Domingos De Sousa Meneses

Contribution of Semi-Quantum Dielectric Function Models to the Analysis of Infrared Spectra

A new approach to modeling using semi-quantum dielectric function models is proposed for the retrieval of the optical functions from infrared spectra. The powerful points of the method are shown throughout the analysis of two semitransparent materials, MgO and KBr. All the results are discussed in light of those obtained with classical techniques. This type of model is able to retrieve, for example, the extinction index in a range that covers at least six orders of magnitude and gives access to highly valuable information about high-order phonon processes.

Structural, dielectric, and optical properties of yttrium calcium borate glasses

Structural and optical properties of stable glasses in the Y2O3–CaO–B2O3 system, containing the same Y/Ca ratio as the YCa4O(BO3)3 (YCOB) crystal, were determined from Raman and reflectance infrared spectroscopy. Changes in optical functions with composition are associated with an increase in the number of non-bridging oxygen and to calcium/yttrium oxides content. Refractive indexes values (from 1.597 to 1.627 at λ=2 μm) are in good agreement with those of the YCOB crystal, an indication that these glasses are potential candidates for optical applications due to their ease of shaping as large bulk samples or fibers.

Prediction of the thermal radiative properties of an x-ray μ-tomographied porous silica glass

A Monte Carlo ray tracing procedure is proposed to simulate thermal optical processes in heterogeneous materials. It operates within a detailed 3D image of the material, and it can therefore be used to investigate the relationship between the microstructure, the constituent optical properties, and the macroscopic radiative behavior. The program is applied to porous silica glass. A sample was first characterized by 3D x-ray tomography; then, its normal spectral emittance was calculated and compared with the experimental spectrum measured independently by high-temperature infrared emittance spectroscopy. We conclude with a discussion of the light-scattering mechanisms occurring in the sample.

Temperature dependence of the dielectric permittivity of CaF2, BaF2 and Al2O3: application to the prediction of a temperature-dependent van der Waals surface interaction exerted onto a neighbouring Cs(8P3/2) atom

The temperature behaviour in the range 22-500 °C of the dielectric permittivity in the infrared range is investigated for CaF(2), BaF(2) and Al(2)O(3) through reflectivity measurements. The dielectric permittivity is retrieved by fitting reflectivity spectra with a model taking into account multiphonon contributions. The results extrapolated from the measurements are applied to predict a temperature-dependent atom-surface van der Waals interaction. We specifically consider as the atom of interest Cs(8P(3/2)), the most relevant virtual couplings of which fall in the range of thermal radiation and are located in the vicinity of the reststrahlen band of fluoride materials.

Dispersion Relations and Phase Retrieval in Infrared Reflection Spectra Analysis

A variant of the maximum entropy method (MEM), recently developed by Vartiainen et al., is proposed for infrared reflectivity spectra analysis. The procedure uses a new interpolation algorithm to estimate the so-called error phase that is the key step of the model. As shown by several examples, this approach is a good alternative to the well known Kramers–Kronig phase retrieval relation for the determination of the dielectric function and proved to be particularly efficient in dealing with conductor materials.

High temperature emissivity, reflectivity, and x-ray absorption of BiFeO3

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and x-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below TN∼640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ∼700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC∼1090 K. Temperature dependent x-ray absorption near edge structure (XANES) at the Fe K edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO wustite XANES profile, the Bi LIII-edge downshift suggests a high temperature very c...

Terahertz and mid-infrared reflectance of epitaxial graphene

Graphene has emerged as a promising material for infrared (IR) photodetectors and plasmonics. In this context, wafer scale epitaxial graphene on SiC is of great interest in a variety of applications in optics and nanoelectronics. Here we present IR reflectance spectroscopy of graphene grown epitaxially on the C-face of 6H-SiC over a broad optical range, from terahertz (THz) to mid-infrared (MIR). Contrary to the transmittance, reflectance measurements are not hampered by the transmission window of the substrate, and in particular by the SiC Reststrahlen band in the MIR. This allows us to present IR reflectance data exhibiting a continuous evolution from the regime of intraband to interband charge carrier transitions. A consistent and simultaneous analysis of the contributions from both transitions to the optical response yields precise information on the carrier dynamics and the number of layers. The properties of the graphene layers derived from IR reflection spectroscopy are corroborated by other techniques (micro-Raman and X-ray photoelectron spectroscopies, transport measurements). Moreover, we also present MIR microscopy mapping, showing that spatially-resolved information can be gathered, giving indications on the sample homogeneity. Our work paves the way for a still scarcely explored field of epitaxial graphene-based THz and MIR optical devices.

Piecewise Polynomial Dielectric Function Model and its Application for the Retrieval of Optical Functions

A new expression of dielectric function model based on piecewise polynomials is introduced. Its association with spline and more recent shape preserving interpolation algorithms allows easy reproduction of every kind of experimental spectra and thus retrieval of all the linear optical functions of a material. Based on a pure mathematical framework, the expression of the model is always applicable and does not necessitate any knowledge of the microscopic mechanisms of absorption responsible for the optical response. The potential of piecewise polynomial dielectric functions is shown through synthetic examples and the analysis of experimental spectra.

Collective phase-like mode and the role of lattice distortions at TN ∼ TC in RMn2O5 (R= Pr, Sm, Gd, Tb, Bi)

We report on electronic collective excitations in RMn(2)O(5) (R =Pr, Sm, Gd, Tb) showing condensation starting at and below ~T(N) ~T(C)~ 40-50 K. Their origin is understood as partial delocalized e(g) electron orbitals in the Jahn-Teller distortion of the pyramid dimer with strong hybridized Mn(3+)-O bonds. Our local probes, Raman, infrared, and x-ray absorption, back the conclusion that there is no structural phase transition at T(N)~T(C). Ferroelectricity is magnetically assisted by electron localization triggering lattice polarizability by unscreening. We have also found phonon hardening as the rare earth is sequentially replaced. This is understood as a consequence of lanthanide contraction. It is suggested that partially f-electron screened rare earth nuclei might be introducing a perturbation to e(g) electrons prone to delocalize as the superexchange interaction takes place.

Retrieval of Linear Optical Functions from Finite Range Spectra

There are many experimental situations in which infrared reflectivity spectra can be acquired only over a limited spectral range. It is therefore necessary to find computing procedures that allow the efficient analysis of such data. In this paper, we propose a new procedure labeled constrained finite range correction (CFRC) that can be advantageously substituted to multiply subtractive Kramers–Kronig relations. The constrained finite range correction is able to produce realistic results even when very little supplementary information is available. For semitransparent crystals, the hypothesis of the phase spectrum positiveness alone is often sufficient to compute satisfactory approximations of the optical functions. The efficiency of the new method is shown through the analysis of several synthetic and experimental spectra.