F. Chávez

Optical fiber sensor based on localized surface plasmon resonance using silver nanoparticles photodeposited on the optical fiber end.

This paper reports the implementation of an optical fiber sensor to measure the refractive index in aqueous media based on localized surface plasmon resonance (LSPR). We have used a novel technique known as photodeposition to immobilize silver nanoparticles on the optical fiber end. This technique has a simple instrumentation, involves laser light via an optical fiber and silver nanoparticles suspended in an aqueous medium. The optical sensor was assembled using a tungsten lamp as white light, a spectrometer, and an optical fiber with silver nanoparticles. The response of this sensor is such that the LSPR peak wavelength is linearly shifted to longer wavelengths as the refractive index is increased, showing a sensitivity of 67.6 nm/RIU. Experimental results are presented.

Selective photodeposition of zinc nanoparticles on the core of a single-mode optical fiber

An experimental and theoretical study about selective photodeposition of metallic zinc nanoparticles onto an optical fiber end is presented. It is well known that metallic nanoparticles possess a high absorption coefficient and therefore trapping and manipulation is more challenging than dielectric particles. Here, we demonstrate a novel trapping mechanism that involves laser-induced convection flow (due to heat transfer from the zinc particles) that partially compensates both absorption and scattering forces in the vicinity of the fiber end. The gradient force is too small and plays no role on the deposition process. The interplay of these forces produces selective deposition of particles whose size is directly controlled by the laser power. In addition, a novel trapping mechanism termed convective-optical trapping is demonstrated.

Pd-decorated ZnO and WO 3 nanowires for sensing applications

In this work the recent advances on Pd nanoparticles deposition by electroless deposition technique (EDT) onto ZnO and WO3 nanowires applied to gas sensing applications is reported. The reported nano-structured materials have been synthesized using combinations of conventional CSVT, thermal oxidation and electroless techniques. The CSVT and thermal oxidation techniques were used to obtain ZnO and WO3 metal-oxide nanowires with excellent gas sensing properties. The sensibility was improved by the deposition of Pd nano-particles using a variant of the nobility of EDT. The metallic nature of Pd particle was corroborated by XRD and Raman Scattering (RS). Dynamical-responses to single hydrogen (H2), water vapor (H2O) and Carbon dioxide (CO2) gaseous species using a 4500 PPMv pulse were satisfactory tested by the measure of their electrical resistance change.

CSVT as a Technique to Obtain Nanostructured Materials: WO3-x

The growth of tungsten oxide nanowires on silicon substrates without using any catalyst is demonstrated by means of close-spaced vapor transport (CSVT) technique at atmospheric pressure. The source was formerly prepared from a tungsten foil to produce a tungsten oxide film. CSVT array is completed with silicon substrates located at a distance of ~350 m over the tungsten oxide source at moderate temperatures (~750°C). Two distinct kinds of nanostructures were produced; a uniform distribution of free standing tungsten oxide wires of several micrometers in length with diameters less than 150 nm; and wires assembled to form nanowire bundle. The X-ray diffraction characterizations show that the phases of WO2.7 and WO2.9 are present.

Photomelting and photofragmentation of silver nanoparticles suspended in ethanol

An optical method to obtain a colloidal solution starting from a mixture of silver nanopowder and ethanol is presented. The particles of the silver nanopowder do not exhibit a specific shape, however in the colloidal solution are spherical. This method is carry out when the mixture is irradiated with a pulsed laser at 532 nm via optical fiber. Due to a stronger absorption of the laser light by silver nanoparticles arise both photofragmentation and photomelting processes. The photomelting process starts when the laser energy is 5 mJ/cm2, inducing an enlargement of nanoparticles whereas the photofragmentation occurs when the laser energy is 25 mJ/cm2 causing a reduction on their sizes (the higher energy is, the smaller nanoparticles are). Results show that it is possible to obtain a colloidal silver solution and to control the particle size by adjusting the laser energy. Experiments were performed at 5 and 25 mJ/cm2, and the results are presented.

