Near-Field Plasmonics of an Individual Dielectric Nanoparticle above a Metallic Substrate

We simulate and discuss the local electric-field enhancement in a system of a dielectric nanoparticle placed very near to a metallic substrate. We use finite-element numerical simulations in order to understand the field-enhancement mechanism in this dielectric NP-on-mirror system. Under appropriate excitation conditions, the gap between the particle and the substrate becomes a "hot spot", i.e., a region of intense electromagnetic field. We also show how the optical properties of the dielectric NP placed on a metallic substrate affect the plasmonic field enhancement in the nanogap and characterize the confinement in the gap. Our study helps to understand and design systems with dielectric NPs on metallic substrates which can be equally as effective for SERS, fluorescence, and nonlinear phenomena as conventional all plasmonic structures.

Studies of the Room-Temperature Multiferroic Pb(Fe0.5Ta0.5)0.4(Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy, Dielectric, and Magnetic Phenomena

Recently, lead iron tantalate/lead zirconium titanate (PZTFT) was demonstrated to possess large, but unreliable, magnetoelectric coupling at room temperature. Such large coupling would be desirable for device applications but reproducibility would also be critical. To better understand the coupling, the properties of all 3 ferroic order parameters, elastic, electric, and magnetic, believed to be present in the material across a range of temperatures, are investigated. In high temperature elastic data, an anomaly is observed at the orthorhombic mm2 to tetragonal 4mm transition, Tot = 475 K, and a softening trend is observed as the temperature is increased toward 1300 K, where the material is known to become cubic. Thermal degradation makes it impos- sible to measure elastic behavior up to this temperature, however. In the low temperature region, there are elastic anomalies near ≈40 K and in the range 160–245 K. The former is interpreted as being due to a magnetic ordering transition and the latter is interpreted as a hysteretic regime of mixed rhom- bohedral and orthorhombic structures. Electrical and magnetic data collected below room temperature show anomalies at remarkably similar temperature ranges to the elastic data. These observations are used to suggest that the three order parameters in PZTFT are strongly coupled.

Combinatorial Frequency Generation in Quasi-Periodic Stacks of Nonlinear Dielectric Layers

Three-wave mixing in quasi-periodic structures (QPSs) composed of nonlinear anisotropic dielectric layers, stacked in Fibonacci and Thue-Morse sequences, has been explored at illumination by a pair of pump waves with dissimilar frequencies and incidence angles. A new formulation of the nonlinear scattering problem has enabled the QPS analysis as a perturbed periodic structure with defects. The obtained solutions have revealed the effects of stack composition and constituent layer parameters, including losses, on the properties of combinatorial frequency generation (CFG). The CFG features illustrated by the simulation results are discussed. It is demonstrated that quasi-periodic stacks can achieve a higher efficiency of CFG than regular periodic multilayers.

Temperature dependence of the mid-infrared dielectric function of YBCO in the ab plane

We have excited mid-infrared surface plasmons in two YBCO thin films of contrasting properties using attenuated total reflection of light and found that the imaginary part of the dielectric function decreases linearly with reduction in temperature. This result is in contrast with the commonly reported conclusion of infrared normal reflectance studies. If sustained it may clarify the problem of understanding the normal state properties of YBCO and the other cuprates. The dielectric function of the films, epsilon = epsilon(1) + i epsilon(2), was determined between room temperature and 80K: epsilon(1) was found to be only slightly temperature dependent but somewhat sample dependent, probably as a result of surface and grain boundary contamination. The imaginary part, epsilon(2), (and the real part of the conductivity, sigma(1),) decreased linearly with reduction in temperature in both films. Results obtained were: for film 1: epsilon(1) = - 14.05 - 0.0024T and epsilon(2) - 4.11 + 0.086T and for film 2: epsilon(1) = - 24.09 + 0.0013T and epsilon(2) = 7.66 + 0.067T where T is the temperature in Kelvin. An understanding of the results is offered in terms of temperature-dependent intrinsic intragrain inelastic scattering and temperature-independent contributions: elastic and inelastic grain boundary scattering and optical interband (or localised charge) absorption. The relative contribution of each is estimated. A key conclusion is that the interband (or localised charge) absorption is only similar to 10%. Most importantly, the intrinsic scattering rate, 1/tau, decreases linearly with fall in temperature, T, in a regime where current theory predicts dependence on frequency, omega, to dominate. The coupling constant, lambda, between the charge carriers and the thermal excitations has a value of 1.7, some fivefold greater than the far infrared value. These results imply a need to restate the phenomenology of the normal state of high temperature superconductors and seek a corresponding theoretical understanding.

