Scielo RSS <![CDATA[Revista mexicana de física]]> http://www.scielo.org.mx/rss.php?pid=0035-001X20140004&lang=en vol. 60 num. 4 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Modulador acusto-óptico en amplitud de fibra óptica basado en ondas acústicas de</b> <b>flexión</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400001&lng=en&nrm=iso&tlng=en Se reporta un estudio experimental de la operación de un modulador acusto-óptico (MAO) de fibra (Óptica basado en la interacción acusto-Óptica producida por ondas acústicas de flexión estacionarias. Concentramos nuestra atención en los efectos de reducir el diámetro en la fibra óptica como mecanismo para optimizar la respuesta espectral del MAO. Como caso particular reportamos un modulador de 70 μm de diámetro con una alta profundidad de modulación (60 %), bajas perdidas por inserción (1.3 dB) y un ancho ancho de banda a 3 dB de 40 nm que opera en el rango de los megahertz. Los resultados obtenidos demuestran que el incluir fibras estrechadas se puede considerar como un grado extra de libertad en el diseño del modulador para controlar el ancho de banda.<hr/>An experimental study of an in-fiber acousto-optic modulator (AOM) based on the acousto-optic interaction produced by standing flexural acoustic waves is reported. We focus our attention in the effects of a gradual reduction in the optical fiber as the mechanisms to improve the spectral response of the AOM. As a particular case we report a 70-μm fiber AOM. Our approach permits the implementation of high modulation depth (60 %), low optical loss (1.3 dB) and a 3 dB broad modulation bandwidth of 40 nm, operating in the MHz frequency range. The experimental results demonstrate that including tapered optical fibers can be regarded as an extra degree of freedom to control the optical bandwidth of the modulator. <![CDATA[<b>A study of the electronic properties of ErB<sub>2</sub> compound by using the PBE approximation and PBE0 hybrid functional</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400002&lng=en&nrm=iso&tlng=en We report calculations from first principles to determine the structural and electronic properties of ErB2 compound using the Density Functional Theory (DFT) and the Full-Potential Linearized Augmented Plane Waves (FP-LAPW) method. For the description of the electron-electron interaction, the Generalized Gradient Approximation Perdew-Burke-Ernzerhof (GGA-PBE) and PBE0 Hybrid Functional were employed. From the density of states (DOS), it is found that the addition of a fraction of the exact Hartree-Fock exchange energy, in the PBE approximation, evidence the localization of the 4f-Er orbitals, which favors electronic spin polarization of these orbitals in the ErB2 compound. The PBE0 scheme is justified because it best describes the electronic and magnetic properties of strongly correlated systems than the PBE approximation.<hr/>Reportamos cálculos de primeros principios para determinar las propiedades estructurales y electrónicas del compuesto ErB2, usando la teoría del funcional densidad (DFT: Density Functional Theory) y el método ondas planas aumentadas y linealizadas en la versión del potencial completo (FP-LAPW: Full-Potential Linearized Augmented Plane Waves). Para la descripción de la interacción electrón-electrón, se emplea la aproximación del gradiente generalizado de Perdew, Burke y Ernzerhof (GGA-PBE: Generalized Gradient Approximation-Perdew-Burke-Ernzerhof) y el funcional híbrido PBE0. A partir de la densidad de estados (DOS: Density of States), se encuentra que la adición de una fracción de la energía de intercambio exacto de Hartree-Fock, en la aproximación PBE, evidencia la localización de los orbitales 4f del Er, lo cual favorece la polarización de los espines electrónicos de estos orbitales en el compuesto ErB2. El esquema PBE0 se justifica porque describe mejor las propiedades electrónicas y magnéticas de los sistemas fuertemente correlacionados que la aproximación PBE. <![CDATA[<b>Fabricación y caracterización de un amplificador mecánico flexible</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400003&lng=en&nrm=iso&tlng=en Las características de desempeño de un mecanismo para amplificar desplazamientos mecánicos, como la ganancia geométrica en régimen forzado, la ganancia geométrica en resonancia y la frecuencia de resonancia, son obtenidas mediante interferometría óptica y comparados con los valores obtenidos mediante elemento finito en ANSyS. Se describe el principio de operación del mecanismo y mediante simulaciones numéricas en ANSyS, se obtienen los rangos de operación del mismo. La similitud entre los resultados experimentales y las simulaciones numéricas muestran que la metodología seguida es adecuada.<hr/>The performance characteristics of a compliant mechanical displacement amplification mechanism, as well as the geometrical gain in forced regime, the geometrical gain in resonance and the resonance frequency are obtained by using optical interferometry and compared with values obtained using the finite element analysis software ANSYS. The operation principle and the limits of the compliant mechanism are described by numerical simulations in ANSYS. The similarity between the experimental results and numerical simulations show that the used methodology is adequate to describe its design and its mechanical performance. <![CDATA[<b>Quasiclassical approach to tunnel ionization in the non relativistic and relativistic regimes</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400004&lng=en&nrm=iso&tlng=en Non relativistic and relativistic transition rates were observed for the deep tunnel regime for the case of a linear polarized laser field. The contribution of the initial momentum of an ejected photoelectron, the ponderomotive potential and the linear Stark shift for the Ar atom and its ions were taken into account. It was shown that, in both regimes, these processes have an influence on the transition rate behavior. The curves obtained for the transition rate show that with increasing laser filed intensities and ion charge the influence of relativistic effects increases. <![CDATA[<b>Influence of the precursors in the morphology, structure, vibrational order and