Saturable and two-photon absorption in zinc nanoparticles photodeposited onto the core of an optical fiber

In this work, the simultaneous presence of saturable (SA) and two-photon absorption (TPA) in zinc nanoparticles (ZnNPs) photodeposited onto the core of an optical fiber was studied in the nanosecond regime with the P-scan method using a high gain pulsed erbium-doped fiber amplifier. An analysis based on Mie theory was carried out to demonstrate the influence of the absorption coefficient with the particles sizes in the proximity of surface plasmon resonance (SPR). The shift from TPA to SA has been observed as the irradiance is increased. It was found that for irradiances lower than 5 MW/cm², TPA is dominant, whereas for irradiances higher than 5 MW/cm², the SA becomes dominant. Furthermore, the values of the nonlinear absorption coefficient and the imaginary part of third-order nonlinear optical susceptibility were calculated numerically from the transmittance measured. Such TPA makes ZnNPs a candidate for optical limiting applications, and SA makes them a candidate for applications in pulsed fiber laser systems.

ZnO nanowires synthesized by CSS and their application as a hydrogen gas sensor

Zinc oxide nanowires (ZnO-NWs) were synthesized through two simple processes on silicon substrates. The first step was to obtain the zinc nanowires (Zn-NWs) by close spaced sublimation (CSS) on a quartz substrate. The second step was the transformation of Zn-NWs to ZnO-NWs by a simple thermal annealing in air environment. In the synthesis process a zinc pellet source was used as the source material, where the temperatures of the source, the temperature of the substrate, and the growth time were fixed at 350°C, 325 °C, and 5 minutes respectively, using a nitrogen environment at atmospheric pressure conditions. Afterwards, the as-prepared Zn-NWs sample was heated at 400 °C for 30 minutes in open tube conditions. In addition, a conductimetric gas sensor was fabricated using the annealed Zn-NWs based film. The sensor was tested to hydrogen at moderate temperatures (200-400 °C) for several concentrations (95-1492 ppm). The results revealed that the ZnO-NWs have a high response to hydrogen at high temperatures and high concentrations. The response and recovery times depend on the hydrogen concentration. The response time for almost all conditions is of the order of 2 minutes. These results are very promising for the development of hydrogen solid state gas sensors based on zinc oxide nanowires.

Synthesis and characterization of In 2 O 3 micro- and nano-structures at low temperatures by the CSVT technique

Indium oxide microstructures were synthesized by close-spaced chemical vapor transport (CSVT) technique at low temperatures and without the use of catalysts. Morphological characterization by scanning electron microscopy (SEM) showed that the CSVT technique provides high mass-transport efficiency at temperatures as low as 650°C, considering that indium oxide powders were used as a source for the synthesis. It is observed that the microstructures are formed mainly of cubes with semi-spheroids attached to their faces. Additional measurements of X-ray (XRD), Raman, and Energy-dispersive X-ray spectroscopy (EDS) showed that the cubes tend to have the stoichiometry of cubic indium oxide while the semi-spheroids have a metallic nature. The microstructures were converted to indium oxide nanobelts by a simple post thermal annealing during 16 hours in a nitrogen environment, according to the morphological and structural characterizations. The results revealed that the indium oxide nanobelts have several micrometers in length with rectangular cross sections in the range from 50 to 250 nanometers and correspond to indium oxide in the cubic and face-centered phases. The growth mechanisms of the micostructures and nanobelts are discussed in detail.

Radiation forces on Au nanoparticles considering infrared beams

The radiation pressure forces for Au nanoparticles in the Rayleigh regime under the influence of a coherent source of infrared light from 0.7-1.5 μm of a Gaussian beam with fundamental mode corresponding to the TEM00 mode are studied. An intensity distribution of the source in terms of the spot size and power are considered to analyze the gradient, scattering and absorption forces on a sphere located arbitrarily on a Gaussian beam. The results have shown, through an analysis stability, the optical manipulation is better suited for longer wavelengths, small particles, and a beam waist significantly reduced.

Tungsten nanostructured thin films obtained via HFCVD

By using the Hot Filament Chemical Vapor Deposition (HFCVD) technique tungsten thin films were deposited on amorphous quartz substrates. To achieve this, a tungsten filament was heated at 1300 °C during 30 minutes maintaining a constant pressure inside the chamber at 460 mTorr and substrate at 700 °C. Transition from tungsten oxide deposits to tungsten thin films, by varying the substrate temperature, were characterized by means of Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), X-Ray Diffraction and, micro-Raman spectroscopy. The SEM micrographs reveal that the tungsten films have no more than 200 nm in thickness while XRD show evidence of the films crystallize in the á-tungsten modification. On the other hand, AFM shows that the tungsten thin films exhibit a uniform and smooth surface composed with semi-spherical shapes whose diameters are below than 50 nm. Furthermore, to the naked eye, the as-deposited tungsten films exhibit a high mirror-like appearance.