DC resistance and dielectric function at 3392 nm of YBCO films from 300 K to 80 K

The dielectric function of a YBCO film was determined at 3392nm at temperatures down to 80K. Results obtained were epsilon(i) = -24.09 - 0.0013T and epsilon(i) = 7.66 + 0.067T. The results for epsilon(i) are compared with the de resistance of the film. Intrinsic intragrain scattering, elastic and inelastic grain boundary scattering and optical interband absorption are estimated as 82%, 5%, 13% and 10% respecively at 0K.

Interwoven spiral arrays on ferrite-dielectric substrates for high-impedance metasurfaces

A high impedance metasurface (HIMS) composed of the arrays of intertwined planar spirals on thin (~0.1λ) ferrite-dielectric substrate is proposed. The HIMS exhibits fractional bandwidth in excess of 10% and excellent angular and polarisation stability of the circular polarised waves at oblique incidence.

Morphology dependence of the dielectric properties of epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers were grown by pulsed laser deposition on vicinal surfaces of (001)-oriented Nb-doped SrTiO3 (SrTiO3:Nb) single-crystal substrates. Atomic force microscopy was used to investigate the surface topography of the deposited films. The morphology of the films, of the BaTiO3/SrTiO3 interfaces, and of the column boundaries was investigated by cross-sectional high-resolution transmission electron microscopy. Measurements of the dielectric properties were performed by comparing BaTiO3 films and BaTiO3/SrTiO3 multilayers of different numbers of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm and linearly decreases with decreasing film thickness below a thickness of 75 nm. At the same thickness of 75 nm, the thickness dependence of the dielectric constant also exhibits a change in the linear slope both for BaTiO3 films and BaTiO3/SrTiO3 multilayers. This behaviour is explained by the change observed in the grain morphology at a thickness of 75 nm. For the thickness dependence of the dielectric constant, two phenomenological models are considered, viz. a 'series-capacitor' model and a 'dead-layer' model.

Microstructure and dielectric properties of epitaxial BaTiO3 films and BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO3 films and BaTiO3/SrTiO3 multilayers were grown by pulsed laser deposition (PLD) on (001)-oriented Nb-doped SrTiO3 (SrTiO3:Nb) substrates. Measurements of the dielectric properties were performed comparing BaTiO3 films and BaTiO3/SrTiO3 multilayers of different number of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm, and linearly decreases with decreasing film thickness below a thickness of 75 nm, and it is independent on the number of multilayers, pointing to some interface effect. The thickness dependence of the dielectric constant of BaTiO3 films and BaTiO3/SrTiO3 multilayers; exhibits a change in the linear slope at a thickness of 75 nm. This behavior is explained by the change observed in the morphology at a thickness of 75 nm. In order to explain the thickness dependence of the dielectric constant, two approaches are considered in this paper, viz. a "series capacitor" model and a "dead layer" model.