optical gap of nanostructured ZnO</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400005&lng=en&nrm=iso&tlng=en The synthesis of ZnO by reaction in solid state from two precursor salts (zinc acetate and zinc sulfate), presented significant differences concerning morphology, structure, vibrational order and optical gap. As well as covering in the size of the compounds, a homogeneous distribution of nanoparticles of 21±3 nm and microstars of 1.03±0.19 μm respectively. The ZnO showed a structural phase with a vibrational state of the hexagonal type (wurtzite). The variation in the morphology due to the precursor is attributed to the disorder within of lattice, which contributes to vibrational changes and is correlated to the degrees of freedom of molecules. Measurements of UV-Vis of nanoparticles displayed a band gap (Eg) lower than the one reported for the bulk material. <![CDATA[<b>Mapping of solutions of the Hamilton-Jacobi equation by an arbitrary canonical transformation</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400006&lng=en&nrm=iso&tlng=en It is shown that given an arbitrary canonical transformation and an arbitrary Hamiltonian, there is a naturally defined mapping that sends any solution of the Hamilton-Jacobi (HJ) equation into a solution of the HJ equation corresponding to the new Hamiltonian.<hr/>Se muestra que dadas una transformación canónica arbitraria y una hamiltoniana arbitraria, existe una aplicación definida en forma natural que envía cualquier solución de la ecuación de Hamilton-Jacobi (HJ) en una solución de la ecuación de HJ correspondiente a la nueva hamiltoniana. <![CDATA[<b>Indoor air quality analysis based on the ventilation effectiveness for CO<sub>2</sub> contaminant removal in ventilated cavities</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400007&lng=en&nrm=iso&tlng=en A theoretical study to determine an optimal configuration for removal a CO2 contaminant from inside a room is presented. The geometry considered was a 2D ventilated cavity in turbulent flow regime and solving the governing equations of mass, momentum and chemical species by the finite volume method. The interval of Reynolds number under study was 0 ≤ Re ≤ 2.5 X 10(4). The air inlet gap is located on the lower side of vertical right wall of the cavity. Three configurations of the cavity varying the air outlet gap on vertical left wall were analysed: in the upper side (case A), in the middle side (case B), and in the lower side (case C). From the results, case A removed the most amount of contaminant from inside room for a Re = 5 x 10³, which had an effect on energy savings. The lower levels of contaminant for higher Reynolds numbers were obtained for the case B. The case C was the less desirable for contaminant removal purposes. <![CDATA[<b>Interfacial properties of inhomogeneous fluids for a smooth profile</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400008&lng=en&nrm=iso&tlng=en We analyze single- and multi-component simple fluids in a liquid-vapor coexistence state, which forms an arbitrarily curved interface. Using an analytical approach, based upon density functional theory, we perform a power series expansion of the density profile on the principal curvatures to calculate the grand potential that represents the free energy of the system. In addition, we derive microscopic expressions for the interfacial properties. All results are in good agreement with other viewpoints at the level of surface tension and spontaneous curvature. <![CDATA[<b>Structure of perfluoroalkane fluids at the liquid-vapor interface</b>: <b>a molecular dynamics study</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400009&lng=en&nrm=iso&tlng=en Structure of perfluoroalkane fluids at the liquid-vapor interface was studied by molecular dynamics simulations at different temperatures. It was observed that fluids formed a freezing layer at the liquid-vapor interface with a strong structure which it was more pronounced at low temperatures, close to the melting point. Whereas molecules did not have any preference orientation in the bulk fluid they developed a monolayer of molecules aligned perpendicular at the interface. Similar results have been observed for alkanes fluids in the same interface, however, the structure of those molecules is weaker than that reported for perfluoroalkanes. As the temperature increased the layer melted until it vanished. Finally, thermodynamic studies were also conducted in terms of surface tensions and good agreement with experimental data was found.<hr/>Se estudia la estructura de los fluidos perfluoroalcanos en la interfase líquido-vapor mediante simulaciones por computadora. Se observó que los fluidos forman una monocapa congelada en la interfase a temperature bajas, cercanas al punto de congelamiento. Las moléculas que se encuentran en la interface se orientan perpendicularmente a la superficie mientras que las moléculas en el centro no lo hacen. Este fenómeno se ha observado en fluidos tipo alcanos, sin embargo, el orden que tienen esas moléculas es menor a la que presentan los perfluoroalcanos. Cuando la temperatura se aumenta la estructura de la capa desaparece. Finalmente, se realizan cálculos de propiedades termodinámicas, como la tensión superficial obteniéndose un buen acuerdo con datos experimentales. <![CDATA[<b>Emissivity measurement of high-emissivity black paint at CENAM</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000400010&lng=en&nrm=iso&tlng=en To measure the temperature of the surface of an opaque object by radiation thermometry, it is necessary to know its surface emissivity. High-emissivity black paint can be applied to the inner walls of a cavity to be used as a blackbody radiator. It can also be applied to some highly reflecting metals so that their temperature can be estimated by radiation thermometry. In this work, it is described the emissivity measurement of high-emissivity black paint that has been used for the two applications described above. The average emissivity of the measured paint in the 8 to 14 micrometers wavelength range was estimated as 0.972 ± 0.012. The results obtained may be of use for those using the paint at temperatures from 50 to 150°C.