2,167 research outputs found
Synchronization in semiconductor laser rings
We examine the dynamics of semiconductor lasers coupled in a ring
configuration. The lasers, which have stable output intensity when isolated,
behave chaotically when coupled unidirectionally in a closed chain. In this
way, we show that neither feedback nor bidirectional coupling is necessary to
induce chaotic dynamics at the laser output. We study the synchronization
phenomena arising in this particular coupling architecture, and discuss its
possible application to chaos-based communications. Next, we extend the study
to bidirectional coupling and propose an appropriate technique to optical chaos
encryption/decryption in closed chains of mutually coupled semiconductor
lasers.Comment: 15 pages, 7 figure
Emergence of spike correlations in periodically forced excitable systems
In sensory neurons the presence of noise can facilitate the detection of weak
information-carrying signals, which are encoded and transmitted via correlated
sequences of spikes. Here we investigate relative temporal order in spike
sequences induced by a subthreshold periodic input, in the presence of white
Gaussian noise. To simulate the spikes, we use the FitzHugh-Nagumo model, and
to investigate the output sequence of inter-spike intervals (ISIs), we use the
symbolic method of ordinal analysis. We find different types of relative
temporal order, in the form of preferred ordinal patterns which depend on both,
the strength of the noise and the period of the input signal. We also
demonstrate a resonance-like behavior, as certain periods and noise levels
enhance temporal ordering in the ISI sequence, maximizing the probability of
the preferred patterns. Our findings could be relevant for understanding the
mechanisms underlying temporal coding, by which single sensory neurons
represent in spike sequences the information about weak periodic stimuli
Numerical and experimental study of the effects of noise on the permutation entropy
We analyze the effects of noise on the permutation entropy of dynamical
systems. We take as numerical examples the logistic map and the R\"ossler
system. Upon varying the noise strengthfaster, we find a transition from an
almost-deterministic regime, where the permutation entropy grows slower than
linearly with the pattern dimension, to a noise-dominated regime, where the
permutation entropy grows faster than linearly with the pattern dimension. We
perform the same analysis on experimental time-series by considering the
stochastic spiking output of a semiconductor laser with optical feedback.
Because of the experimental conditions, the dynamics is found to be always in
the noise-dominated regime. Nevertheless, the analysis allows to detect
regularities of the underlying dynamics. By comparing the results of these
three different examples, we discuss the possibility of determining from a time
series whether the underlying dynamics is dominated by noise or not
Coherence and synchronization in diode-laser arrays with delayed global coupling
The dynamics of a semiconductor-laser array whose individual elements are
coupled in a global way through an external mirror is numerically analysed. A
coherent in-phase solution is seen to be preferred by the system at
intermediate values of the feedback coupling strength. At low values of this
parameter, a strong amplification of the spontaneous emission noise is
observed. A tendency towards chaos synchronization is also observed at large
values of the feedback strength.Comment: 8 pages, LaTeX, 6 PS figures, to appear in International Journal of
Bifurcation and Chao
A new approach to local hardness
The applicability of the local hardness as defined by the derivative of the
chemical potential with respect to the electron density is undermined by an
essential ambiguity arising from this definition. Further, the local quantity
defined in this way does not integrate to the (global) hardness - in contrast
with the local softness, which integrates to the softness. It has also been
shown recently that with the conventional formulae, the largest values of local
hardness do not necessarily correspond to the hardest regions of a molecule.
Here, in an attempt to fix these drawbacks, we propose a new approach to define
and evaluate the local hardness. We define a local chemical potential,
utilizing the fact that the chemical potential emerges as the additive constant
term in the number-conserving functional derivative of the energy density
functional. Then, differentiation of this local chemical potential with respect
to the number of electrons leads to a local hardness that integrates to the
hardness, and possesses a favourable property; namely, within any given
electron system, it is in a local inverse relation with the Fukui function,
which is known to be a proper indicator of local softness in the case of soft
systems. Numerical tests for a few selected molecules and a detailed analysis,
comparing the new definition of local hardness with the previous ones, show
promising results.Comment: 30 pages (including 6 figures, 1 table
The parent Diels-Alder cycloaddition reaction: a DFT study of the concerted and stepwise mechanism
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