Oxygen vacancy formation in CeO2 and Ce1-xZrxO2 solid solutions:electron localization, electrostatic potential and structural relaxation

Ceria (CeO2) and ceria-based composite materials, especially Ce1-xZrxO2 solid solutions, possess a wide range of applications in many important catalytic processes, such as three-way catalysts, owing to their excellent oxygen storage capacity (OSC) through the oxygen vacancy formation and refilling. Much of this activity has focused on the understanding of the electronic and structural properties of defective CeO2 with and without doping, and comprehending the determining factor for oxygen vacancy formation and the rule to tune the formation energy by doping has constituted a central issue in material chemistry related to ceria. However, the calculation on electronic structures and the corresponding relaxation patterns in defective CeO2-x oxides remains at present a challenge in the DFT framework. A pragmatic approach based on density functional theory with the inclusion of on-site Coulomb correction, i.e. the so-called DFT + U technique, has been extensively applied in the majority of recent theoretical investigations. Firstly, we review briefly the latest electronic structure calculations of defective CeO2(111), focusing on the phenomenon of multiple configurations of the localized 4f electrons, as well as the discussions of its formation mechanism and the catalytic role in activating the O-2 molecule. Secondly, aiming at shedding light on the doping effect on tuning the oxygen vacancy formation in ceria-based solid solutions, we summarize the recent theoretical results of Ce1-xZrxO2 solid solutions in terms of the effect of dopant concentrations and crystal phases. A general model on O vacancy formation is also discussed; it consists of electrostatic and structural relaxation terms, and the vital role of the later is emphasized. Particularly, we discuss the crucial role of the localized structural relaxation patterns in determining the superb oxygen storage capacity in kappa-phase Ce1-xZr1-xO2. Thirdly, we briefly discuss some interesting findings for the oxygen vacancy formation in pure ceria nanoparticles (NPs) uncovered by DFT calculations and compare those with the bulk or extended surfaces of ceria as well as different particle sizes, emphasizing the role of the electrostatic field in determining the O vacancy formation.

A Model to Understand the Oxygen Vacancy Formation in Zr-Doped CeO2:Electrostatic Interaction and Structural Relaxation

Using density functional theory with the inclusion of on-site Coulomb Correction, the O vacancy formation energies of CexZr1-xO2 solid solutions with a series of Ce/Zr ratios are calculated, and a model to understand the results is proposed. It consists of electrostatic and structural relaxation terms, and the latter is found to play a vital role in affecting the O vacancy formation energies. Using this model, several long-standing questions in the field, such as why ceria with 50% ZrO2 usually exhibit the best oxygen storage capacity, can be explained. Some implications of the new interpretation are also discussed.

Non-reciprocity in doubly periodic strip arrays on ferrite-dielectric substrate

Doubly periodic arrays of strip conductors printed on a composite ferrite-dielectric substrate have been investigated at oblique incidence of linear polarized plane waves. The simulation results revealed strong non-reciprocity of wave reflectance and transmittance at positive and negative angles of incidence. It is also shown that the non-reciprocity is further enhanced by the strip conductor pattern. 

Determining adsorbate configuration on alumina surfaces with 13C nuclear magnetic resonance relaxation time analysis

Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved 13C nuclear magnetic resonance (NMR) T1 relaxation times. The ratio of relaxation times for the adsorbed atoms T1,ads to the bulk liquid relaxation time T1,bulk provides an indication of the mobility of the atom. Hence a low T1,ads/T1,bulk ratio indicates a stronger surface interaction. The carbon atoms associated with unsaturated bonds in the molecules are seen to exhibit a larger reduction in T1 on adsorption relative to the aliphatic carbons, consistent with adsorption occurring through the carbon-carbon multiple bonds. The relaxation data are interpreted in terms of proximity of individual carbon atoms to the alumina surface and adsorption conformations are inferred. Furthermore, variations of interaction strength and molecular configuration have been explored as a function of adsorbate coverage, temperature, surface pre-treatment, and in the presence of co-adsorbates. This relaxation time analysis is appropriate for studying the behaviour of hydrocarbons adsorbed on a wide range of catalyst support and supported-metal catalyst surfaces, and offers the potential to explore such systems under realistic operating conditions when multiple chemical components are present at the surface.