Method for determination of the dielectric function of a thin absorbing film on variable substrates from transmission spectra

Transmission spectra of bilayers of a strongly absorbing dye molecule on thin semitransparent metallic films show a pronounced variation of the shape as a function of the thickness of the metal film. The shape changes with increasing thickness of the metal film from the form of an absorption spectrum as determined by the imaginary part of the dielectric function to an antisymmetric shape characteristic of the dispersion of the real part of the dielectric function in the vicinity of a resonance. These different spectra shapes were exploited to derive the complex dielectric function of a dye layer from transmission spectra of the layer on metal films of a different thickness. This method proved to be a simple alternative to determination of the dielectric function of a thin film of a dye by spectroscopic ellipsometry.     

Reliable deposition of induced transmission filters with a single metal layer

The sensitivity of an induced transmission filter (ITF) design to deposition errors is analyzed for the case of a single metal layer ITF. Theoretical knowledge of the least and most sensitive layers within the ITF design improves deposition reliability when using broadband optical monitoring of only the dielectric part of such metal-dielectric filters. Linearly variable ITFs have been successfully fabricated using this developed approach for error compensation.     

Heat or mass transfer from single spheres in a low reynolds number flow

The problem of forced heat or mass transfer from a single sphere in low Reynolds number flow is studied. In Stokes flow the Nusselt number $\text{N}$ can depend only upon the Peclet number $\text{P}$. For $\text{P ?}$ a general “master-formula” is derived which involves Stokes resistance $\text{F}{}_0$. On the other hand, the$\text{P?l}$ limit requires different formulas, depending upon whether the tangential velocity vanishes at the sphere surface (rigid sphere) or not (slip flow and droplets). Although these differences do not disappear when first-order inertia effects are included, it is found that inertia enhances the heat or mass transfer in either case.     

Tuning the properties of microwave dielectric resonators in a stack configuration

The microwave properties of ceramic dielectric resonators in a stack configuration are investigated using two different dielectric materials with opposite temperature coefficients of resonant frequency in the temperature range 50–300 K. The two dielectrics, which have been studied, are Ba₅Nb₄O₁₅ and 5ZnO–Nb₂O₅, synthesized by the conventional solid-state ceramic route. The temperature coefficient of resonant frequency of the stack resonator is tuned by varying the volume fraction of 5ZnO–Nb₂O₅. Temperature coefficient of resonant frequency slowly changed from a non-linear to almost a linear behavior with increase in the volume fraction of 5ZnO–Nb₂O_5.     

Wideband dielectric characterization using a dielectric filledcavity adapted to the end of a transmission line

The concept, analysis, and testing of a technique for characterizing dielectric materials over a wide band of frequencies are presented. The technique utilizes a cylindrical cavity completely filled with a sample of the dielectric material under test and adapted to the end of the precision air line. The complex permittivity of the filling dielectric is derived from the measured reflection coefficient. This technique offers great advantages over conventional methods where the sample is placed in a resonant cavity or inserted in a transmission line     

Absorption of electromagnetic radiation in a polar dielectric layer

Conditions are investigated for initiation of nonreflecting absorption of electromagnetic radiation in a polar dielectric layer applied to a reflecting metal substrate.     

The effect of temperature of a cone nose-tip on the spectra of disturbances in the hypersound boundary layer

The results of an experimental study of the influence of local heating/cooling of a blunted cone nose-tip on the development of perturbations in the boundary layer for Mach number M = 6 are presented. A comparative analysis of the pressure-pulsation spectra on the cone surface has been carried out. It is shown that heating/cooling in the boundary layer of an acute cone leads to destabilization/stabilization of the perturbations of the second mode. For blunted cones, the opposite effect has been observed: heating leads to weak stabilization of disturbances, and cooling leads to destabilization. This effect is enhanced by increasing the bluntness radius of the cone nose-tip.     

Anomalous dispersion in the dielectric spectra of conductivematerials

A high-frequency impedance analyzer is used to study the dielectric permittivity of a conductor-filled composite. An anomalous dielectric dispersion is observed at high frequencies. Comparison to dielectric probe measurements indicates that this dispersion is not reproducible with other measurement techniques and may be a result of instrumental artifacts. Theoretical circuit models show that this dispersion is most likely caused by finite inductances and resistances that exist in the electrode fixture     

Optical trapping of 12 nm dielectric spheres using double-nanoholes in a gold film

Optical tweezers have found many applications in biology, but for reasonable intensities, conventional traps are limited to particles >100 nm in size. We use a double-nanohole in a gold film to experimentally trap individual nanospheres, including 20 nm polystyrene spheres and 12 nm silica spheres, at a well-defined trapping point. We present statistical studies on the trapping time, showing an exponential dependence on the optical power. Trapping experiments are repeated for different particles and several nanoholes with different gap dimensions. Unusually, smaller particles can be more easily trapped than larger ones with the double-nanohole. The 12 nm silica sphere has a size and a refractive index comparable to the smallest virus particles and has a spherical shape which is the worst case scenario for trapping.     

