Marcos Oriá

Predictability and suppression of extreme events in a chaotic system.

In many complex systems, large events are believed to follow power-law, scale-free probability distributions so that the extreme, catastrophic events are unpredictable. Here, we study coupled chaotic oscillators that display extreme events. The mechanism responsible for the rare, largest events makes them distinct, and their distribution deviates from a power law. On the basis of this mechanism identification, we show that it is possible to forecast in real time an impending extreme event. Once forecasted, we also show that extreme events can be suppressed by applying tiny perturbations to the system.

Absorption-induced trapping in an anisotropic magneto-optical trap

We report on a simple anisotropic magneto-optical trap for neutral atoms that produces a large sample of cold atoms confined in a cylindrically-shaped volume with a high aspect ratio (100:1). Due to the large number of trapped atoms, the laser beams that propagate along the optically thick axis of the trap to cool the atoms are substantially attenuated. We demonstrate that the resulting intensity imbalance produces a net force that spatially localizes the atoms. This limits both the trap length and the total number of trapped atoms. Rotating the cooling beams by a small angle relative to the trap axis avoids the problem of attenuation, and atoms can be trapped throughout the entire available trapping volume. Numerical and experimental results are reported that demonstrate the effects of absorption in an anisotropic trap, and a steady-state, line-center optical path length of 55 is measured for a probe beam propagating along the length of the trap.

Backward-emitted sub-Doppler fluorescence from an optically thick atomic vapor

Literature mentions only incidentally a sub-Doppler contribution in the excitation spectrum of the backward fluorescence of a dense vapor. This contribution is here investigated on Cs vapor, both on the first resonance line (894 nm) and on the weaker second resonance line (459 nm). We show that in a strongly absorbing medium, the quenching of excited atoms moving towards a window irradiated under near normal incidence reduces the fluorescence on the red side of the excitation spectrum. Atoms moving slowly towards the window produce a sub- Doppler velocity-selective contribution, whose visibility is here improved by applying a frequency-modulation technique. This sub-Doppler feature, induced by a surface quenching combined with a short absorption length for the incident irradiation, exhibits close analogies with the narrow spectra appearing with thin vapor cells. We also show that a normal incidence irradiation is essential for the sub-Doppler feature to be observed, while it should be independent of the detection geometry

Tunable power law in the desynchronization events of coupled chaotic electronic circuits

We study the statistics of the amplitude of the synchronization error in chaotic electronic circuits coupled through linear feedback. Depending on the coupling strength, our system exhibits three qualitatively different regimes of synchronization: weak coupling yields independent oscillations; moderate to strong coupling produces a regime of intermittent synchronization known as attractor bubbling; and stronger coupling produces complete synchronization. In the regime of moderate coupling, the probability distribution for the sizes of desynchronization events follows a power law, with an exponent that can be adjusted by changing the coupling strength. Such power-law distributions are interesting, as they appear in many complex systems. However, most of the systems with such a behavior have a fixed value for the exponent of the power law, while here we present an example of a system where the exponent of the power law is easily tuned in real time.

Measurement of the Kerr nonlinear refractive index of Cs vapor

Atomic vapors are systems well suited for studies of opticalnonlinearities. First of all, they are easy to saturate, whichenables the observation of nonlinear effects with low intensitycontinuous-wave laser light [1,2]. At the same time, atomicvapors are damage-free which is important, for instance, forfilamentation studies [3]. Second, as the resonances are sharpthe nonlinear parameters can be easily modified by finelytuningthefrequencynearoracrossaresonance[4].Thisallowsto play with the relative contributions of linear and nonlineareffects by changing the laser wavelength. Third, atomicsystems allow for a variety of level schemes exploring fine,hyperfine, and Zeeman levels such as two-level systems [5,6], three-level schemes [4], double- four-level schemes [7,8],five-level schemes [9], and so on. Fourth, in most experiments,when one can ignore radiation trapping and collisional effects,atomic vapors behave as locally saturable media and are thuseasy to model [10].As atomic vapors are isotropic media, the first nonlinearcontribution to the polarization is a third-order term in theelectric field (

Laser-induced atomic adsorption: A mechanism for nanofilm formation

We demonstrate and interpret a technique of laser-induced formation of thin metallic films using alkali atoms on the window of a dense-vapour cell. We show that this intriguing photo-stimulated process originates from the adsorption of Cs atoms via the neutralization of Cs+ ions by substrate electrons. The Cs+ ions are produced via two-photon absorption by excited Cs atoms very close to the surface, which enables the transfer of the laser spatial intensity profile to the film thickness. An initial decrease of the surface work function is required to guarantee Cs+ neutralization and results in a threshold in the vapour density. This understanding of the film growth mechanism may facilitate the development of new techniques of laser-controlled lithography, starting from thermal vapours.

Effect of attractor on the desynchronization events in coupled chaotic circuits

The phenomenon of attractor bubbling consists in incomplete or imperfect synchronization between two coupled chaotic oscillators, which synchronization regime is broken for brief escapes of different sizes.Here, we use as oscillators two chaotic electronic circuits coupled through unidirectional linear feedback in the regime of moderate coupling to study the occurrence of attractor bubbling in this system. For two different dynamical states, the system exhibits different chaotic attractors. Analysing temporal series, we build empirical distributions of the amplitudes of desynchronization events for different values of the coupling parameter. We observe that in the regime of attractor bubbling the distributions are characterized by a heavy tail, bearing similarity to the ones observed in complex systems with self-organized criticality. A given heavy-tailed distribution is exhibited by the two chaotic states for different coupling strengths. We explain this effect of the attractor shape on the statistics of the desynchronization as caused by a region of instability, which is more often visited for one of the attractors than it is by the other.

CONDIÇÕES PARA BIESTABILIDADE EM FREQUÊNCIA DE LASER SEMICONDUTOR SOB REALIMENTAÇÃO ÓTICA ORTOGONAL

Biestabilidade na frequencia de emissao de um laser semicondutor sujeito a realimentacao otica com polarizacao ortogonal foi observada experimentalmente por Farias et al. em 2005. Um modelo de equacoes de taxa para esse sistema dinâmico, apresentado posteriormente por Masoller et al. em 2007, que leva em conta os efeitos termicos e ganho de saturacao, preve uma variacao linear da frequencia do laser com a intensidade do campo de realimentacao. Nesse trabalho, usando o mesmo modelo, estudamos o processo de biestabilidade otica em frequencia nesses sistemas com realimentacao filtrada, determinando as condicoes espectrais do filtro de realimentacao necessarias para o aparecimento de histerese que leva a biestabilidade.

Model for neural signaling leap statistics

We present a simple model for neural signaling leaps in the brain considering only the thermodynamic (Nernst) potential in neuron cells and brain temperature. We numerically simulated connections between arbitrarily localized neurons and analyzed the frequency distribution of the distances reached. We observed qualitative change between Normal statistics (with T = 37.5°C, awaken regime) and Levy statistics (T = 35.5°C, sleeping period), characterized by rare events of long range connections.

BIDIRECTIONAL SYNCHONIZATION OF COUPLED TIME-DELAY ELECTRONIC CIRCUITS

In this work we studied the behavior of bidirectionally-coupled time-delay electronic circuits. We observed that synchronization is stronger and more robust than in the unidirectional configuration. We were able to numerically reproduce the main observed synchronization behavior.