Alfredo Franco

Spin-1/2 particle on a cylinder with radial magnetic field

We study the motion of a quantum charged particle, constrained on the surface of a cylinder, in the presence of a radial magnetic field. When the spin of the particle is neglected, the system essentially reduces to an infinite family of simple harmonic oscillators, equally spaced along the axis of the cylinder. Interestingly enough, it can be used as a quantum Fourier transformer, with convenient visual output. When the spin-1/2 of the particle is taken into account, a non-conventional perturbative analysis results in a recursive closed form for the corrections to the energy and the wavefunction, for all eigenstates, to all orders in the magnetic moment of the particle. A simple two-state system is also presented, the time evolution of which involves an approximate precession of the spin perpendicularly to the magnetic field. A number of plots highlight the findings while several three-dimensional animations have been made available on the web.

Hybrid Organic-Inorganic Photodriven Nanoimpellers for Drug Release

Mesostructured SiO2 films functionalized with the azo-chromophore Disperse Red 1 were synthesized by sol-gel method. The mesostructured long-range order in the films was determined by X-Ray Diffraction. The azo-chromophores in the films work as nano-impellers through their photo-induced trans-cis reversible isomerization. When the films are doped, they are able to control the release of the dopant by all-optical processes. We used the dye laser Rhodamine 6G as dopant, its very distinctive luminescence around 550 nm allows to follow the release. Polarized green and infrared laser light were used as pump sources to direct the movement of the nano-impellers. 299 nm light was used as a probe to induce the Rhodamine 6G luminescence, which was measured as function of the pumping time with a photomultiplier coupled to a monochromator. The results corresponding to the green and to the infrared pumping sources are compared in order to determine the feasibility to photo-control the nano-impellers movement through a two-photon absorption process.

Photoconductivity on nanostructured sol-gel thin films with silver nanoparticles

2d-hexagonal nanostructured sol-gel thin films were prepared by dip-coating method. A neutral surfactant Brij58 was used as template to produced channels into the film. The structure was identified by X-ray diffraction and TEM. Silver nanoparticles were obtained by spontaneous reduction process of Ag+ ions to Ag0 at room temperature. A broad band located at 430 nm was detected by optical absorption; it corresponds to the surface plasmon. Photoconductivity studies were performed on films with ions and with silver nanoparticles to characterize their mechanisms of charge transport in the darkness and under illumination at 420 and 633 nm wavelengths. Straight lines showing an ohmic behavior fit the experimental data. Films without colloids possess normal photoconductivity behavior. But films with colloids present an abnormal response. Transport parameters were calculated. The films with silver nanoparticles exhibit a photovoltaic effect stronger than the films without nanoparticles, except to high concentrations. A theoretical model is proposed to predict these processes as function of the silver concentration.

Corona poling assisted second harmonic genertion in nanostructured polymethylmetacrylate films

Films made of Polymethylmetacrylate (PMMA) and doped with dipolar second order nonlinear optical chromophores were studied by UV-visible spectroscopy and transmitted resonant Second Harmonic Generation technique. The chromophores were non-centrosymmetrically oriented by a Corona poling field. The UV-visible spectra were measured in poled films at normal incidence as function of their poling temperatures (60, 80 and 100°C) and they were analyzed in terms of the Second Order Parameter (A2). The Second Harmonic Generation (SHG) signals in the films were measured in-situ as function of the poling time at several incidence angles for each poling temperature. The stability of the SHG signal was also determined, by turning off the Corona field but leaving the films at their original poling temperature. The films were of two different kinds: amorphous and nanostructured. All the films doped simultaneously with chromophores and surfactants showed long-range ordered nanostructures. Two kinds of surfactants were used during the synthesis of the films: ionic and neutral, both of them induced long-range order in the structure of the PMMA, but only some of the nanostructured films exhibited enough large SHG signals. The second order nonlinear optical response of the nanostructured films was compared with the corresponding response of the amorphous films. The long-range order in the films was detected by X-Ray Diffraction (XRD).

