F. Pérez-Rodríguez

Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors

Uniform, spherical-shaped TiO2:Eu nanoparticles with different doping concentrations have been synthesized through controlled hydrolysis of titanium tetrabutoxide under appropriate pH and temperature in the presence of EuCl3·6H2O. Through air annealing at 500°C for 2 h, the amorphous, as-grown nanoparticles could be converted to a pure anatase phase. The morphology, structural, and optical properties of the annealed nanostructures were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy [EDS], and UV-Visible diffuse reflectance spectroscopy techniques. Optoelectronic behaviors of the nanostructures were studied using micro-Raman and photoluminescence [PL] spectroscopies at room temperature. EDS results confirmed a systematic increase of Eu content in the as-prepared samples with the increase of nominal europium content in the reaction solution. With the increasing dopant concentration, crystallinity and crystallite size of the titania particles decreased gradually. Incorporation of europium in the titania particles induced a structural deformation and a blueshift of their absorption edge. While the room-temperature PL emission of the as-grown samples is dominated by the 5D0 - 7Fj transition of Eu+3 ions, the emission intensity reduced drastically after thermal annealing due to outwards segregation of dopant ions.

Nonlocal effects in the electrodynamics of metallic slabs

The influence of spatial dispersion and size on the interaction of a metallic slab with electromagnetic radiation has been studied in the model of the Boltzmann kinetic equation. It has been shown that the results are qualitatively different from those obtained in the Drude-Lorentz approximation. In particular, in the high-frequency region, the absorption oscillates with the radiation frequency and sample thickness. The absorption becomes sensitive to the Fermi velocity of electrons and depends nontrivially on the electron relaxation rate. The results may be useful for the analysis of the electromagnetic response of metal-dielectric micro- and nanostructures in the terahertz and/or infrared frequency range.

Anisotropy effects in homogenized magnetodielectric photonic crystals

Exact analytic formulas for calculating the effective permittivity, permeability, and crossed magnetoelectric tensors for photonic crystals in the long-wavelength limit are presented. The formulas are valid for arbitrary Bravais lattice and form of inclusions, which can be dielectric, magnetic, or chiral. We have applied them to study the optical anisotropy of homogenized magnetodielectric three-dimensional photonic crystals, which can be induced by the type of Bravais lattice or the form of the inclusion even in the case when the photonic crystal is composed of isotropic materials. It is established that the electromagnetic modes, propagating in such anisotropic magnetodielectric metamaterials, are extraordinary, unlike the modes in homogenized nonmagnetic dielectric composites, where at least one mode is ordinary.

Enhanced transmission of terahertz radiation through a periodically modulated slab of layered superconductor

We predict the enhanced transparency of a modulated slab of layered superconductor for terahertz radiation due to the diffraction of an incident wave and the resonance excitation of eigenmodes. The electromagnetic field is transferred from the irradiated side of the slab to the other by excited waveguide modes (WGMs) which do not decay in layered superconductors, in contrast to metals, where the enhanced light transmission is caused by the excitation of evanescent surface waves. We show that a series of resonance peaks can be observed in the dependence of transmittance on the incidence angle when the dispersion curve of the diffracted wave crosses successive dispersion curves for the WGMs.

Effective Permittivity Tensor for a Metal-Dielectric Superlattice

We have derived simple analytical expressions for the frequency-dependent effective permittivity tensor of a one-dimensional metal-dielectric photonic crystal in the long wavelength limit. Our results describe the transition between the regime, described by Rytov’s formulas for sufficiently long waves, and that predicted by Xu et al. [6], where the effective plasma frequency is independent of the metallic-layer parameters. The derived expressions can be useful for determining the frequency intervals where such an anisotropic system can exhibit metamaterial behavior. Received 13 February 2011, Accepted 28 March 2011, Scheduled 31 March 2011 Corresponding author: Felipe Perez-Rodriguez (fperez@ifuap.buap.mx). 166 Zenteno-Mateo et al.

