The first direct experimental evidence of topological defects in nonlinear optics is presented. For increasing Fresnel numbers F, the two-dimensional field is characterized by an increasing number of ...topological defects, from a single vortex up to a large number of vortices with zero net topological charge. At variance with linear scattering from a fixed phase plate, here the defect pattern evolves in time according to the nonlinear dynamics. The scaling exponents are assigned for the mean number of defects, their mean separation, and the charge unbalance as functions of F, as well as the correlation time of the defect pattern. (Author)
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We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates by a femptosecond Ti: Sapphire laser. As for their optical characterization, a spatially ...resolved microscopy interferometric technique is used to investigate the two-dimensional distribution of the refractive index change produced by the irradiation process. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated PMMA, providing information on how the fabrication process depends on the laser irradiation.
► Phase gratings written in PMMA samples by a femtosecond laser are studied. ► Two dimensional phase maps of the refractive index changes are shown. ► Interferometric techniques are used to measure the refractive index changes. ► Femtosecond laser pulses induce localized optical phase variation in PMMA samples. ► Digital holography is used to reconstruct amplitude and phase of the field.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Cognizione implica percezione e giudizio. La percezione consiste nell'interpretare uno stimolo sensoriale: è un processo comune a tutti gli animali con cervello e si può descrivere come una inferenza ...di Bayes in cui l'algoritmo interpretativo è immagazzinato nella memoria a lungo termine. Il giudizio riguarda, invece, il confronto fra due percezioni codificate in un linguaggio, in cui la precedente viene ripresentata dalla memoria a breve termine e confrontata con la successiva. L'operazione – chiamata Bayes inverso – non presuppone un algoritmo, ma ne costruisce uno nuovo attraverso il confronto. Nel libro si mostra come i salti algoritmici legati a operazioni linguistiche catturino aspetti della realtà non raggiungibili attraverso catene di inferenze di Bayes guidate dallo stesso algoritmo.
Oggi sperimentiamo i successi dell'Intelligenza Artificiale (AI), la quale, però, opera per Bayes diretto, velocizza le catene ricorsive, ma non ricorre a salti algoritmici; pertanto, non contribuisce al linguaggio umano. Fortunato Tito Arecchi è professore emerito di Fisica presso l'Università di Firenze. Dal 1975 al 2001 ha diretto l'Istituto Nazionale di Ottica, orientandolo verso l'ottica quantistica (fisica delle sorgenti laser e delle applicazioni). Come ricercatore, ha introdotto la statistica dei fotoni per caratterizzare la coerenza del laser (1965); nel 1982 ha dato la prima evidenza di caos deterministico nei laser. È autore e co-autore di oltre 400 lavori scientifici e di 12 volumi. Testo dell'editore
In this paper, we estimate the errors due to observational noise on the recurrence quantification analysis (RQA). Based on this estimation, we present ways to minimize these errors. We give a ...criterion to choose the threshold
ε needed for the optimal computation of the recurrence plot (RP). One important point is to show the limits of interpretability of the results of the RQA if it is applied to measured time series. We show that even though the RQA is very susceptible to observational noise, it can yield reliable results for an optimal choice of
ε if the noise level is not too high. We apply the results to typical models, such as white noise, the logistic map and the Lorenz system, and to experimental laser data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We present the role of the discharge current and noise on the polarization dynamics in the CO
2
laser, showing a transition from stable to bistable states and the presence of coherent resonance. We ...also explore the role of an optical feedback to achieve polarization stabilization or alternation between first-order transverse modes.
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The output of a dynamical system in a regime of homoclinic chaos transforms from a continuous train of irregularly spaced spikes to clusters of regularly spaced spikes with quiescent periods in ...between (bursting), provided a low frequency portion of the output is fed back. We provide experimental evidence of such an autonomous bursting by a CO2 laser with feedback. The phenomena here presented are extremely robust against noise and display qualitative analogies with bursting phenomena in neurons.
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We report Stochastic Resonance and Coherence Resonance phenomena in experiments using CO
2
lasers. First, we consider a polarized laser with feedback; for suitable feedback parameters, the laser ...intensity undergoes homoclinic chaos consisting in the return to a saddle focus, where noise controls the permanence time around the saddle. Second, we discuss a quasi-isotropic laser where noise induces switching between two intensity components with mutually orthogonal polarization.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We study the properties of a homoclinic model of neuron by introducing a suitable one-dimensional map. We show that the system is characterized by a response time to external signals which is a ...decreasing function of the signal strength, in contrast to excitable models whose response time is signal-independent. In a one-dimensional array of these systems with bidirectional coupling, we observe a sudden transition to a synchronized state at a certain value of the coupling strength. The transition occurs when the response time of a site to the signals of the adjacent sites is of the order of refractory time. Near the transition, we find an intermittent behavior due to the competition between a turbulent and a synchronized state. The observed behavior distinguishes homoclinic systems from excitable systems.
We examine the mutual synchronization of a one-dimensional chain of chaotic identical objects in the presence of a stimulus applied to the first site. We first describe the characteristics of the ...local elements, and then the process whereby a global nontrivial behavior emerges. A propensity criterion for networking is introduced, consisting in the coexistence within the attractor of a localized chaotic region, which displays high sensitivity to external stimuli, and an island of stability, which provides a reliable coupling signal to the neighbors in the chain. Based on this criterion, we compare homoclinic chaos, recently explored in lasers and conjectured to be typical of a single neuron, with Lorenz chaos.
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