrafast plasmonic nanolasers.     

CdTe量子点-罗丹明B荧光共振能量转移猝灭法测定金银花中的微量铜

研究CdTe量子点(供体)和罗丹明B(受体)之间荧光共振能量转移的最佳条件,建立了荧光共振能量转移猝灭法测定金银花中微量铜的新方法.采用十六烷基三甲基溴化铵,在pH 6.00的Tris-HCl缓冲液中,Cu2+能对能量转移体系中罗丹明B荧光峰强猝灭从而测定铜的含量.Cu2+浓度在1.3×10-4～3.1×10-2 μg...     

Higher Landau levels contribution to the energy of interacting electrons in a quantum dot

Properly regularized second-order degenerate perturbation theory is applied to compute the contribution of higher Landau levels to the low-energy spectrum of interacting electrons in a disk-shaped quantum dot. At “filling factor” near , this contribution proves to be larger than energy differences between states with different spin polarizations. After checking convergence of the method in small systems, we show results for a 12-electron quantum dot, a system which is hardly tractable by means of exact diagonalization techniques.     

On the two-dimensional thermoelectric power in quantum wells of non-parabolic materials under magnetic quantization

We present a simplified theoretical formulation of the thermoelectric power (TP) under magnetic quantization in quantum wells (QWs) of nonlinear optical materials on the basis of a newly formulated magneto-dispersion law. We consider the anisotropies in the effective electron masses and the spin–orbit constants within the framework of k.p formalism by incorporating the influence of the crystal field splitting. The corresponding results for III–V materials form a special case of our generalized analysis under certain limiting conditions. The TP in QWs of Bismuth, II–VI, IV–VI and stressed materials has been studied by formulating appropriate electron magneto-dispersion laws. We also address the fact that the TP exhibits composite oscillations with a varying quantizing magnetic field in QWs of n- Cd₃As₂, n- CdGeAs₂, n-InSb, p-CdS, stressed InSb, PbTe and Bismuth. This reflects the combined signatures of magnetic and spatial quantizations of the carriers in such structures. The TP also decreases with increasing electron statistics and under the condition of non-degeneracy, all the results as derived in this paper get transformed into the well-known classical equation of TP and thus confirming the compatibility test. We have also suggested an experimental method of determining the elastic constants in such systems with arbitrary carrier energy spectra from the known value of the TP.     

常压介质阻挡放电间隙的选择及其在材料表面改性中的应用

使用自制介质阻挡放电装置，分析了常压等离子体放电电流和放电功率与放电间隙的变化关系，提出了“放电临界间隙”的概念，并应用该装置对PBT熔喷非织造布进行表面改性，讨论了放电间隙和放电气体等因素对改性效果的影响．     

Structural and optical properties of In0.5Ga0.5As/GaAs quantum dots in an In0.1Ga0.9As well using repeated depositions of InAs/GaAs short-period superlattices for the application of optical communication

We report structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) in a 100 Å-thick In_0.1Ga_0.9As well grown by repeated depositions of InAs/GaAs short-period superlattices with atomic force microscope, transmission electron microscope (TEM) and photoluminescence (PL) measurement. The QDs in an InGaAs well grown at 510 °C were studied as a function of n repeated deposition of 1 monolayer thick InAs and 1 monolayer thick GaAs for n=5–10. The heights, widths and densities of dots are in the range of 6–22.0 nm, 40–85 nm, and 1.6–1.1×10¹⁰/cm², respectively, as n changes from 5 to 10 with strong alignment along [1 −1 0] direction. Flat and pan-cake-like shape of the QDs in a well is found in TEM images. The bottoms of the QDs are located lower than the center of the InGaAs well. This reveals that there was intermixing—interdiffusion—of group III materials between the InGaAs QD and the InGaAs well during growth. All reported dots show strong 300 K-PL spectrum, and 1.276 μm (FWHM: 32.3 meV) of 300 K-PL peak was obtained in case of 7 periods of the QDs in a well, which is useful for the application to optical communications.     

On the confinement of a Dirac particle to a two-dimensional ring

In this contribution, we propose a new model for studying the confinement of a spin-half particle to a two-dimensional quantum ring for systems described by the Dirac equation by introducing a new coupling into the Dirac equation. We show that the introduction of this new coupling into the Dirac equation yields a generalization of the two-dimensional quantum ring model proposed by Tan and Inkson [W.-C. Tan, J.C. Inkson, Semicond. Sci. Technol. 11 (1996) 1635] for relativistic spin-half quantum particles.     

Using two-dimensional photon echo spectroscopy to probe the fine structure of the ground state biexciton of CdSe nanocrystals

The origin of the fine structure of the ground state single- and biexciton of CdSe nanocrystals is reviewed, along with the theoretical framework used to describe these states. Calculations were performed to determine the transition dipole moments of optically allowed transitions from the single- to biexciton fine structure states. Two-dimensional photon echo spectroscopy measurements for a sample of CdSe nanocrystals are reported. The two-dimensional electronic spectrum at a population time of 0 fs is analyzed using a simulation based on k.p theory predictions of the exciton and biexciton manifolds of states. The analysis suggests that a particular excited state absorption transition from the single- to biexciton fine structure dominates the 2D spectra. These excited state absorptions are clearly resolved in 2D spectra and the method therefore has promise for gaining clearer insights into quantum dot spectroscopy.     

