Extensive radiative transfer calculations to fit the more significant average spectral features displayed in measured reflectance spectra by cuticles of Chrysina resplendes scarabs have been carried out. A simulated annealing approach is the starting point to optimize the values of the spatial average uric acid volume fraction, the thickness of the retarder layer located between the two left-handed chiral structures in the cuticle, and the slope of the parameter describing the variation with depth of the uric acid volume fraction through the retarder layer. Both twisted arrangements are characterized in terms of their photonic behaviors to display the variation with depth of the photonic band gaps, effective refractive index and birefringence, wave numbers and relative densities of states.

Measured reflectance spectra by cuticles of C. resplendens scarabs, in the spectral range 300 to 1000 nm, show high variability depending on the magnitude of the illuminated area, the position of the illuminated section, the elytron, a single specimen, and on the different specimens considered. This fact suggests variability in the depth dependence of the pitch and of the average uric acid concentration, and on the thickness of the retarder layer located between the two helicoidal structures characterizing the cuticle of these beetles.

A parametric Drude–Lorentz (DL) model is used to describe the spectral variation of the dielectric functions of bulk palladium samples at low and room temperature. In addition to the contribution of conduction electrons, the contribution of holes is also explicitly accounted for in the model. A simulated annealing method is applied to obtain the optimized values of the parameters involved in the model: volume plasma frequency of conduction electrons, high frequency dielectric constant, collision frequency of holes and corresponding relaxation time, and two additional parameters from which the effective mass of holes and collision frequency of conduction electrons are evaluated. Oscillatior strengths, resonance frequencies, and widths entering in the Lorentz contribution to the dielectric function are also optimized. Renormalization of the oscillator strengths requires the introduction of a new parameter in the context of the DL model: the ratio between number density of conduction electrons and number density of metal atoms, whose optimized value fits very well with its evaluation from band structure calculations and from independent measurements. Inclusion of this parameter in the framework allows us to evaluate additional quantities related to the charge-carrier transport: average effective masses, Fermi energies and electronic densities of states at the corresponding Fermi energies, intrinsic electrical resistivity, intrinsic mean free paths, heat capacities, mobilities, as well as paramagnetic and diamagnetic susceptibilities, for both electrons and holes. The optimized resonance frequencies are compared with energy differences between plausible interband transitions, in accordance with reported band structure diagrams and with our own band structure obtained from density functional theory calculations.

A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles (Chrysina chrysargyrea and Chrysina optima), measured in the wavelength range 300–1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman’s formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz’s equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.

We report measurements of the optical transmission, between 240 and 1040 nm, and electrical resistivity of polycrystalline zirconium thin films as they absorb hydrogen. Both are measured as H-2 pressure is increased up to 880 mbar, at room temperature. Films, 20-22 nm thick, are deposited on fused quartz substrates by e-beam evaporation at 5.3 x 10(-7) mbar base pressure and covered with a 8.0 nm Pd over-coat. The morphology of the films is studied by means of AFM images. The complex refractive indices of Zr and Pd are extracted numerically from the transmission spectra by using a spectral projected gradient method for different hydrogen pressures. The corresponding dielectric functions for various Zr hydrogen concentrations are described with the parametric Drude-Lorentz and Brendel-Bormann (DL & BB) models. The Acceptance-Probability-Controlled Simulated Annealing approach is applied to calculate the parameters of the DL & BB model. This allows us to describe the effect of increasing hydrogen absorption on these parameters and in derived quantities, like the relaxation time and the effective mass of conduction electrons, the electrical resistance, the Fermi energy, and the electronic density of states at the Fermi level. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Cuticles of some Chrysina scarabs are characterized by flat, graded, and twisted structures of nanosized chitin fibrils. As inferred from SEM images, each species has its own spatial period or pitch P which is dependent on the depth z through the cuticle. From Berreman’s formalism, taking into account the corresponding P(z) dependence, we evaluate reflection spectra of C. aurigans and C. chrysargyrea scarabs. The spectra display the main spectral features observed in the measured ones when small sections of the cuticles are illuminated with non-polarized light, for wavelengths between 300 and 1100 nm. By considering these twisted structures as 1D photonic crystals, an approach is developed to show how the broad band characterizing the reflection spectra arises from a narrow intrinsic photonic band width, whose spectral position moves through visible and near infrared wavelengths. The role of the epicuticle that covers the twisted structures is analyzed in terms of a waxy layer acting as an anti-reflecting coating that also shows low levels of light scattering.

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.

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 scarab's 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.

