Flavio Horowitz

First hyperpolarizability in proton-transfer benzoxazoles: computer-aided design, synthesis and study of a new model compound

Abstract With regard to second-order nonlinear optics (NLO) applications, a new class of 2-(2′-hydroxyphenyl)benzoxazoles (HBO) was designed for a combination of high first hyperpolarizability, β, with good photothermal stability, in association with a fast excited state intramolecular proton transfer (ESIPT) mechanism. Semi-empirical optimization of molecular structures and ab initio calculations of dipole moments were performed. Clear evidence was found that conditions such as conjugation efficiency and electron donor/acceptor strength cannot be evaluated separately, due to structural changes in molecular spatial distribution. Experimentally, a new fluorescent molecule of the HBO family, 2-(2′-hydroxy-4′-aminophenyl)-6-nitrobenzoxazole (BO6), was synthesized, purified and characterized, including solvent environments of distinct polarities. Hyper-Rayleigh scattering, UV–Vis absorption and emission spectroscopy, differential scanning calorimetry and thermogravimetric analysis of BO6 show a significant β (213.4±25.7×10−30 esu in acetone, at 1064 nm) and thermal stability up to 270 °C. Such results, in this first study of ESIPT dyes for second-order NLO to our best knowledge, indicate that the HBO family well deserves further attention towards promising application materials.

Computer simulation of thin film growth: applying the results to optical coatings

Computer simulation of thin film growth has been used extensively to gain insight into the origin and nature of the microstructure of vapor-deposited thin films. Usually, however, no attempts are made to predict film properties other than column angle and film density from such simulations. The aim of our work is to derive quantitative data from computer simulations in order to be able to predict relevant properties of optical coatings. The deposition of 2500 to 25,000 particles has been simulated on different computers by random deposition of two-dimensional hard disks, using a simple relaxation scheme. Statistical analysis of the results yields quantitative data for the density, column angle, and column period. On the basis of these results, a simple model has been developed for the microstructure of a three-dimensional film. The birefringence and the shape of water-penetration fronts in evaporated optical coatings, predicted from this model, are confirmed by experiment.

Silver migration at the surface of ion-exchange waveguides: a plasmonic template

The formation and evolution of metallic-silver nanoparticles capped with silver oxide, in the surface of Ag-doped waveguides produced by ion-exchange, were characterized. The samples were exposed to air atmosphere for periods lasting until 35 days and their aging process was investigated by optical and Atomic Force Microscopy (AFM) measurements. The results evidence migration of the Ag+ cations from inside the glass to the surface at room temperature, followed by aggregation of the silver nanoparticles (NPs) and oxidation, creating a nanometric-thick layer over the waveguide surface. This layer was employed for surface-enhanced Raman scattering (SERS) signal and for the fabrication of holographic diffraction gratings (HDG), which are presented as application examples of this material as a new plasmonic template.

Envelope and waveguide methods : a comparative study of PbF2 and CeO2 birefringent films

We have characterized low-birefringence, PbF(2) coatings to permit, first, agreement between envelope and prism-coupler waveguide methods under the standard isotropic assumption. In essentially the same measurement conditions, for obliquely deposited (58.3°) CeO(2) coatings the isotropic model becomes unsustainable. Explicit consideration of the film microstructure is then required for good correlation between thickness results from TE (503 ± 9 nm) and TM (504 ± 10 nm) modes in the waveguide experiment as well as between refractive-index results from envelope (n(2) = 1.78 ± 0.03) and waveguide (n(2) = 1.794 ± 0.002) techniques. We considered uniaxial and biaxial models to achieve consistency, and the refractive indices along the principal axes of symmetry were determined.

Visible and near infrared, wide-angle, anti-reflection coatings with self-cleaning on glass

In this work self-cleaning and transparent surfaces were produced on glass surface with simultaneous wide-angle and good optical transmittance on the visible region. These properties are pursued by combination of multi-scale surface topology based on silica nanoparticles (SNPs), index grading and interference coating, as well as polytetrafluoroethylene (PTFE) self-assembly, using two approaches. In the first, two-layer approach (glass/SNPs/PTFE), the resulting samples presented a water contact angle (WCA) of 169° ± 2° with very low hysteresis, as well as significant antireflection. The second, three-layer approach (glass/SNPs/silica aerogel/PTFE), produced surfaces with WCA of 158° ± 2° with also very low hysteresis (<5°), in addition to normal transmittance of 99% or higher, which decreased less than 2% at 20° incidence. These results show that proper structure-coated glass, with a combination of interference and graded-index effects, may provide simultaneous wide-angle antireflection and self-cleaning properties.

