Vol. 63 No. 2 Mar-Apr (2017): Revista Mexicana de Física

Published: 2017-01-01

Articles

  • Optimization for maximum Raman frequency conversion in supercontinuum sources using genetic algorithms

    F.R. Arteaga-Sierra, C. Milián, I. Torres-Gómez, M. Torres-Cisneros, H. Plascencia-Mora, G. Moltó, A. Ferrando
    111-0
    Abstract:
    A frequency convertor based on the soliton self-frequency shift by the supercontinuum generation is obtained by optimization of only three parameters of a Ti:Sapphire laser pulse, namely, carrier wavelength, peak power and time duration. The frequency conversion is performed and calculated by simulating the propagation of the pulse in a simple piece of $25$ cm long commercial photonic crystal fiber pumped by the femtosecond Ti:Sapphire laser, whose only pre-requisite is to exhibit the standard supercontinuum. The resulting spectral broadening has a maximum spectral conversion in the anomalous region just by playing with the three realistic controllable parameters. Optimization is performed using pre-defined functions of genetic algorithms. Our results indicate that the efficiencies of Raman conversion achieved by merely optimizing the pulse parameters in a commercial fiber are comparable with those obtained in more elaborated Raman convertor devices.
    ⬇️ Scroll down to see the full summary

  • Experimental multi-scroll attractor driven by switched systems

    I. Campos-Cantón, E. Campos-Cantón, S. González-Bautista, R.E. Balderas-Navarro
    117-0
    Abstract:
    This article deals with an electronic implementation of a 3-D dynamical system that comprises multiple scrolls and is regarded as unstable dissipative system. Such a system is dissipative in one of its components but unstable in the other two. The proposed electronic circuit is implemented with resistors, capacitors and comparators and has the capability to generate two or three scrolls
    ⬇️ Scroll down to see the full summary

  • Cinética inicial de formación de rejillas de Bragg en fibras ópticas estándar

    I. Santiago Nuñez, M.G. Shlyagin, G. Beltrán-Pérez
    124-0
    Abstract:
    Results on experimental study on kinetics of fiber Bragg gratings formation in the standard telecom optical fiber SMF-28 at initial stage of irradiation are presented. For low fluence per pulse, it was found that the fast formation of color centers Ge(1) is a dominant mechanism of the refractive index change at initial stage for accumulated fluence of below 0.02 J/cm$^2$ at the wavelength of 266 nm. At low fluence per pulse, the induced change of the refractive index does not depend on UV light intensity and saturates at a level of $ \sim 2 \times 10^{-6} $. Dominance of the Ge(1) color center mechanism in the FBG formation at the initial stage makes possible a cost-effective recording of large arrays of weak FBGs for sensors utilizing lasers of low energy per pulse.
    ⬇️ Scroll down to see the full summary

  • Precise analytic approximation for the modified Bessel function $I_1(x)$

    P. Martin, J. Olivares, A. Sotomayor
    130-0
    Abstract:
    Precise analytic approximations have been found for the modified Bessel function $I_1(x)$ of order one. The approximant is simple and with good accuracy for every positive value of the variable $x$. The method to obtain the approximant is an improvement of the multipoint quasi-rational approximation technique, MPQA. This is obtained combining rational functions with elementary auxiliary functions as a bridge function between the power series and asymptotic expansion.
    ⬇️ Scroll down to see the full summary

  • Effects of lubrication in mhd mixed convection stagnation point flow of a second grade fluid adjacent to a vertical plate

    Khalid Mahmood., Muhammad Sajid., Nasir Ali., Tariq Javed.
    134-0
    Abstract:
    The present manuscript describes effects of mixed convection on MHD flow of a second grade fluid above a vertical plate. The fluid impinges orthogonally on the plate which is lubricated by a slim coating of power-law fluid. A system of ordinary differential equations is obtained by employing the similarity transformations to the original partial differential equations. To handle the present flow situation, it is assumed that velocity and shear stress of the second grade fluid and the lubricant are continuous at the interface. A well reputed numerical technique called Keller-box method is utilized to solve coupled non-linear equations. Influence of slip, magnetic and mixed convection parameters, Weissenberg and Prandtl numbers on the velocity, skin friction coefficient, temperature and heat transfer rate at the surface is presented in the form of graphs and tabular data for both assisting and opposing flows. The results in the case of no-slip condition are compared with the available numerical data. A good agreement of these results certifies our effort.
    ⬇️ Scroll down to see the full summary