An optimized parametric description of the dielectric function of thin granular Pd thin films is reported. The optical transmission spectra of 10, 15 and 20nm thick films are measured and by means of an inversion procedure, the dielectric function of nano-sized Pd grains in the films is determined. Measurements are carried out for hydrogen pressures of up to 1atm. The Brendel–Bormann model is used to describe in parametric form the dielectric function under these conditions.

Under normal incidence of non-polarized light, reflection spectra from the cuticle of golden-like C. aurigans scarabs shows a broad band displayed from 525 to 950nm, with a spectral ripple structure that consists of a uniform sequence of peaks superimposed on the main reflection band. Cross sectional Scanning Electron Microscope (SEM) images of the cuticle initially suggest the presence of a multilayered structure. A radiative transfer matrix formalism is first applied to describe as much as possible the main features of coherent reflection spectra, by assuming optically homogenous layers distributed through the exocuticle, with chitin as the major constituent material. Additional non-coherent multiple reflections due to layers in the endocuticle are also evaluated from this approach. The presence of a pigmented micron sized structure beneath the procuticle requires the evaluation of a diffuse light contribution to the reflection. This was carried out from a four-flux radiative transfer model. Optical anisotropy is introduced by interpreting the SEM images in terms of a twisted Bouligand-type structure, and reflection spectra are evaluated from an implementation of the so-called 4×4 Berreman׳s formalism. We have been able to approach the main features characterizing the reflection spectra of C. aurigans׳ elytra following this progressive way.

Polycrystalline indium films have been deposited at room temperature by electron beam evaporation on amorphous quartz substrates. Once deposited, electrical resistance and specular reflection of the films, for wavelengths between 250 and 920 nm, has been measured in situ. For these as-deposited films, these parameters decrease with time for several minutes until equilibrium is reached. At this state, additional specular and diffuse reflection, and direct transmission measurements were carried out ex situ, as well as atomic force microscope characterizations of the films’ surface. The specular reflection spectra are inverted to obtain the intrinsic scattering and absorption coefficients including their time variations. These parameters are successfully correlated with corresponding volumetric scattering and absorption cross sections of submicron- and nano-sized indium particles calculated from Mie theory. The decrease of electrical resistance with time is explained in terms of an increase of the average size of some of the nano-sized grains due to mass transfer from large submicron-sized grains, a manifestation of an inverse Ostwald ripening process. The decrease of specular reflection is related to the decoupling of the surface plasmons oscillating in neighboring nano-sized particles due to the growth of some of them.

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.

A parametric description of the dielectric function of Pd thin films with thicknesses between 10 and 30 nm is reported. These films were grown at room temperature on amorphous quartz substrates by electron beam evaporation, with a base pressure of 7.0×10(-7)  mbar. By using nonpolarized normal incident light, transmission spectra were measured for wavelengths between 240 and 1050 nm. Inversion of the spectra by means of a projected gradient method enables us to obtain the mean dielectric function of the Pd grains in the films. We follow the Brendel-Bormann model to describe the frequency dependence of the dielectric function, with the plasma frequency, collision frequency, and screening factor as parameters in the free electron term. The contributions of bound electrons and their interband transitions, described in terms of Lorentz oscillators, involve the resonance frequencies, decay times, strengths, and Gaussian widths as parameters of the model. All these parameters have been optimized from the Pd grains' dielectric function, which fits the transmission spectra. A similar procedure was followed for Pd films exposed to a hydrogen atmosphere close to one bar. Thus, the dielectric functions of palladium and palladium hydride can easily be calculated through spectral ranges covering near-ultraviolet, visible, and near-infrared wavelengths. This can be used to model the behavior of nano-sized structures in which palladium particles or thin films are exposed to hydrogen pressures close to one bar.

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 mu m 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.