Stiffness matrix invariants to validate the characterization of composite materials with ultrasonic methods

This paper presents a method of testing the ultrasonic measurements of the stiffness matrix, and the identification of the anisotropic behaviour, of composite materials. Some linear combinations of elastic constants are invariants for a rotation around an axis of symmetry. If the stacking sequence is the only parameter which changes in a set of long-fibre composites made of the superimposition of plies, the composites must own these invariants. So, PEEK-carbon fibre composite samples were constructed in this way to measure their elastic properties by immersion and contact ultrasonic methods, and to compare the results with predicted invariants. By changing the stacking sequence of plies three anisotropic models are tested: orthotropic, hexagonal and quadratic. Measurements of ultrasonic velocities in various planes of propagation permit the identification of the elastic constant and invariants. From the invariance of these linear combinations, the precision of the three-dimensional effective moduli can be estimated.     

Energy of excitons and acceptor-exciton complexes to explain the origin of ultraviolet photoluminescence in ZnO quantum dots embedded in a SiO2 matrix

Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor-exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO₂ matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor-exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good agreement between the matrix element calculation method and the experimental results.     

On the surface contribution to the grand-canonical pressure of free quantum gases

The surface term in the thermodynamic pressure of free quantum gases is proved to exist and is evaluated. Detailed proofs are given for parallelepipedic domains with Dirichlet, periodic, and Neumann boundary conditions and for more general domains with Dirichlet boundary conditions.     

Application of the method of equivalent edge currents to composite scattering from the cone-cylinder above a dielectric rough sea surface

Compared with scattering from a rough surface only, composite scattering from a target above a rough surface has become so practical that it is a subject of great interest. At present, this problem has been solved by some numerical methods which will produce an enormous calculation amount. In order to overcome this shortcoming, the reciprocity theorem (RT) and the method of equivalent edge currents (MEC) are used in this paper. Due to the advantage of RT, the difficulty in computing the secondary scattered fields is reduced. Simultaneously, MEC, a high-frequency method with edge diffraction considered, is used to calculate the scattered field from the cone-cylinder target with a high accuracy and efficiency. The backscattered field and the polarization currents of the rough sea surface are evaluated by the Kirchhoff approximation (KA) method and physical optics (PO) method, respectively. The effects of the backscattering radar cross section (RCS) and the Doppler spectrum on the size of the target and the windspeed of the sea surface for different incident angles are analysed in detail.     

Theory of the transmission of metal strip gratings on a dielectric substrate application to submillimeter laser coupling

The advantages of supported metal strip gratings with respect to free-standing metal meshes are pointed out for applications as laser output couplers. A new, simple formula for the power transmission through the strip grating on a dielectric plate is derived from a line equivalent-circuit model. This formula is in good agreement with measurements performed on several samples, at different submillimeter wavelengths. An application to output couplers of far-infrared gas discharge lasers allows to improve the reliability of such lasers.x Unité associée au CNRS. U.A. 836.     

On the use of perforations to improve the sound absorption of porous materials

The concept of meso-perforations in appropriately chosen porous media can help enhance their sound absorption performance. The meso-perforated materials are also referred to as “double porosity materials” since they are made up of two interconnected networks of pores of different characteristic size. Several theoretical, numerical and experimental works have been accomplished on the subject by the authors. The purpose of this paper is to give a synthetic review of these works and establish practical design rules to develop optimized noise control solutions based on this concept. The paper presents two complementary models to deal with this kind of materials: an analytical model based on homogenization techniques and a numerical model relying on a finite element discretization of the domains. The limits of these models are discussed. The choice of the design parameters is then been investigated in order to provide practical design rules. This choice relies on a criterion which is evaluated from the knowledge of the resistivity, porosity and tortuosity of the micropous medium, and the calculation of a geometrical parameter defined from the chosen mesoscopic structure. Experimental and numerical results regarding the influence of the mesopore profile along the thickness performed in a appropriately chosen substrate microporous medium are presented. The agreement between the models and the experiments is satisfactory. Results show that significant enhancements of the absorption properties can be obtained over a selected frequency band by adjusting the mesopore profile. It is also shown that interesting absorbing properties can be obtained when coating a double porosity medium with an impervious screen.     

Localization of heat in the shock compressed matter and its application to materials processing

Abstract

Both the heterogeneous deformation and the localization of heat in the shock compressed materials are supported by in situ measurements and textural observations. Additional shock-induced radiation measurements on diamond powder aggregates confirm such a current aspect and the proposed model, namely the skin model, that allows us to estimate quantitatively both the amount of localized heat and the subsequent thermal diffusion process. The parameter, ratio of the skin thickness to the cube size or particle size, is a convenient, universal parameter to representthe shock state of powder and is independent of particle size but dependent on shock pressures. The skin temperatures are approximately constant and equal to the melting temperature. The shock processing, especially shock consolidation of powders, effectively utilizes these phenomena and can be designed based on the skin model.