Arturo Mendoza-Galván

Effect of metal-ion doping on the optical properties of nanocrystalline ZnO thin films

Optical properties of metal (Al, Ag, Sb, and Sn)-ion-implanted ZnO films have been studied by ultraviolet-visible spectroscopy and spectroscopic ellipsometric techniques. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n), and extinction coefficient (k) of nanocrystalline ZnO films have been studied for the similar implantation dose of all the metal ions. The ellipsometric spectra of the ion-implanted samples could be well described by considering an air/roughness/ZnO–M (layer 1)/ZnO (layer 2)/glass model. The band gap of ZnO films increases with Al ion doping and decreases with doping of Ag, Sb, and Sn ions. The refractive index of ZnO films in the visible spectral region increases substantially on Sb and Sn ion doping, while it decreases to some extent with Al ion doping.

Optical response of supported gold nanodisks

It is shown that the ellipsometric spectra of short range ordered planar arrays of gold nanodisks supported on glass substrates can be described by modeling the nanostructured arrays as uniaxial homogeneous layers with dielectric functions of the Lorentz type. However, appreciable deviations from experimental data are observed in calculated spectra of irradiance measurements. A qualitative and quantitative description of all measured spectra is obtained with a uniaxial effective medium dielectric function in which the nanodisks are modeled as oblate spheroids. Dynamic depolarization factors in the long-wavelength approximation and interaction with the substrate are considered. Similar results are obtained calculating the optical spectra using the island-film theory. Nevertheless, a small in-plane anisotropy and quadrupolar coupling effects reveal a very complex optical response of the nanostructured arrays.

Evidence for a dispersion relation of optical modes in the cuticle of the scarab beetle Cotinis mutabilis

Variable angle Mueller matrix spectroscopic ellipsometry is used to study the properties of light reflected from the exoskeleton (cuticle) of the scarab beetle Cotinis mutabilis. For unpolarized incident light, the ellipticity and degree of polarization of the reflected light reveal a left-handed helical structure in the beetle cuticle. Analysis of the spectral position of the maxima and minima in the interference oscillations of the Mueller-matrix elements provides evidence for a dispersion relation similar to that of optical modes in chiral nematic liquid crystals calculated within a two-wave approximation. Additionally, a structural model for the cuticle of C. mutabilis is derived from the properties of the optical modes for non-attenuated propagation or selective reflection.

Spectroscopic ellipsometry study on the dielectric function of bulk Ti2AlN,Ti2AlC, Nb2AlC, (Ti0.5,Nb0.5)2AlC, and Ti3GeC2 MAX-phases

The averaged complex dielectric function =2+ /3 of polycrystalline Ti2AlN, Ti2AlC,Nb2AlC, Ti0.5,Nb0.52AlC, and Ti3GeC2 was determined by spectroscopic ellipsometry coveringthe mid infrared to the u ...

Graded pitch profile for the helicoidal broadband reflector and left-handed circularly polarizing cuticle of the scarab beetle Chrysina chrysargyrea

The cuticle of the beetle Chrysina chrysargyrea reflects left-handed polarized light in the broad spectral range from 340 to 1000 nm. Interference oscillations in the experimental Mueller-matrix spectroscopic ellipsometry data reveal that transparent materials comprise the cuticle. A spectral analysis of the interference oscillations makes evident that the pitch profile across the cuticle is graded. The graded pitch and effective refractive indices are determined through non-linear regression analysis of the experimental Mueller matrix by using a cuticle model based on twisted biaxial dielectric slices. Non-uniformity in cuticle thickness as well as in pitch profile near the cuticle surface account for depolarizance of the Mueller matrix. Transmission electron microscopy supports the reliability of the results.

Fano interference in supported gold nanosandwiches with weakly coupled nanodisks.

We studied the far-field optical response of supported gold-silica-gold nanosandwiches using spectroscopic ellipsometry, reflectance and transmittance measurements. Although transmittance data clearly shows that the gold nanodisks in the sandwich structure interact very weakly, oblique reflectance spectra of s- and p-polarized light show clearly asymmetric line-shapes of the Fano type. However, all experimental results are very well described by modeling the gold nanodisks as oblate spheroids and by employing a 2 × 2 scattering matrix formulation of the Fresnel coefficients provided by an island film theory. In particular, the Fano asymmetry can be explained in terms of interference between the scattered waves from the decoupled nanodisks in the spectral range limited by their respective plasmon resonances. We also show that the reflectance and ellipsometry spectra can be described by a three-layer system with uniaxial effective dielectric functions.

Birefringence of nanocrystalline chitin films studied by Mueller-matrix spectroscopic ellipsometry

Birefringent chitin films were prepared by a dipping technique from aqueous suspensions of chitin nanocrystals in a nematic liquid crystal phase. In the films, chitin nanocrystals are preferentially oriented along the withdrawal direction. Normal incidence transmission Mueller-matrix (M) spectroscopic ellipsometry measurements as a function of sample rotation were used to investigate the optical birefringence in the spectral range 0.73 to 5 eV. Analysis of eigenvalues and depolarization data reveal that the Mueller matrix corresponds to a pure retarder for photon energies below 4 eV and is depolarizing in the range 4 to 5 eV. By modeling the chitin film as a slab with in-plane anisotropy the birefringence was determined. The determination of birefringence was extended to include the range of 4 to 5 eV by a differential decomposition of M.

Electrochromic Properties of Nanoporous α and β Nickel Hydroxide Thin Films Obtained by Chemical Bath Deposition

Nickel hydroxide nanoporous electrochromic thin films were obtained by chemical bath deposition method using nickel nitrate in two different formulations. In one case, ammonium hydroxide controlled the precipitation of the β-Ni (OH)2 phase through the formation of coordination compounds. In the second case, the decomposition of urea yielded the α-Ni (OH)2 phase. After thermal annealing in air in between 250 and 300 °C, the electrochromic behavior of the films was examined by cyclic voltammetry and single wavelength transmittance. The optical contrast and structural transformations between colored and bleached states of the samples were studied by ex-situ optical transmittance, X-ray diffraction, as well as Raman and infrared reflectance spectroscopies. The α-Ni (OH)2 films showed higher optical contrast and reversibility, properties associated with their porous morphology which is revealed by scanning electron microscopy studies.

Electrochromic Nickel Oxide-Based Thin Films Deposited by Chemical Bath

Nickel oxide-based thin films were deposited onto indium-tin oxide (ITO) coated glass substrates by the chemical bath method. The synthesis, based on the decomposition of urea in an aqueous nickel nitrate solution, promotes the deposition of the turbostratic -Ni(OH)2 phase. The electrochromic behavior of films obtained from low and high urea concentrations was tested after air annealing at temperatures of 250 and 300 °C. Using cyclic voltammetry, chronoamperometry and in-situ single wavelength transmittance it is shown that the films exhibit good reversibility and coloration efficiency. Transmittance spectra in the visible range show the high optical contrast of the films. It was found that in films annealed at 250 °C the bleached-colored switching is between the Ni(OH)2 and -NiOOH phases. Films annealed at 300 °C are comprised by a Ni(OH)2-NiO mixture and Ni2O3 units or the -NiOOH phase are responsible for coloring depending on urea concentration in solution. The -NiOOH phase was clearly identified in over-colored films. X-ray diffraction, Raman scattering, and infrared reflectance were used for phase identification.