  • Dependence of exchange bias in NiFe/NiO bilayers on film thickness

    José R. Fermin
    145-0
    Abstract:
    Here we report on the effect of the ferromagnetic (FM) and antiferromagnetic (AF) films thicknesses on the exchange bias field in a FM/AF bilayer. For this, a series of NiFe($t_{\textrm{NiFe}}$)/NiO($t_{\textrm{NiO}}$) bilayers were grown by DC magnetron sputtering onto commercial Si(001) wafers. Magneto-optical hysteresis loops were used as probes to measure the exchange-bias field, and the coercivity field, as functions of the in-plane angle, $\varphi_H$, and the films' thicknesses, $t_{\textrm{NiFe}}$ and $t_{\textrm{NiO}}$. The in-plane symmetry of the exchange field and coercivity display unidirectional and uniaxial anisotropies, with angular dependences different from the simple $\cos \varphi_H$ and $cos^2 \varphi_H$, respectively. These symmetries are intrinsically sensitive to the thickness of both NiFe and NiO layers. With respect to the FM layer thickness, the exchange bias and coercivity field follow the usual 1/$t_{\textrm{NiFe}}$, while the dependence on the thickness of the AF layer is more complicated, and is characterized by a critical behavior.
    ⬇️ Scroll down to see the full summary

  • Positive even-odd effects in the maximal kinetic energy and negative even-odd effects in the minimal excitation energy of fragments from thermal neutron induced fission of ${}^{235}$U

    M. Montoya, V. Collin
    151-0
    Abstract:
    Based on the Coulomb effect hypothesis it is shown that positive even-odd effects of the maximal total kinetic energy ($K_{\max}$) and negative even-odd effects of the total minimal excitation energy ($X_{\max}$), as a function of charge ($Z$) and neutron number ($A$) of fragments, respectively, are not in contradiction. According to the Coulomb effect hypothesis, $K_{\max}$ is equal to the maximal Coulomb interaction energy ($C_{\max}$) reached by the most compact scission configuration. The fragmentation corresponding to $Z =41$ and $A=103$ is an exceptional case for which scission configuration is formed by complementary fragments in their corresponding ground states. However, more symmetrical o more asymmetrical fragmentations than that need to be out of their ground states, which implies that $K_{\max}=C_{\max}< Q $ % $.
    ⬇️ Scroll down to see the full summary

  • Growth of carbon fibres, sheets and tubes on diamond films under high power plasma etching conditions

    I. Villalpando, P. John, J.I.B. Wilson
    155-0
    Abstract:
    The application of diamond as a plasma facing material for fusion reactors can be limited by unknown reactions between diamond and the chamber materials transported by the plasma. Transformation of diamond to other structures can cause problems such as contamination of the plasma with loose particles or retention of gases. We have seen that diamond thin films are eroded under hydrogen plasma etching, but if silicon is present the growth of various carbon structures on diamond films is observed. We have produced carbon with different morphologies on diamond films including fibres, sheets with flower-like shapes and tubes and proposed growth mechanisms based on the results of Scanning Electron Microscopy, X-Ray Photoelectron Spectroscopy and Raman Spectroscopy. Sample surfaces contain silicon and are oxidised having COO and CO groups as seen by XPS analysis. Raman analyses revealed a spectrum typical for graphite combined with that from diamond that remains on the surface after hydrogen bombardment. The results of this study show the experimental conditions in which carbon fibres, sheets and tubes are produced under high-power hydrogen etching of diamond films and open the possibility to other applications such as catalysts, sensors and the production of electrodes.
    ⬇️ Scroll down to see the full summary

  • Ermakov--Lewis invariants for a class of parametric anharmonic oscillators

    A. Gallegos, H. Vargas-Rodríguez, J.E. Macías-Díaz
    162-0
    Abstract:
    In this letter, we investigate a general class of damped anharmonic oscillators with time-dependent coefficients. The model is a second-order ordinary differential equation in which the driving is a general function of the solution and time. Several well-known equations of mathematical physics are generalized by our model. The equation is presented then as a generalized Ray--Reid system, and an invariant of the Ermakov--Lewis type is derived next. Particular forms of this invariant are obtained for the classical harmonic oscillator and the Ermakov equation. In this form, this work opens the investigation on the determination of Ermakov--Lewis invariants of anharmonic systems.
    ⬇️ Scroll down to see the full summary

