Daniel E. Azofeifa

Retrieved optical properties of thin films on absorbing substrates from transmittance measurements by application of a spectral projected gradient method

The refractive index and the extinction coefficient of thin films deposited on absorbing substrates, as well as their thicknesses, are obtained from measurements of direct transmittance. The inversion approach is based on application of a spectral projected gradient method (SPGM). First, simulated transmittance spectra are used to compare the performance of the method with published results considering films with different degrees of absorption and thickness, and then inversions from real measurements taken over the solar spectral range are carried out. It is shown that the use of the SPGM, to minimize the difference between measured and computed transmittance spectra, becomes a powerful tool to solve efficiently the problem of obtaining the optical constants of thin films from spectroscopic measurements.

Visible light reflection spectra from cuticle layered materials

The brilliant metallic appearance of two kinds of beetles, the Chrysina aurigans and the Chrysina limbata, is displayed visually as well as from normalized reflection measurements of non polarized visible light under normal incidence. The C. limbata is reflective over the visible spectral wavelength range, with a silver-like aspect, while the golden-like C. aurigans is reflective for wavelengths larger than 525 nm, and with a well defined sequence of minima and maxima depicted in the reflection spectra. Both specimens show selective reflection of circular polarized light with the effect being significantly weaker for the C. limbata. A transfer matrix formalism is applied to approach the main features displayed in the measured reflection spectra, by assuming a chirped multilayer structure with decreasing thicknesses of successive layers through the cuticle of the beetles.

Light reflection by the cuticle of C. aurigans scarabs: a biological broadband reflector of left handed circularly polarized light

Measured reflection spectra from elytra of Chrysina aurigans scarabs are reported. They show a broad reflection band for wavelengths from 0.525 to 1.0 μm with a sequence of maxima and minima reflection values superimposed on a mean value of around 40% for the high reflection band. Different mechanisms contributing to the reflection spectra have been considered, with the dominant effect, reflection of left handed circularly polarized light, being produced by a laminated left handed twisted structure whose pitch changes with depth through the procuticle in a more complex way than that characterizing broad band circular polarizers based on cholesteric liquid crystals.

Optical and electrical changes of hydrogenated Dy films

Abstract Electrical and optical measurements on hydrogenated Pd coated (20 nm) Dy films (250–300 nm) are presented. The films are prepared in a high vacuum chamber and hydrogenated, at room temperature, by exposing them to a hydrogen atmosphere. Maximum hydrogen concentration of the order of 3 is achieved as measured by QCM. Changes in electrical resistance and optical transmission (at λ =632.8 nm) in the films indicate the dielectric nature of the DyH 3 . The Hall coefficient measured also as a function of concentration, gives an insight into the evolution of the density of charge carriers as hydrogen is absorbed.

Qualitative correlation between structural chirality through the cuticle of Chrysina aurigans scarabs and left-handed circular polarization of the reflected light

Measured reflection spectra from elytra of C. aurigans scarabs are reported for wavelengths between 300 and 1100 nm. They show a broad reflection band for wavelengths from 525 to 950 nm with a ripple structure consisting of a sequence of maxima and minima reflection values superimposed on a mean value of around 30% of the reflection band. To our knowledge, this is one the first reports on measurements of a natural broad band reflector in which the spectral features of the band are completely contained in the measuring range, including the mentioned ripple structure. What seems to be a multilayer structure of the cuticle of the C. aurigans is displayed from SEM analysis showing a layer’s thickness dependence with the perpendicular depth through the procuticle. Additional optical measurements are carried out to establish the polarization of the reflected light which is circularly polarized to the left, with lower contributions of diffuse and non-coherent light. These findings require an interpretation of the structure displayed by the SEM images, in terms of a Bouligand-type (twisted helical) structure characterized by a depth-dependent spatial period distribution more complex than those previously reported in the literature for others biological systems.

Collective response of silver islands on a dielectric substrate when normally illuminated with electromagnetic radiation

Normal reflectance measurements of silver island films, for wavelengths between 250 and 1050 nm, are inverted to obtain the effective dielectric function of the inhomogeneous films. An effective medium model is applied to evaluate the dielectric function of the metallic islands, which are modelled in terms of oblate spheroidal particles whose average values for the semiaxes have been estimated from atomic force microscopy images. Effective depolarization factors, which account for substrate effects and dipolar interactions between islands, are used in the context of the effective medium model. The dielectric function of the system of dipoles assigned to the silver island film is also obtained from an approach based on evaluating renormalized polarizabilities of the metallic islands also taking into account dipolar interparticle interactions and substrate effects.

Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarabs cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.

On the optical absorption coefficient of Gd hydride thin films: coexistence of the Gd (GdH2) and GdHx (GdH2+y) phases

Electron beam evaporated Pd-capped Gd thin films were deposited on quartz substrates and subsequently hydrided. Transmittance spectra of these bilayer systems, under unpolarized normal illumination, were obtained and used in an inversion approach from which the spectral dependence of the effective optical constants is obtained. The optical absorption coefficient of the β-phase GdHx (with 1 < x ≤ 2) is reported. The semiconductor GdH2+y state (with 0 < y ≤ 1) is characterized in terms of the optical absorption coefficient and the optical gap. By assuming a granular morphology of the films, with some of the grains in the metal GdH2 phase and other grains covered with a transition layer containing the semiconducting GdH2+y phase, the hydrogen concentration dependence of the optical gap is shown and the optical absorption edge of the stoichiometric γ-phase GdH3 is reported.

The effect of hydrogen uptake on the Hall resistivity and the electrical resistivity of gadolinium films

Abstract The Hall resistivity and electrical resistivity of Gd films (300 nm) coated with Pd (10 nm) are studied as a function of hydrogen pressure above and below the Gd Curie temperature. The hydrogen pressure varies from 0 to 6.0 Torr and the concentrations obtained indicate the formation of a mixed phase of Gd and GdH 2 . The electrical and Hall resistivities decrease as a function of hydrogen pressure. The Hall measurements are made in magnetic fields varied from 0 to 0.60 T and they indicate a significant change on the magnetic properties of the films.

Room temperature gas-solid reaction of titanium on glass surfaces forming a very low resistivity layer

Titanium films were deposited on quartz, glass, polyamide and PET substrates in a high vacuum system at room temperature and their electrical resistance monitored in vacuo as a function of thickness. These measurements indicate that a low electrical resistance layer is formed in a gas-solid reaction during the condensation of the initial layers of Ti on glass and quartz substrates. Layers begin to show relative low electrical resistance at around 21 nm for glass and 9nm for quartz. Samples deposited on polyamide and PET do not show this low resistance feature.