Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 63 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[Optimization for maximum Raman frequency conversion in supercontinuum sources using genetic algorithms]]> 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. <![CDATA[Experimental multi-scroll attractor driven by switched systems]]> 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.<hr/>Resumen Este artículo trata de la instrumentación de un sistema dinámico 3-D que puede presentar múltiples enroscados, cual se ha denominado sistema disipativo inestable. Este sistema es disipativo en uno de sus componenets pero inestable en los otros dos. El circuito eléctrico propuesto esta constituido por resistencias, capacitores y comparadores. Este circuito es capaz de generar dos y tres enroscados. <![CDATA[Cinética inicial de formación de rejillas de Bragg en fibras ópticas estándar]]> Resumen Se presentan resultados de un estudio experimental sobre la cinética inicial de formación de rejillas de Bragg en una fibra óptica estándar de telecomunicaciones SMF-28. Se encontró que para un régimen de irradiación de bajo flujo luminoso por pulso, a una longitud de onda de 266 nm y con una dosis de irradiación acumulada inferior a 0.02 J/cm2, la formación de centros de color tipo Ge(1) es el mecanismo principal de cambio fotoinducido del índice de refracción. Dicho valor de cambio fotoinducido no depende de la intensidad de la luz y se satura rápidamente a ∼ 2 × 10 - 6. La dominancia de este mecanismo de fotosensibilidad en la etapa inicial de irradiación, hace que sea posible grabar, de forma económica, arreglos (redes) extensos de rejillas de Bragg de baja reflexión, utilizando láseres de baja energía por pulso.<hr/>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/cm2 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 ∼ 2 × 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. <![CDATA[Precise analytic approximation for the modified Bessel function <em>I</em> <sub>1</sub> (<em>x</em>)]]> 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. <![CDATA[Effects of lubrication in mhd mixed convection stagnation point flow of a second grade fluid adjacent to a vertical plate]]> 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. <![CDATA[Dependence of exchange bias in NiFe/NiO bilayers on film thickness]]> 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 NiFe)/NiO(t 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, φ H, and the films’ thicknesses, t NiFe and t NiO. The in-plane symmetry of the exchange field and coercivity display unidirectional and uniaxial anisotropies, with angular dependences different from the simple c o s φ H and c o s 2 φ 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 NiFe, while the dependence on the thickness of the AF layer is more complicated, and is characterized by a critical behavior. <![CDATA[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 <sup>235</sup>U]]> 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 &lt; Q. <![CDATA[Growth of carbon fibres, sheets and tubes on diamond films under high power plasma etching conditions]]> 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. <![CDATA[Ermakov-Lewis invariants for a class of parametric anharmonic oscillators]]> 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.<hr/>Resumen En este artículo investigamos una clase general de osciladores anarmónicos con coeficientes dependientes del tiempo. El modelo es una ecuación diferencial ordinaria de segundo orden en el cual el término forzado es una función general de la solución y el tiempo. Varias ecuaciones bien conocidas de la física matemática son generalizadas por nuestro modelo. La ecuación es presentada como un sistema Ray-Reid generalizado y posteriormente es obtenido un invariante del tipo Ermakov-Lewis. Las formas particulares de este invariante son obtenidas para el oscilador armónico clásico y la ecuación de Ermakov. De esta manera, se abre una línea de investigación en la determinación de invariantes de Ermakov-Lewis para sistemas anarmónicos. <![CDATA[XPS, DRUV-VIS and ESR characterization of the non-stoichiometric compound Ge<sub>0.74</sub>V<sub>0.21</sub> □<sub>0.05</sub>O<sub>2</sub>]]> Abstract Microcrystalline powders of the nonstoichiometric Ge0.74V0.21 □0.05O2 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 V4+ and V5+ vanadium ions, the DRUV-VIS spectra show absorption bands in the 200-800 nm wave length interval and the ESR analysis confirms that the V4+ ions are within microcrystals, hosted as VO2+ at sites of rhombic (C 2v ) symmetry. <![CDATA[Border collision bifurcations in tantalus oscillator]]> 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. <![CDATA[Transformation of a wavefunction under changes of reference frame]]> 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.<hr/>Resumen Se aplica a algunos ejemplos un procedimiento simple para obtener la transformación de una función de onda bajo un cambio de marco de referencia y se estudia su relación con la transformación de la función principal de Hamilton. <![CDATA[Cross sections for electron-impact excitation of neutral atoms]]> 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. <![CDATA[Eigenestados exactos y estados entrelazados de un ión atrapado oscilando en dos dimensiones]]> Resumen Demostramos que es posible encontrar eigenestados exactos de la interacción de un ión atrapado oscilando en dos dimensiones y que cada uno de los eigenestados exactos son estados entrelazados. También demostramos que el hamiltoniano que describe dicha interacción puede ser mapeado a un escenario tipo Rabi cuántico (un átomo de dos niveles y dos campos electromagnéticos cuantizados) mediante una transformación unitaria.<hr/>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. <![CDATA[Spatial average symmetry associated to unconventional polarization]]> 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.<hr/>Resumen Se propone el promedio espacial del vector de Stokes como un criterio cuantitativo para evaluar el promedio de la simetría axial asociado a la sección transversal de luz polarizada no-convencional. Utilizando representaciones algebráicas e imágenes se presentan resultados experimentales y numéricos para polarización acimutal y radial.