Mobility of DR1 Molecules Embedded in Nanostructured PMMA Films

The kinetics of the orientation of Disperse Red 1 (DR1) molecules embedded in nanostructured Polymethylmetacrylate (PMMA) films was studied under the effect of an intense constant electric poling field. The changes in the orientation distribution of the DR1 molecules were followed by Second Harmonic Generation (SHG) measurements. The SHG signal was recorded as function of time at three different temperatures. We focused on both, the signal increases under the presence of the poling field and the signal decays without the poling field. The studied PMMA films were nanostructured by the incorporation of ionic surfactants as the Sodium Dodecyl Sulfate (SDS) and the Cetyl Trimethyl Ammonium Bromide (CTAB) during their preparation. The kinds of nanostructures obtained in the films were determined by means of X-ray diffraction (XRD) measurements. Substantial differences in signal intensity and in growth and decay rates between amorphous and nanostructured films were found.

Optical absorption and SHG in PMMA:DR1 thin films as function of poling time

Amorphous, PMMA: DR1: surfactant and PMMA:DR1:TEOS thin films were prepared by dip-coating. All films were calcined at 70 °C for 3 hours. For nanostructured thin films two ionic surfactants were used, Sodium Dodecyl Sulfate (SDS) and Cetyltrimethyl ammonium bromide (CTAB) to obtain two different nanostructures of the PMMA matrix: lamellar, and hexagonal, respectively. X-ray diffraction studies were performed to determine the long-order structure tailored in the films. The measurements of the optical absorption and the second harmonic generation (SHG) intensity were carried out at different orientation arrangements of the chromophores embedded in the films. The chromophore orientation distributions were obtained by means of the corona technique. These distributions depend on the corona poling time. We physically modeled the optical absorption and the second harmonic generation experimental results as function of the corona poling time, employing only one fitting parameter related to the matrix-chromophore interactions. The physical model and the experimental results were in an excellent agreement. The experimental results fitted by the model are shown in plots of order parameter against corona poling time and SHG intensity against corona poling time. The amorphous films provide a larger SHG intensity values than those obtained from the nanostructured films. Thin films with lamellar structure have a SHG intensity bigger than those from hexagonal and PMMA:TEOS thin films.

Optical absorption and second harmonic generation in SiO2:DR1 sol-gel films as function of poling time

Amorphous and nanostructured SiO2:DR1 sol-gel films were prepared by dip-coating. X-ray diffraction studies were performed to determine the long-order structure obtained in the films. The optical absorption (AO) measurements were done in three different nanostructures of the SiO2 network: lamellar, hexagonal and mixed. The AO measurements and the second harmonic generation (SHG) intensity were carried out at different orientation steps of the chromophores embedded in the films. These chromophore orientation distributions were obtained by means of the corona technique, and they depend on the corona poling time. We physically model the optical absorption and the second harmonic generation experimental results as function of the corona poling time, employing only one fitting parameter related to the matrix-chromophore interactions. The physical model and the experimental results were in an excellent agreement. The experimental results fitted by the model are shown in plots of order parameter against corona poling time and SHG intensity against corona poling time. The lamellar structure provides a larger order parameter values than those obtained for the other structures. A minimum value for the order parameter was detected by means of the optical absorption measurements at short poling times. For the SHG measurements, four different chromophore concentrations were used. As the concentration increases the measured SHG intensity increases too, but the increment is limited by the electrostatic interactions among the chromophores, which is also considered in our model.

A concetration-dependent model for silver colloids in nanostructured sol-gel materials

We report on the physical modelling of the photoconductive response of nanostructured sol-gel films in function of the silver nitrate concentration (ions and colloids). This model considers several factors as the silver nitrate concentration and the transport parameters obtained. The model is compared with others commonly used. 2d-hexagonal nanostructured sol-gel thin films were prepared by dip-coating method using a non-ionic diblock copolymer Brij58 (surfactant) to produce channels into the film. Silver colloids (metallic Ag0 nanoparticles ) were obtained by spontaneous reduction process of Ag+ ions to Ag0. These nanoparticles were deposited into the channels formed by the surfactant. The structure was identified by X-ray diffraction and TEM. An absorption band located at 430 nm was detected by optical absorption; it corresponds to the plasmon surface. Fit to this band with modified Gans theory is presented. Photoconductivity studies were performed on films with silver ions and films with silver colloids to characterized their mechanisms of charge transport in the darkness and under illumination at 420, 633 nm wavelengths. Transport parameters were calculated. The films with silver colloids exhibit a photovoltaic effect stronger than the films with silver ions. While, the last ones possesses a photoconductivity behaviour.