C. P. Cristescu

Analysis of the intermittent behavior in a low-temperature discharge plasma by recurrence plot quantification

The paper presents applications of the recurrence plot quantification to the analysis of type 1 intermittency observed in a discharge plasma. The main measures of recurrence plot analysis considered are the laminarity and the determinism. First, it is demonstrated that the laminarity measure can be related to the average time between chaotic bursts in this type of intermittency. Second, it is shown that the determinism measure introduced by the recurrence plot quantification depends on the control parameter as a decreasing exponential. The test of the results is performed by analyzing the determinism measure for type 1 intermittency observed in the logistic map. It is also shown that the role of the intrinsic noise of the plasma can easily be identified by this technique.

Investigation on series of length of coding and non-coding DNA sequences of bacteria using multifractal detrended cross-correlation analysis

In the framework of multifractal detrended cross-correlation analysis, we investigate characteristics of series of length of coding and non-coding DNA sequences of some bacteria and archaea. We propose the use of a multifractal cross-correlation series that can be defined for any pair of equal lengths data sequences (or time series) and that can be characterized by the full set of parameters that are attributed to any time series. Comparison between characteristics of series of length of coding and non-coding DNA sequences and of their associated multifractal cross-correlation series for selected groups is used for the identification of class affiliation of certain bacteria and archaea. The analysis is carried out using the dependence of the generalized Hurst exponent on the size of fluctuations, the shape of the singularity spectra, the shape and relative disposition of the curves of the singular measures scaling exponent and the values of the associated parameters. Empirically, we demonstrate that the series of lengths of coding and non-coding sequences as well as the associated multifractal cross-correlation series can be approximated as universal multifractals.

Similarity analysis for DNA sequences based on chaos game representation. Case study: the albumin.

Using chaos game representation we introduce a novel and straightforward method for identifying similarities/dissimilarities between DNA sequences of the same type, from different organisms. A matrix is associated to each CGR pattern and the similarities result from the comparison between the matrices of the sequences of interest. Three different methods of analysis of the resulting difference matrix are considered: a 3-dimensional representation giving both local and global information, a numerical characterization by defining an n-letter word similarity measure and a statistical evaluation. The method is illustrated by implementation to the study of albumin nucleotides sequences from eight mammal species taking as reference the human albumin.

Dynamics of the plasma of a twin discharge system induced by external perturbations

In this work we present new results on the dynamics of charge structures generated in the inter-anode space of a twin electric discharge system both for free running and for sinusoidal forcing. We study the change of plasma dynamics as function of the amplitude of the ac forcing for two dc biasing values. In a certain range of the control parameter, proportional to the amplitude of the sinusoidal perturbation, the dynamics can be dramatically changed. The results of the computational model proposed in previous works for the system are in reasonable agreement with the experiment.

Local electric fields in optical glasses during field-assisted ionic exchanges

We study the phenomena connected with high concentration of incoming ions emerging during field-assisted migration in otpical glass. We find that ion dynamics are very different at concentrations higher and lower than a certain parameter called the transition concentration. To explain anomalies at high ionic concentrations, we introduce a supplementary local electric field. This field opposes to the field E0 existing in the glass at all concentrations of incoming ions and is connected with a local space charge. We investigate its dependence on concentration and on E0. These effects are studied using a model with concentration-dependent diffusion coefficients and mobilities. We present a method to obtain the concentration dependence of the relevant quantities. Theoretical curves are compared with experimental results measured in usual silicate glasses, during Ag+-Na+ exchanges.

Magneto-mechanical oscillations revisited

Here we study the temporal evolution of a magnetemechanical, inverted spherical pendulum. The oscillator is a rigid, stainless steel, hollow needle. The needle has one end fixed in a spherical-like articulation, while the other extremity has no mechanical contact. The oscillation is driven by a longitudinal, periodically magnetic field added to a constant value of a static field. A video camera takes simultaneously the optical images of the projections of the oscillating needle along two mutually normal directions. The pairs of temporal sequences are analyzed in the real space, phase space and Fourier space. Among all the external parameter that can be usually varied, like the strength of the static magnetic field, the amplitude and the frequency of the driving magnetic field, or the momentum of inertia of the oscillating body, of a crucial importance seems to be the magnetc-mechanical feed-back of the oscillating system.

3D Analysis of the dynamics of a metal-ferroelectric-semiconductor heterostructure

A series RLC electric circuit with the metal-ferroelectric-semiconductor (MFS) heterwdructure playing the role of nonlinear capacity shows various nonlinear dynamics such as period doubling and Ruelle-Takens scenarios of evolution to chaos. Experimental investigations and preliminary theoretical analysis on the dynamics of this structure are considered. We mainly focus on a period-doubhng bifurcation evolution to chaos induced by the change in the amplitude of a sinusoidal driving with given frequency and by the change in the frequency of driving signal of fixed amplitude. In this paper we concentrate on an integrated image of these phenomena as reflected in 3D graphs of spectral analysis. A theoretical model of this MFS structure as nonlinear element of an electrical circuit based on a system of two coupled nonlinear Duffing oscillators is proposed. Numerical simulations for electrical variables in the circuit were performed both by specific methods of nonlinear investigations and by 3D plots representing Fourier power spectra as function of the frequency and the amplitude of the forcing as control parameters. The dynamics of the theoretical model are in reasonable agreement with the experimental results.

Surface phenomena in ionic inter-diffusion in optical glasses

We present theoretical results in modeling ionic inter-diffusion in optical glasses. Several interface phenomena are studied and their influence on the shape and the position of the waveguides is explained. The most important process is re-diffusion. We investigate the influence of the maximum surface concentration, of the clustering and of the time constant for reaching interface equilibrium. Their influence is taken into account by introducing supplementary terms in the inter-diffusion equation or by modifying boundary and initial conditions. All the parameters of the final buried guides depend on surface phenomena. The total amount of ions in the guide, the width at half the maximum, the burying depth and the shape of the profiles depend on surface processes. The influence appears for the mono-mode, as well as for multi-mode structures.

Interferometric study of the laser-induced melting stage of an absorbing glass

We study the processes of glass heating and melting by absorption of laser radiation using profilometry in reflected light and interferometric real time analysis. The sample irradiation is obtained by an argon ion laser and the interference pattern is generated by a He-Ne laser or by another argon ion laser. In the reflected beam we observe a change of the ring pattern caused by the increasing of pumping power. Above a certain level an additional ring pattern appears. We study the dependence of the structure of the ring patterns as function of the power of the pumping laser beam. From the diffraction pattern and the interference pattern given by a wedge shearing interferometer we estimate the shape and dimension of the mound and respectively the dimple generated for different pumping powers.

Interferometric detection of photothermal effects in uranyl optical glass

In this paper we present a sensitive photothermal interferometric detection technique using a Wollaston prism. The interference fringe pattern is formed by overlapping of the ordinary and extraordinary beams with adjustable intensities by a linear polarizer. The interferometric montage is made both for the pump beam (Ar-ion laser) and the probe beam (He-Ne laser). From the analysis of the interference patterns obtained separately for the two beams, we calculate the absolute change of the refractive index in the middle of the heated zone of an uranyl-doped optical glass, as well as the average transverse refractive index gradient induced by the pump laser. The experimental results are interpreted in terms of a Gaussian beam propagating through a weak absorptive medium whose refractive index is thermally modified by the pump beam.