Evaluation of Bulk and Surfaces Absorption Edge Energy of Sol-Gel-Dip-Coating SnO2 Thin Films

The absorption edge and the bandgap transition of sol-gel-dip-coating SnO2 thin films, deposited on quartz substrates, are evaluated from optical absorption data and temperature dependent photoconductivity spectra. Structural properties of these films help the interpretation of bandgap transition nature, since the obtained nanosized dimensions of crystallites are determinant on dominant growth direction and, thus, absorption energy. Electronic properties of the bulk and (110) and (101) surfaces are also presented, calculated by means of density functional theory applied to periodic calculations at B3LYP hybrid functional level. Experimentally obtained absorption edge is compared to the calculated energy band diagrams of bulk and (110) and (101) surfaces. The overall calculated electronic properties in conjunction with structural and electro-optical experimental data suggest that the nature of the bandgap transition is related to a combined effect of bulk and (101) surface, which presents direct bandgap transition.

Effect of hydrogenation on the optical and structural properties of GaAs thin films prepared by rf-magnetron sputtering

We have utilized infra-red and optical absorption measurements, grazing incidence X-ray diffraction (GIXRD) and extended X-ray absorption fine structure (EXAFS) measurements to investigate the influence of hydrogenation on the optical and structural properties of GaAs thin films prepared by rf-magnetron sputtering. Hydrogenation induces distinct changes in the optical properties, namely shifts in the absorption edges and reduction of the Urbach energy. Such modifications are correlated to a reduction in structural disorder as determined by EXAFS and the increase of crystallinity determined by GIXRD. (c) 2005 Elsevier B.V. All rights reserved.

Annealing effects on optical properties of natural alexandrite

Natural alexandrite (BeAl2O4:Cr3+) crystals are investigated as regards the effects of annealing on their optical properties. Optical absorption spectra are measured from the ultraviolet (190 nm) to the near infrared (900 nm), for a sample subjected to consecutive annealing processes, where time and temperature are varied. Besides this, luminescence spectra are simultaneously obtained for this sample, excited with a Kr+ laser source, tuned on an ultraviolet multi-line mode (337.5, 350.7 and 356.4 nm). We observe from absorption as well as from emission data that annealing mainly influences the distribution of Cr3+ and Fe3+ ions, located on sites of a mirror plane (C-s symmetry), which are responsible for the optical properties of alexandrite. The results obtained lead to the conclusion that annealing induces a modification of the population of Cr3+ on C-s sites as well as on sites located on an inversion plane (C-i). Annealing could improve the optical properties of this material, as regards its application as a tunable laser.

Synthesis, structural refinement and optical behavior of CaTiO3 powders: A comparative study of processing in different furnaces

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Electrical and optical characteristics of SnO2 thin films prepared by dip coating from aqueous colloidal suspensions

Starting from aqueous colloidal suspensions, undoped and Nb5+ doped SnO2 thin films have been prepared by using the dip-coating sol gel process. X-ray diffraction results show that films are polycrystalline with crystallites of average size1-4nm. Decreasing the thickness of the films and increasing the Nb5+ concentration limits the crystallite size growth during firing. Complex impedance measurements reveal capacitive and resistive effects between adjacent crystallites or grains, characteristic of electrical potential barriers. The transfer of charge throughout these barriers determines the macroscopic electrical resistance of the layer. The analysis of the optical absorption spectra shows that the samples present more than 80% of their transmittance in the visible region and the value of the band gap energy increases with decreasing crystallite size. © 1997 Chapman & Hall.

Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses

Optical absorption, Stokes, and anti-Stokes photoluminescence were performed on Er3+-Yb3+ co-doped fluoroindate glasses. For compounds prepared with a fixed 2 mol % ErF3 concentration and YbF3 contents ranging from 0 to 8 mol %, important upconversion processes were observed as a function of temperature and photon excitation energy. Based on the experimental data, two mechanisms for the upconversion (or anti-Stokes photoluminescence) processes were identified and analyzed in detail. At high Yb contents, the upconversion mechanisms are mostly determined by the population of the 2F5/2 levels of Yb3+ ions (or 4I11/2 levels of Er3+ ions, by energy transfer) regardless of the photon excitation energy and temperature of measurement. Moreover, green and red light emission have similar intensities when a large Yb3+ content is present. © 1998 American Institute of Physics.

Cr+3 distribution in Al1 and Al2 Sites of alexandrite (BeAl2O4: Cr3+) induced by annealing, investigated by optical spectroscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Tricritical-like behavior of the nonlinear optical refraction at the nematic-isotropic transition in the E7 thermotropic liquid crystal

We use Z-scan technique to investigate the nonlinear optical response of the thermotropic liquid crystal E7 in the neighborhood of the nematic-isotropic phase transition. The analysis of the data for the nonlinear optical birefringence is compatible with an effective critical exponent of the order parameter, beta = 0.28 +/- 0.03, which is close to the classical value, beta = 0.25, for a tricritical point. The nonlinear optical absorption in the nematic range depends on the geometrical configuration of the nematic director with respect to the polarization beam, and vanishes in the isotropic phase.