A study of direct optical transitions in silicon single crystals based on transmission spectra

The transmission spectrum of single-crystal silicon in the range of 2.75?C3.25 eV has been obtained experimentally. An empirical formula is proposed to describe the spectral dependence of the optical density. The energy of the direct transition at the saddle point is found to be E(??₁₅) ? E(??_25??) = 3.43 eV. Analysis of the spectral dependence showed that the observed absorption corresponds to direct transitions to the van Hove singularity M ₁ of the conduction band.     

Synthesis of porous carbon spheres derived from lignin through a facile method for high performance supercapacitors

Porous carbon spheres (PCS) derived from lignin have been prepared through a facile method and fabricated as electrodes for electric double-layer capacitors. Spherical shaped mixtures of lignosulfonate and crystalized KOH are formed by spray drying of a solution of lignosulfonate and KOH. Activation by KOH is performed at high temperatures along with lignosulfonate carbonization. With an appropriate pore structure, the obtained PCS have a specific surface area of 1372.87 m² g^?1 and show a capacitance of 340 F g^?1 in 3 M KOH at a current density of 0.5 A g^?1. Moreover, a symmetric supercapacitor fabricated using the PCS as electrodes show a maximum capacitance of 68.5 F g^?1, and an energy density of 9.7 W h kg^?1 at a power density of 250 W kg^?1. The capacity retention is more than 94.5% after 5000 galvanostatic charge-discharge cycles. The excellent characteristics seem to be ascribed to the pore structures of PCS that have a large specific surface area and a low electrical resistance.     

Effect of leakage current and dielectric constant on single and double layer oxides in MOS structure

MOS structure of Al/Al₂O₃/n-Si, Al/TiO₂/n-Si and Al/Al₂O₃/TiO₂/n-Si was obtained by deposition of Al₂O₃ and TiO₂ on silicon substrate by RF Magnetron Sputtering system. The total thickness of the oxide layer ~ 40 ± 5 nm in the MOS structure was kept constant. Samples were characterized by X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Impedance analyzer and Current-voltage (J-V) characteristics. The variations in the dielectric constant and tan δ of the MOS capacitor in the frequency range of 1000Hz-1MHz were measured by impedance analyzer. The variation in dielectric constant of the Al/Al₂O₃/TiO₂/n-Si multilayer compared to single layer of Al/Al₂O₃/n-Si and Al/TiO₂/n-Si is due to high probability of defects, lattice mismatch and interface interactions. The steep rise of Tan δ values in the Al/Al₂O₃/TiO₂/n-Si structure is due to the resonance effect of both Al₂O₃ and TiO₂ layers. The leakage current mechanisms of MOS structures were extracted from Schottky coefficient and Poole-Frenkel coefficient. Theoretical values of Schottky coefficients (β_SC) and Poole-Frenkel coefficients (β_PF) for each sample were estimated using the real part of the dielectric constant. The experimental values were calculated from J-V characteristics and compared with theoretical values. The appropriate model has been proposed. It was found that Schottky and Poole-Frenkel mechanisms are applicable at low and high field respectively for all MOS structures. The combination of Al/Al₂O₃/TiO₂/n-Si is found to be a promising structure with high dielectric constant and low leakage current suitable for MOS devices.     

Evaluation of the dielectric parameters from TSDC spectra: application to polymeric systems

The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analy-sis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (ΔW≈± 0.06 eV). Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Measurement of the temperature-field distribution in the volume of a dielectric material treated in a microwave resonator chamber

The problem of producing a uniform temperature-field distribution in the volume of a dielectric material, treated in a microwave chamber, is considered. An experimental procedure for determining the temperature distribution inside a heated object is described. Translated from Metrologiya No. 1, pp. 22–27, January, 2009.     

Advantages of the dilatometer investigation of the heat-transfer coefficient of a dielectric layer

A dilatometer method for the comparative determination of the heat-transfer coefficient of dielectrics which is fast, accurate, and simple is described.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 31, No. 6, pp. 981–985, December, 1976.     

Radiation of a charge moving in a waveguide with a resonantly dispersive dielectric layer

The radiation of an ultrarelativistic point charged particle moving along the axis of a cylindrical waveguide containing a layer of resonantly dispersive dielectric medium is studied theoretically. It is shown that, in the case of a sufficiently thin waveguide, the dispersion leads to a significant contraction of the radiation, a decrease in the energy of harmonics, and an increase in the relative contribution due to the first mode as compared to the higher harmonics. These effects are less pronounced in waveguides with relatively large radii; still, they have to be taken into account, especially in determining the frequencies of harmonics. The role of the resonance dispersion is most significant in cases where the dielectric layer thickness is small compared to the waveguide radius.     

Right bipyramids of silver: a new shape derived from single twinned seeds

Silver nanoparticles with a single (111) twin were selectively nucleated and grown for the first time to produce right bipyramids 75-150 nm in edge length. Key to the production of single twinned seeds was the addition of NaBr to a polyol synthesis in which AgNO3 is reduced by ethylene glycol in the presence of poly(vinyl pyrrolidone). Examination of nanoparticles at different stages of the reaction with electron microscopy revealed that the spherical, single twinned seeds grew to become right bipyramids through enlargement of their (100) facets. The UV-vis-NIR spectrum exhibited by right bipyramids is distinct from that of any other silver nanostructure, and their sharp corners make these new nanoparticles especially promising for enhancement of Raman scattering.