  • XPS, DRUV-VIS and ESR characterization of the non-stoichiometric compound Ge$_{0.74}$V$_{0.21}$ $\square_{0.05}$O$_{2}$

    J.L. Boldú, J. Barreto, I. Rosales, L. Bucio, E. Orozco
    166-0
    Abstract:
    Microcrystalline powders of the nonstoichiometric Ge$_{0.74}$V$_{0.21}$ $\square_{0.05}$O$_{2}$ compound were prepared by conventional high temperature solid-state reactions. The powders were characterized by X-ray diffraction (XRD); scanning electron microscopy (SEM); X-ray photoelectron spectroscopy (XPS); diffuse reflectance ultraviolet-visible spectroscopy (DRUV-VIS) and electron spin resonance (ESR) spectroscopy. From the analysis performed on compound, it was found that: The powders showed a rutile type crystalline structure with a rectangular prismatic crystalline habit. The XPS analysis, confirm the presence of V$^{4+}$ and V$^{5+}$ vanadium ions, the DRUV-VIS spectra show absorption bands in the 200-800 nm wave length interval and the ESR analysis confirms that the V$^{4+}$ ions are within microcrystals, hosted as VO$^{2+}$ at sites of rhombic (C$_{2v}$) symmetry.
    ⬇️ Scroll down to see the full summary

  • Border collision bifurcations in tantalus oscillator

    H. Arce, A. Torres, A. Falcón-Neri, O. Mimila, G.H. González
    171-0
    Abstract:
    The Tantalus oscillator is a nonlinear system having a stable limit cycle. In this work we analytically obtain the Phase Transition Curve (PTC) finding a one-dimensional piecewise map which has a discontinuity. The map is defined by a function which was experimentally verified with an excellent consistency between theoretical and experimental results. We iterate the obtained map to predict the coupling behavior of the system under periodic perturbations, finding that it presents Periodicity Diagrams that display a high number of bistabilities. We experimentally show the occurrence of the predicted behaviors. Bifurcations among periodicities resulted Border Collision Bifurcations. Finally, by studying the Two-parametric Bifurcations Diagram we conjecture that there is at least one point in the diagram which corresponds to a Big Bang Bifurcation. This point appears when the perturbation intensity leads to the discontinuity loss in the PTC.
    ⬇️ Scroll down to see the full summary

  • Transformation of a wavefunction under changes of reference frame

    G.F. Torres del Castillo, B.C. Nájera Salazar
    185-0
    Abstract:
    A simple procedure to derive the transformation of a wavefunction under a change of reference frame is applied to some examples and its relation with the transformation of the Hamilton principal function is studied.
    ⬇️ Scroll down to see the full summary

  • Cross sections for electron-impact excitation of neutral atoms

    J.L.S. Lino
    190-0
    Abstract:
    We present a study theoretical on the scaling of plane wave Born cross section for electron impact by atoms, more specifically, integral cross sections for He, Na, K, and Hg. The calculations were performed using a simple modification of the original scaling method proposed by Y.K. Kim (Phys. Rev. A 64 (2001) 032713) and comparison of our calculated results with experimental data shown qualitative agreement.
    ⬇️ Scroll down to see the full summary

  • Eigenestados exactos y estados entrelazados de un ión atrapado oscilando en dos dimensiones

    I. Ramos Prieto, H.M. Moya-Cessa
    194-0
    Abstract:
    We show that it is possible to find exact eigenstates in the interaction between a trapped ion oscillating in two dimensions and they will be entangled states. We also show that the Hamiltonian that describes this interaction may be mapped to a quantum Rabi interaction Hamiltonian (for a two-level atom and two quantized fields) by means of a unitary transformation.
    ⬇️ Scroll down to see the full summary

  • Spatial average symmetry associated to unconventional polarization

    R. Espinosa-Luna, G. López-Morales, V.M. Rico-Botero, E. Aguilar-Fernández
    205-0
    Abstract:
    The spatial average Stokes vector is proposed as a quantitative criterion to test the spatial average symmetry associated to the cross section of unconventional polarized light. Experimental and numerical results for azimuthal and radial polarization are presented using images and algebraic representations.
    ⬇️ Scroll down to see the full summary