Low-Dimensional Chaotic Attractors for an Unstable, Inhomogeneously Broadened, Single-Mode Laser

Quantitative characterization of the intensity pulsations from an inhomogeneously broadened laser confirm that observed irregular pulsing has its origins in deterministic chaos corresponding to motion on a strange attractor of low fractal dimensionality. The pointwise information dimension and the Grassberger-Procaccia K2 (estimators from below of the fractal dimensionality of the attractor and the Kolmogorov entropy, respectively) have been determined for digitized time series from parameter regions identified qualitatively by power spectra as representing periodic, period-doubled, quasi-periodic, and chaotic behavior. Some amount of chaos seems present for almost all operating conditions

Low-Dimensional Chaotic Attractors for an Unstable, Inhomogeneously Broadened, Single-Mode Laser

Quantitative characterization of the intensity pulsations from an inhomogeneously broadened laser confirm that observed irregular pulsing has its origins in deterministic chaos corresponding to motion on a strange attractor of low fractal dimensionality. The pointwise information dimension and the Grassberger-Procaccia K2 (estimators from below of the fractal dimensionality of the attractor and the Kolmogorov entropy, respectively) have been determined for digitized time series from parameter regions identified qualitatively by power spectra as representing periodic, period-doubled, quasi-periodic, and chaotic behavior. Some amount of chaos seems present for almost all operating conditions

MBE GROWTH AND PROPERTIES OF ZnYbTe LAYERS

The MBE grown ZnYbTe layers were characterized by X-ray diffraction, photoluminescence and reflectivity measurements. The MBE growth conditions allowing to obtain monocrystalline ZnYbTe layers were found to be metal-rich (MBE growth with excess of Zn flux). In optical measurements (photoluminescence, reflectivity), both transitions connected with ternary ZnYbTe compound and with Yb3+ ions were detected. The quality of ZnYbTe layers with Yb content of 3% and 1% is inferior to the quality of pure ZnTe MB.E layers, which is clearly seen in the results of photoluminescence and reflectivity measurements. In the ZnYbTe layers with 3% Yb, exhibiting monocrystalline character in reflection high-energy electron diffraction and X-ray diffraction measurements, optical transitions characteristic of pure YbTe were detected. In ZnYbTe layers with 1% Yb, no transítions connected with YbTe were observed

Comparison of Random Walk Based Techniques for Estimating Network Averages

International audienceFunction estimation on Online Social Networks (OSN) is an important field of study in complex network analysis. An efficient way to do function estimation on large networks is to use random walks. We can then defer to the extensive theory of Markov chains to do error analysis of these estimators. In this work we compare two existing techniques, Metropolis-Hastings MCMC and Respondent-Driven Sampling, that use random walks to do function estimation and compare them with a new reinforcement learning based technique. We provide both theoretical and empirical analyses for the estimators we consider

Learning Algorithms for Markov Decision Processes with Average Cost

Abstract. This paper gives the first rigorous convergence analysis of analogues of Watkins’s Q-learning algorithm, applied to average cost control of finite-state Markov chains. We discuss two algorithms which may be viewed as stochastic approximation counterparts of two existing algorithms for recursively computing the value function of the average cost problem—the traditional relative value iteration (RVI) algorithm and a recent algorithm of Bertsekas based on the stochastic shortest path (SSP) formulation of the problem. Both synchronous and asynchronous implementations are considered and analyzed using the ODE method. This involves establishing asymptotic stability of associated ODE limits. The SSP algorithm also uses ideas from two-time-scale stochastic approximation. Key words. simulation-based algorithms, Q-learning, controlled Markov chains, average cost control, stochastic approximation, dynamic programming AMS subject classification. 93E20 PII. S0363012999361974 1. Introduction. Q-learning algorithms are simulation-based reinforcement learning algorithms for learning the value function arising in the dynamic programming approach to Markov decision processes. They were first introduced for the discounte

Optical and magneto-optical differential spectroscopy of a ZnCdSe-ZnSe superlattice