Nonlocal effect on optic spectrum of a periodic dielectric-metal stack

On the basis of the formalism of the Boltzmann kinetic equation for the distribution function of the conduction electrons, the photonic band structure of binary dielectric-metal superlattice is theoretically studied. Using the constitutive nonlocal relation between the electrical current density and the electric field inside the metallic layer, the dispersion equation for photonic eigenmodes in the periodic stack is analytically expressed in terms of the surface impedances at the interfaces of the metal and dielectric layers. In the case of very thin metallic layers, the optic spectrum for the superlattice exhibits narrow pass bands as a result of the strong contrast between the impedances of the dielectric and the metal. The narrow pass bands are attributed to Fabry-Perot resonances in the relatively-thick dielectric layer. The metal nonlocality is well pronounced in the infrared and, therefore, the nonlocal effect upon the photonic band structure of the superlattice can be strong when the Fabry-Perot resonance bands are in that frequency range. Our results for the photonic spectrum have been compared with those obtained within the local Drude-Lorentz model. Noticeably differences not only in the the magnitude, but also in the sign of the real part of the Bloch wave number in the Fabry-Perot resonance bands, have been found.

Double critical-state model for the weak-link regime of granular high-Tc superconductors

Abstract The phenomenological double critical-state model, which uses critical values for the components of the current-density both perpendicular and parallel to the local magnetic induction B, is applied to tubes of granular high-Tc superconductors in the presence of axial and azimuthal magnetic fields. We develop a theory valid for the weak-link regime of these superconductors. In this theory not only intergranular but also intragranular currents are taken into account. The behavior of the magnetic induction inside the wall of a superconducting tube cooled below the critical temperature in the axial magnetic field and, later, subjected to an increasing azimuthal field, is analyzed. Our theoretical results are compared with the experiment.

Landau damping of electromagnetic transport via dielectric–metal superlattices

We discuss the propagation of electromagnetic waves through a one-dimensional periodic array of bilayers with metal inclusions. We show that the nonlocality of metal conductivity leads to the emergence of the fundamental collisionless Landau damping. It cannot be neglected, not only when prevailing over ordinary collision damping, but even when these two kinds of electromagnetic absorption are of the same order. Landau damping always exists and considerably alters the photonic transmission of the array within the THz and near-infrared frequency range.

THz photonic bands of periodic stacks composed of resonant dielectric and nonlocal metal

The photonic band structures of superlattices composed of spatially-dispersive metal and polaritonic dielectric are theoretically investigated. The nonlocal relation between the electric current density and the electric field inside the metal layers is defined within the formalism of the Boltzmann kinetic equation, whereas the frequency dependent permittivity of the polar layers is modeled by a Lorentz-oscillator. Due to the large dielectric contrast between metal and polar components, the photonic band structure exhibits flat pass bands associated with Fabry-Perot resonances in the dielectric layers. There is also a wide stop band because of the existence of the polaritonic gap. We have compared our results with the predictions of the Drude-Lorentz model for the frequency-dependent metal permittivity. It is found that the nonlocal effect on the Fabry-Perot resonance bands is strong if their corresponding frequencies are within the interval where the difference between the impedances at both metal surfaces, predicted by the nonlocal and local formalisms, is maximal.

Bianisotropic metamaterials based on twisted asymmetric crosses

The effective bianisotropic response of 3D periodic metal-dielectric structures, composed of crosses with asymmetrically-cut wires, is investigated within a general homogenization theory using the Fourier formalism and the form-factor division approach. It is found that the frequency dependence of the effective permittivity for a system of periodically-repeated layers of metal crosses exhibits two strong resonances, whose separation is due to the cross asymmetry. Besides, bianisotropic metamaterials, having a base of four twisted asymmetric crosses, are proposed. The designed metamaterials possess negative refractive index at frequencies determined by the cross asymmetry, the gap between the arms of adjacent crosses lying on the same plane, and the type of Bravais lattice.