Magnetic and optical investigation of 40SiO(2)center dot 30Na(2)O center dot 1Al(2)O(3)center dot(29-x) B2O3 center dot xFe(2)O(3) glass matrix

Samples of 40SiO(2)center dot 30Na(2)O center dot 1Al(2)O(3)center dot(29 - x)B2O3 center dot xFe(2)O(3) (mol%), with 0.0 <= x <= 17.5, were prepared by the fusion method and investigated by electron paramagnetic resonance (EPR), optical absorption (OA) and Mossbauer spectroscopy (MS). The EPR spectra of the as-synthesized samples exhibit two well-defined EPR signals around g = 4.27 and g = 2.01 and a visible EPR shoulder around g = 6.4, assigned to isolated Fe3+ ion complexes (g = 4.27 and g = 6.4) and Fe3+-based clusters (g = 2.01). Analyses of both EPR line intensity and line width support the model picture of Fe3+-based clusters built in from two sources of isolated ions, namely Fe2+ and Fe3+; the ferrous ion being used to build in iron-based clusters at lower x-content (below about x = 2.5%) whereas the ferric ion is used to build in iron-based clusters at higher x-content (above about x = 2.5%). The presence of Fe2+ ions incorporated within the glass template is supported by OA data with a strong band around 1100 nm due to the spin-allowed E-5(g)-T-5(2g) transition in an octahedral coordination with oxygen. Additionally, Mossbauer data (isomer shift and quadrupole splitting) confirm incorporation of both Fe2+ and Fe3+ ions within the template, more likely in tetrahedral-like environments. We hypothesize that ferrous ions are incorporated within the glass template as FeO4 complex resulting from replacing silicon in non-bridging oxygen (SiO3O-) sites whereas ferric ions are incorporated as FeO4 complex resulting from replacing silicon in bridging-like oxygen silicate groups (SiO4). (C) 2012 Elsevier Masson SAS. All rights reserved.

Laboratory study on the optical properties of absorbing atmospheric aerosols and field applications

The interaction between aerosols and sun light plays an important role in the radiative balance of Earth’s atmosphere. This interaction is obtained by measuring the removal (extinction), redistribution (scattering), and transformation into heat (absorption) of light by the aerosols; i.e. their optical properties. Knowledge of these properties is crucial for our understanding of the atmospheric system. rn Light absorption by aerosols is a major contributor to the direct and indirect effects on our climate system, and an accurate and sensitive measurement method is crucial to further our understanding. A homebuilt photoacoustic sensor (PAS), measuring at a 532nm wavelength, was fully characterized and its functionality validated for measurements of absorbing aerosols. The optical absorption cross-sections of absorbing polystyrene latex spheres, to be used as a standard for aerosol absorption measurements, were measured and compared to literature values. Additionally, a calibration method using absorbing aerosol of known complex refractive index was presented.rn A new approach to retrieve the effective broadband refractive indices (mbroad,eff) of aerosol particles by a white light aerosol spectrometer (WELAS) optical particle counter (OPC) was achieved. Using a tandem differential mobility analyzer (DMA)-OPC system, the nbroad,eff are obtained for both laboratory and field applications. This method was tested in the laboratory using substances with a wide range of optical properties and it was used in ambient measurements to retrieve the nbroad,eff of biomass burning aerosols in a nationwide burning event in Israel. The retrieved effective broadband refractive indices for laboratory generated scattering aerosols were: ammonium sulfate (AS), glutaric acid (GA), and sodium chloride, all within 4% of literature values. For absorbing substances, nigrosine and various mixtures of nigrosine with AS and GA were measured, as well as a lightly absorbing substance, Suwannee river fulvic acid (SRFA). For the ambient measurements, the calibration curves generated from this method were to follow the optical evolution of biomass burning (BB) aerosols. A decrease in the overall aerosol absorption and scattering for aged aerosols during the day after the fires compared to the smoldering phase of the fires was found. rn The connection between light extinction of aerosols, their chemical composition and hygroscopicity for particles with different degrees of absorption was studied. The extinction cross-section (σext) at 532nm for different mobility diameters was measured at 80% and 90% relative humidity (RH), and at an RH<10%. The ratio of the humidified aerosols to the dry ones, fRHext(%RH,Dry), is presented. For purely scattering aerosols, fRHext(%RH,Dry) is inversely proportional with size; this dependence was suppressed for lightly absorbing ones. In addition, the validity of the mixing rules for water soluble absorbing aerosols is explored. The difference between the derived and calculated real parts of the complex RIs were less than 5.3% for all substances, wavelengths, and RHs. The obtained imaginary parts for the retrieved and calculated RIs were in good agreement with each other, and well within the measurement errors of retrieval from pulsed CRD spectroscopy measurements. Finally, a core-shell structure model is also used to explore the differences between the models, for substances with low growth factors, under these hydration conditions. It was found that at 80% RH and for size parameters less than 2.5, there is less than a 5 % difference between the extinction efficiencies calculated with both models. This difference is within measurement errors; hence, there is no significant difference between the models in this case. However, for greater size parameters the difference can be up to 10%. For 90% RH the differences below a size parameter of 2.5 were up to 7%.rn Finally, the fully characterized PAS together with a cavity ring down spectrometer (CRD), were used to study the optical properties of soot and secondary organic aerosol (SOA) during the SOOT-11 project in the AIDA chamber in Karlsruhe, Germany. The fresh fractal-like soot particles were allowed to coagulate for 28 hours before stepwise coating them with SOA. The single scattering albedo for fresh fractal-like soot was measured to be 0.2 (±0.03), and after allowing the soot to coagulate for 28 hours and coating it with SOA, it increased to 0.71(±0.01). An absorption enhancement of the coated soot of up to 1.71 (±0.03) times from the non-coated coagulated soot was directly measured with the PAS. Monodisperse measurements of SOA and soot coated with SOA were performed to derive the complex refractive index (m) of both aerosols. A complex refractive index of m = 1.471(±0.008) + i0.0(±0.002) for the SOA-αO3 was retrieved. For the compact coagulated soot a preliminary complex refractive index of m = 2.04(+0.21/-0.14) + i0.34(+0.18/-0.06) with 10nm(+4/-6) coating thickness was retrieved.rn These detail properties can be use by modelers to decrease uncertainties in assessing climatic impacts of the different species and to improve weather forecasting.rn