International audienceWe report on optical and magneto-optical studies of a ZnCdSe-ZnSe strained layer superlattice. This structure was grown by metal-organic vapour phase epitaxy (MOVPE) and consisted of 40 periods of 72 Angstrom ZnCdSe layer and 49 Angstrom ZnSe layer with cadmium composition equal to 4%. Photoluminescence, reflectivity and magnetocircular dichroism experiments at 2 K were used to identify e(1)-hh(1) and e(1)-lh(1) excitonic transitions. By comparing these experimental transitions with the calculated ones, we have determined the valence and the conduction band offsets. In these calculations, we have included values of the binding energies related to the different transitions. These binding energies are calculated within the formalism of fractional dimensional space. Zeeman splitting of \+/-3/2, +/-1/2] and \+/-1/2, -/+ 1/2] transitions are calculated at 5.5 T and the corresponding effective g-values g(3/2) and g(1/2) are deduced. (C) 1997 Elsevier Science S.A

MBE Growth and Properties of ZnYbTe Layers

The MBE grown ZnYbTe layers were characterized by X-ray diffraction, photoluminescence and reflectivity measurements. The MBE growth conditions allowing to obtain monocrystalline ZnYbTe layers were found to be metal-rich (MBE growth with excess of Zn flux). In optical measurements (photoluminescence, reflectivity), both transitions connected with ternary ZnYbTe compound and with Yb$\text{}^{3+}$ ions were detected. The quality of ZnYbTe layers with Yb content of 3% and 1% is inferior to the quality of pure ZnTe MBE layers, which is clearly seen in the results of photoluminescence and reflectivity measurements. In the ZnYbTe layers with 3% Yb, exhibiting monocrystalline character in reflection high-energy electron diffraction and X-ray diffraction measurements, optical transitions characteristic of pure YbTe were detected. In ZnYbTe layers with 1% Yb, no transitions connected with YbTe were observed

Optical properties of Zn1−xCdxSe-ZnSe superlattices grown by metal organic vapour phase epitaxy

International audienceAn investigation of the reflectivity and photoluminescence spectra of 40-period Zn1-xCdxSe-ZnSe strained layer superlattices is reported. Superlattices with cadmium composition between 3% and 10% were grown by metal organic vapour phase epitaxy. The well and barrier widths were varied in order to study the tunnelling effect in the superlattices. This effect was shown to be large not only for the excited states (e(2)hh(2) transitions for the 10% cadmium composition) but also for e(1)hh(1) transitions despite the large heavy hole effective mass. The band discontinuities were determined by calculating the transition energies within the envelope function framework generalized to strained layers. It was found that for all superlattices the mechanical state is pseudomorphic and that the heavy hole band offset is 32% of the energy difference of the strained band gaps. The binding energies and the oscillator strengths of the various excitonic transitions were calculated within the formalism of fractional-dimensional space and compared with variational calculations reported in the literature

REFLECTIVITY AND PHOTOLUMINESCENCE MEASUREMENTS IN ZNS EPILAYERS GROWN BY METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION

International audienceThin films of high-quality ZnS were grown on (001) GaAs and (111) Si substrates by metal-organic chemical-vapor deposition. 2-K reflectivity was used to analyze various samples grown at different temperatures. The spectra show two structures at 3.801 eV corresponding to the free exciton and at 3.871 eV corresponding to the E0+Δ0 transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model with two oscillators. Thus, the transverse energies, the longitudinal transversal splitting, the oscillator strengths, and the damping parameters were determined for both the free exciton and the split-off exciton of ZnS. Photoluminescence measurements were also carried out using an excimer laser (308 nm). Both light-hole and heavy-hole excitons were observed, which allow for the determination of the strain that exists in the layer. This strain is due only to the difference between the thermal-expansion coefficients of the GaAs substrate and the ZnS epilayer, and was demonstrated to be a tensile strain. This paper also presents results on the band-gap energy variation as a function of the temperature and photoluminescence spectra when the excitation was varied from weak to very high densities (15 MW/cm2)