espanolLas propiedades opticas de los polimeros naturales quitina y quitosano, asi como las del acido urico, han sido consideradas para longitudes de onda entre 0.25 y 0.75 μm. En primer lugar, mediante la inversion de datos publicados del indice de refraccion para polimeros compuestos tanto de quitosano como de quitina, se ha podido obtener la dependencia espectral de los indices de refraccion de ambos materiales. En segundo lugar, se llevaron a cabo mediciones de reflexion y transmision de luz no polarizada a muestras obtenidas de camarones frescos. A partir de estas mediciones espectrofotometricas se ha obtenido el indice de refraccion del medio compuesto por quitina y acido urico, y su correspondiente coeficiente de extincion para el intervalo espectral antes mencionado. La absorcion de luz por la quitina es despreciable para las longitudes de onda consideradas. El coeficiente de extincion del acido urico muestra bandas de absorcion en el ultravioleta cercano. Las mediciones de absorbancia confirman la presencia de acido urico en las muestras de quitina obtenidas de camarones frescos. A manera de ejemplo ilustrativo, las constantes opticas de la quitina se utilizan para modelar el color estructural de un chinche asiatico: el Poecilocoris lewisi. EnglishThe optical properties of chitin and chitosan natural polymers, as well as uric acid, have been considered for wavelengths between 0.25 and 0.75 μm. First, by inverting refractive index data published for chitin-chitosan composite samples, it was possible to obtain the spectral dependence of the refractive indices of both materials. Reflection and transmission measurements of non-polarized light were carried out from samples of fresh shrimp shells. From these spectrophotometric measurements we obtained the refractive index of the composite medium chitin-uric acid as well as the corresponding extinction coefficient through the mentioned spectral wavelength range. The absorption of light by the chitin is negligible for the wavelengths considered. The extinction coefficient of uric acid shows absorption bands in the near ultraviolet. Absorbance measurements confirm the presence of uric acid in the chitin samples obtained from fresh shrimps. By way of illustrative example, the optical constants of chitin are used to model the structural color of a shield bug from Asia: the Poecilocoris lewisi.

Optical properties of 50 nm thick single crystal vanadium films deposited on double side polished MgO substrates have been obtained from spectrophotometric measurements. The films were coated with a polycrystalline Pd layer (5 nm thick) to protect them from oxidation and to favor absorption of atomic hydrogen. Electrical resistance was recorded while hydrogen pressure was increased slowly up to 750 mbar keeping temperature constant. Simultaneously, under normal incidence of non-polarized radiation [350-950 nm], transmittance spectra of this Pd/V/MgO system were measured. These were numerically inverted to obtain the spectral behavior of the Pd, V. PdHy, and VHx dielectric functions at 22 and 140 degrees C, and at 750 mbar. Hydrogen concentration in V film was first determined from a resisto-optical method. Finally, we demonstrate the possibility to determine the concentration in the Pd and the V layers independently, solely using the changes in the optical transmission. (C) 2013 Elsevier B.V. All rights reserved.

The optical properties of chitin and chitosan natural polymers, as well as uric acid, have been considered for wavelengths between 0.25 and 0.75. m. First, by inverting refractive index data published for chitin-chitosan composite samples, it was possible to obtain the spectral dependence of the refractive indices of both materials. Reflection and transmission measurements of non-polarized light were carried out from samples of fresh shrimp shells. From these spectrophotometric measurements we obtained the refractive index of the composite medium chitin-uric acid as well as the corresponding extinction coefficient through the mentioned spectral wavelength range. The absorption of light by the chitin is negligible for the wavelengths considered. The extinction coefficient of uric acid shows absorption bands in the near ultraviolet. Absorbance measurements confirm the presence of uric acid in the chitin samples obtained from fresh shrimps. By way of illustrative example, the optical constants of chitin are used to model the structural color of a shield bug from Asia: the Poecilocoris lewisi.

Optical properties of V thin films deposited on MgO substrates have been obtained from spectrophotometric measurements. The V films were coated with a thin Pd layer to protect them from oxidation and to favor absorption of atomic hydrogen. Electrical resistance was recorded while hydrogen pressure was increased slowly up to 750mbar keeping the temperature constant. Simultaneously, visible and near-infrared transmittance spectra of this Pd/V/MgO system were measured. The spectra were numerically inverted to obtain the spectral behavior of the Pd and V dielectric functions at 22 and 140 degrees C. Hydrogen concentrations were first determined from the combined effect of hydrogen content on the electrical resistance and on the optical direct transmission of the system. Then, determination of these concentrations was improved using retrieved values of the absorption coefficients of the hydrides and taking into account the structural change of V and the volumetric expansion of Pd. Good agreement is established when considering qualitative correlations between spectral features of the optimized PdHy and VHx dielectric functions and band structure calculations and densities of states for these two transition metal hydrides.

Optical properties of the biopolymers chitin and chitosan have been considered for wavelengths between 250 and 750nm. First, by inverting published refractive index data for composite chitosan–chitin samples of two independent sources, we have been able to obtain the spectral dependence of both the chitosan and chitin refractive indices. Then light reflection and transmission measurements were carried out for samples obtained from fresh shrimp shells. From these spectrophotometric measurements the chitin refractive index and its extinction coefficient have been obtained for the mentioned spectral range. Absorption of light by chitin is negligible for visible wavelengths. Chitin extinction coefficient displays absorption bands in the near ultraviolet, and it is attributed to the proteinaceous content of chitin. Cuticle proteins have been isolated from these chitin samples, and absorbance measurements support the presence of the aforementioned absorption band. The chitin optical constants are used to model the structural color of a shield bug: the Poecilocoris lewisi.

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.