Micro and Nano-Texturization of Intermetallic Oxide Alloys by a Single Anodization Step: Preparation of Artificial Self-Cleaning Surfaces

Micro- and nanostructures of Ti-γCu (γ = 0, 30, 50, 70, and 100 wt %) intermetallic alloys were produced through a single anodization step. It was found that the original alloy composition influences the final oxide morphology obtained after anodization which presented formation of a microstructure with nanotubes, nanoparticles or nanopillars on the surface. Pure Ti and Cu oxide metals and their alloys presented hydrophilic or superhydrophilic properties immediately after anodization. When the anodized pure metal and/or Ti-γCu surfaces were functionalized with trimethoxypropylsilane (TPMSi), by dipping and coating with a thin perfluorinated layer, the treated substrates became in all cases superhydrophobic (water contact angles in the range of 152-166°), showing excellent self-cleaning properties with hysteresis below 3°. These results can be explained by a combination of nanomicro morphologies with low surface energy compounds in the topmost monolayers. The decrease in hysteresis was associated with a higher M-OH bond concentration on the anodized surfaces, which allowed for more complete TMPSi coating coverage. This study also indicates that easy and effective fabrication of superhydrophobic surfaces in pure metals and alloys is possible without involving traditional multistep processes.

Optical monitoring of the sol-to-gel transition in spinning silica films

Temporal evolution during spin coating is interferometrically monitored, with close regard to gelation at the final stages of the process. Uniform silica films on silicon were produced from an established sol composition, from which slight deviations were taken to produce nonuniform, cracked films. A distinct evolution of the sol to gel transition could then be detected, at speed of rotations of 1000 - 3000 rpm and data acquisition at 100 Hz. Interpretation of a set of experimental results is discussed in terms of a two-phase system, in the light of, and as a test of, current theoretical models.

Simple modeling of plasmon resonances in Ag/SiO2 nanocomposite monolayers.

Normal incidence transmittance and reflectance spectra of sputtered nanocomposite monolayer films of Ag in SiO2, buried and unburied, showed significant redshifted plasmon resonances from 410 to 455 nm, which could be well interpreted with a simple model that starts from the Maxwell Garnett theory and the Kreibig extension of the Drude-Lorentz equation, but with a further extension related to the dipolar interaction between the metal particles distributed on a surface.

Refractive index control in bicomponent polymer films for integrated thermo-optical applications

Optical properties of transparent polymer thin films, produced by spin-coating on silicon and constituted of polycarbonate (PC), poly(methyl methacrylate) (PMMA), and PC/PMMA, were investigated with regard to integrated thermo-optical (TO) device applications. Refractive index dependences on wavelength, temperature, and film composition were measured by spectroscopic ellipsometry with a dedicated autocontrolled heater setup, in the ranges of 400 to 800 nm, 25 to 85 °C and 0 to 100 wt % PC, respectively, with determination of Cauchy and Lorentz-Lorenz parameters. Within these intervals, thermomechanical compatibility and pronounced index contrast of around 0.12 between PC and PMMA, as well as their TO coefficients one order of magnitude higher than that of silica, allow convenient tailoring for specific TO requirements. In addition, wide-range fine-tuning of refractive index variation is found to be facilitated by the weak dependence of isothermal dispersion curves and TO coefficients on film composition.

Interferometric monitoring of dip coating

Dip-coated films, which are widely used in the coating industry, are usually measured by capacitive methods with micrometric precision. For the first time to our knowledge, we have applied an interferometric determination of the evolution of thickness in real time to nonvolatile Newtonian mineral oils with several viscosities and distinct dip withdrawing speeds. The evolution of film thickness during the process depends on time as t(-1/2), in accordance with a simple model. Comparison with measured results with an uncertainty of +/- 0.007 microm) showed good agreement after the initial steps of the process had been completed.