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We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm −2 , and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation.

We present investigations on Degenerate Four Wave Mixing in bulk GaAs Fabry-Pérot microcavities at room temperature excited with femtosecond pulses. Due to increased interaction length and amplification of light fields in the cavity, the measured diffraction efficiency η≈2×10−3 at 878 nm is enhanced by a factor of roughly 30 compared to bulk GaAs without cavity. The results may be useful for exploiting the cavity enhancement effect for investigation of carrier kinetics at small excitation densities. For application purposes, the results renew the interest in ultrafast coherent deflectors for all-optical routing.

We study the subnanosecond reflectivity kinetics of distributed Fabry-Pérot cavities composed of two GaAs/AlAs Bragg reflectors enclosing a central GaAs layer. The structure reflectivity exhibits a sharp minimum in the absorption edge of GaAs at the temperature of 300 K, around the wavelength of 885 nm. The photogeneration of an electron-hole plasma causes a shift of the cavity resonance. The switch energy and the recovery time are investigated. Switching capabilities are observed with excitation energies of few tens of μJ/cm2. We correlate the reflectivity kinetics to the plasma dynamics in the central GaAs zone of the structure, especially to the density and temperature relaxation.

We study the time-resolved reflectivity spectra of distributed Fabry-Perot (DFP) cavities following a picosecond laser excitation. The structures, which are composed of two GaAs/AlAs Bragg reflectors enclosing a central GaAs layer, exhibit a sharp reflectivity minimum slightly below the GaAs band gap. Due to the photogeneration of carriers in the central zone, we observe a red shift of 5–6 nm of the reflectivity minimum on a subnanosecond time scale. This effect enables one to deduce the plasma density from the spectra analysis. This method still works at low plasma density, i.e., for nondegerated plasma, when the standard luminescence method fails. The reflectivity of a photoexcited DFP is calculated by combining the absorption and index theory in the presence of thermalized carriers with a many-layer description of the structures (Abeles formalism). We trace the relaxation kinetics of the plasma density in the central zone of the microcavity and conclude the stimulated emission in the microcavity.

We study the temporal evolution of populations including thousands of mobile repulsive agents. In the first part, the agents follow the rules of a deterministic automaton. We show that, the population may evolve toward different types of high-symmetry steady distributions, depending on the agent-agent repulsion law, and on the presence (or absence) of borders around the playground. We observe the formation of hexagonal-condensed lattices that maximize the potential function of the agent distribution. In the second part, agents are faulty. They violate the repulsion law and follow the rule of a non-deterministic automaton. We observe fast and random oscillations between the ordered phase and a strongly disorganized state when the number of agents is small (say for population involving a few tens of agents). These oscillations are essentially averaged and disappear in the large populations, resulting in a partially-ordered homogeneous distribution

We studied pico‐ and femtosecond degenerate four‐wave mixing in bulk GaAs Fabry–Perot microcavities at room temperature. For wavelengths below the GaAs band edge, a cavity with a finesse of approximately 20 yields an enhancement of the diffracted signal by more than two orders of magnitude as compared to a cavityless GaAs layer of the same thickness (685 nm). The cavity‐enhanced nonlinearity of the interband transition results in a diffraction efficiency η≊0.5% for 150 pJ pulses with a duration of 900 fs at 878 nm, being 400 times larger than that for the bare GaAs reference sample.

The temporal and spectral response of bulk GaAs Fabry-Perot microcavities have been investigated in the subpicosecond regime at room temperature. Time resolved reflectivity measurements show a strong reshaping of a Gaussian fs-pulse after reflection from a microcavity caused by group velocity dispersion. The pulse distortion agrees fairly well with model calculations using Fourier transform analysis. Time integrated degenerate four-wave mixing experiments show that response times of the cavity as short as 300 fs were achieved.

We calculate the effective electron-hole interaction Vre in the presence of an exciton gas, which reads in real space: V re (r)= −e 2 r {1+ ∑ i=1 4 (−1) i C i exp (−Z i r a } The parameters Ci and Zi are given explicitly for GaAs. For this material, we show the binding energy of the exciton is weakly modified so long as 8πR 0 ϱ ex a 0 3 kT⩽1 . (R0, exciton Rydberg, a0 exciyon radius, ϱex exciton density, T temperature).

The operation of picosecond NOR gates using induced absorption is described. No Fabry-Pérot cavity is required. Cascadability through frequency doubling is discussed. Preliminary experiments are reported.

This thesis is a contribution at the architectural level to the improvement of fault-tolerance in massively defective multi-core chips fabricated using nanometer transistors. The main idea of this work is that a chip should be organized in a replicated architecture and become as autonomous as possible to increase its resilience against both permanent defects and transient faults occurring at runtime. Therefore, we introduce a new chip self-configuration methodology, which allows detecting and isolating the defective cores, deactivating the isolated cores, configuring the communications and managing the allocation and execution of tasks. The efficiency of the methods is studied as a function of the fraction of defective cores, of defective interconnects and soft error rate.; Cette thèse est une contribution au niveau architectural à l'amélioration de la tolérance aux fautes dans les puces multi-coeurs massivement défectueuses fabriquées à partir de transistors nanométriques. L'idée principale de ce travail est qu'une puce devrait être organisée en une architecture réplicative et devenir aussi autonome que possible pour augmenter sa résilience contre les défauts permanents et les erreurs transitoires apparaissant en opération. C'est pourquoi nous introduisons une nouvelle méthodologie d'autoconfiguration de la puce qui permet de détecter et isoler les coeurs défectueux, de désactiver les coeurs isolés, de configurer les communications et de diriger l'allocation et l'exécution des tâches. L'efficacité des méthodes est étudiée en fonction de la fraction de coeurs ou d'interconnections défectueux et du taux d'erreurs transitoires.