RESUMO
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.
RESUMO
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.