Conformationally Controlled Electron Delocalization in n-Type Rods: Synthesis, Structure, and Optical, Electrochemical, and Spectroelectrochemical Properties of Dicyanocyclophanes

A series of dicyanobiphenyl-cyclophanes 1-6 with various pi-backbone conformations and characteristic n-type semiconductor properties is presented. Their synthesis, optical, structural, electrochemical, spectroelectrochemical, and packing properties are investigated. The X-ray crystal structures of all n-type rods allow the systematic correlation of structural features with physical properties. In addition, the results are supported by quantum mechanical calculations based on density functional theory. A two-step reduction process is observed for all n-type rods, in which the first step is reversible. The potential gap between the reduction processes depends linearly on the cos(2) value of the torsion angle phi between the pi-systems. Similarly, optical absorption spectroscopy shows that the vertical excitation energy of the conjugation band correlates with the cos(2) value of the torsion angle phi. These correlations demonstrate that the fixed intramolecular torsion angle phi is the dominant factor determining the extent of electron delocalization in these model compounds, and that the angle phi measured in the solid-state structure is a good proxy for the molecular conformation in solution. Spectroelectrochemical investigations demonstrate that conformational rigidity is maintained even in the radical anion form. In particular, the absorption bands corresponding to the SOMO-LUMO+i transitions are shifted bathochromically, whereas the absorption bands corresponding to the HOMO-SOMO transition are shifted hypsochromically with increasing torsion angle phi.

Multi-software modeling technique for field distribution propagation through an optical vertical interconnect assembly

Embedded siloxane polymer waveguides have shown promising results for use in optical backplanes. They exhibit high temperature stability, low optical absorption, and require common processing techniques. A challenging aspect of this technology is out-of-plane coupling of the waveguides. A multi-software approach to modeling an optical vertical interconnect (via) is proposed. This approach utilizes the beam propagation method to generate varied modal field distribution structures which are then propagated through a via model using the angular spectrum propagation technique. Simulation results show average losses between 2.5 and 4.5 dB for different initial input conditions. Certain configurations show losses of less than 3 dB and it is shown that in an input/output pair of vias, average losses per via may be lower than the targeted 3 dB.

Optical and structural properties of InAlN/GaN Bragg reflectors examined by transmission electron microscopy and electron energy loss spectroscopy

Molecular beam epitaxy growth of ten-period lattice-matched InAlN/GaN distributed Bragg reflectors (DBRs) with peak reflectivity centered around 400nm is reported including optical and transmission electron microscopy (TEM) measurements [1]. Good periodicity heterostructures with crack-free surfaces were confirmed, but, also a significant residual optical absorption below the bandgap was measured. The TEM characterization ascribes the origin of this problem to polymorfism and planar defects in the GaN layers and to the existence of an In-rich layer at the InAlN/GaN interfaces. In this work, several TEM based techniques have been combined.

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy

Photopyroelectric (PPE) spectroscopy, in the 350-1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength lambda, V (n)(lambda) and its phase F (n)(lambda) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient beta of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.