Extreme Events Prediction from Nonlocal Partial Information in a Spatiotemporally Chaotic Microcavity Laser.

The forecasting of high-dimensional, spatiotemporal nonlinear systems has made tremendous progress with the advent of model-free machine learning techniques. However, in real systems it is not always possible to have all the information needed; only partial information is available for learning and forecasting. This can be due to insufficient temporal or spatial samplings, to inaccessible variables, or to noisy training data. Here, we show that it is nevertheless possible to forecast extreme event occurrences in incomplete experimental recordings from a spatiotemporally chaotic microcavity laser using reservoir computing. Selecting regions of maximum transfer entropy, we show that it is possible to get higher forecasting accuracy using nonlocal data vs local data, thus allowing greater warning times of at least twice the time horizon predicted from the nonlinear local Lyapunov exponent.

Pulse propagation in a 1D array of excitable semiconductor lasers.

Nonlinear pulse propagation is a major feature in continuously extended excitable systems. The persistence of this phenomenon in coupled excitable systems is expected. Here, we investigate theoretically the propagation of nonlinear pulses in a 1D array of evanescently coupled excitable semiconductor lasers. We show that the propagation of pulses is characterized by a hopping dynamics. The average pulse speed and bifurcation diagram are characterized as a function of the coupling strength between the lasers. Several instabilities are analyzed such as the onset and disappearance of pulse propagation and a spontaneous breaking of the translation symmetry. The pulse propagation modes evidenced are specific to the discrete nature of the 1D array of excitable lasers.

Strong Coupling between Self-Assembled Molecules and Surface Plasmon Polaritons.

We experimentally demonstrate strong coupling between self-assembled PTCDI-C7 organic molecules and the electromagnetic mode generated by surface plasmon polaritons (SPPs). The system consists of a dense self-assembly of ordered molecules evaporated directly on a thin gold film, which stack perpendicularly to the metal surface to form H-aggregates, without a host matrix. Experimental wavevector-resolved reflectance spectra show the formation of hybrid states that display a clear anticrossing, attesting the strong coupling regime with a Rabi splitting energy of ΩR ≃ 102 meV at room temperature. We demonstrate that the strength of the observed strong coupling regime derives from the high degree of organization of the dense layers of self-assembled molecules at the nanoscale that results in the concentration of the oscillator strength in a charge-transfer Frenkel exciton, with a dipole moment parallel to the direction of the maximum electric field. We compare our results to numerical simulations of a transfer matrix model and reach good qualitative agreement with the experimental findings. In our nanophotonic system, the use of self-assembled molecules opens interesting prospects in the context of strong coupling regimes with molecular systems.

Spike latency and response properties of an excitable micropillar laser.

We present experimental measurements concerning the response of an excitable micropillar laser with saturable absorber to incoherent as well as coherent perturbations. The excitable response is similar to the behavior of spiking neurons but with much faster time scales. It is accompanied by a subnanosecond nonlinear delay that is measured for different bias pump values. This mechanism provides a natural scheme for encoding the strength of an ultrafast stimulus in the response delay of excitable spikes (temporal coding). Moreover, we demonstrate coherent and incoherent perturbations techniques applied to the micropillar with perturbation thresholds in the range of a few femtojoules. Responses to coherent perturbations assess the cascadability of the system. We discuss the physical origin of the responses to single and double perturbations with the help of numerical simulations of the Yamada model and, in particular, unveil possibilities to control the relative refractory period that we recently evidenced in this system. Experimental measurements are compared to both numerical simulations of the Yamada model and analytic expressions obtained in the framework of singular perturbation techniques. This system is thus a good candidate to perform photonic spike processing tasks in the framework of novel neuroinspired computing systems.

Spatiotemporal Chaos Induces Extreme Events in an Extended Microcavity Laser.

Extreme events such as rogue waves in optics and fluids are often associated with the merging dynamics of coherent structures. We present experimental and numerical results on the physics of extreme event appearance in a spatially extended semiconductor microcavity laser with an intracavity saturable absorber. This system can display deterministic irregular dynamics only, thanks to spatial coupling through diffraction of light. We have identified parameter regions where extreme events are encountered and established the origin of this dynamics in the emergence of deterministic spatiotemporal chaos, through the correspondence between the proportion of extreme events and the dimension of the strange attractor.

Temporal summation in a neuromimetic micropillar laser.

Neuromimetic systems are systems mimicking the functionalities or architecture of biological neurons and may present an alternative path for efficient computing and information processing. We demonstrate here experimentally temporal summation in a neuromimetic micropillar laser with an integrated saturable absorber. Temporal summation is the property of neurons to integrate delayed input stimuli and to respond by an all-or-none kind of response if the inputs arrive in a sufficiently small time window. Our system alone may act as a fast optical coincidence detector and paves the way to fast photonic spike-processing networks.

Relative refractory period in an excitable semiconductor laser.

We report on experimental evidence of neuronlike excitable behavior in a micropillar laser with saturable absorber. We show that under a single pulsed perturbation the system exhibits subnanosecond response pulses and analyze the role of the laser bias pumping. Under a double pulsed excitation we study the absolute and relative refractory periods, similarly to what can be found in neural excitability, and interpret the results in terms of a dynamical inhibition mediated by the carrier dynamics. These measurements shed light on the analogy between optical and biological neurons and pave the way to fast spike-time coding based optical systems with a speed several orders of magnitude faster than their biological or electronic counterparts.

Transient chirp in high-speed photonic-crystal quantum-dot lasers with controlled spontaneous emission.

We report on a series of experiments on the dynamics of spontaneous emission controlled nanolasers. The laser cavity is a photonic-crystal slab cavity, embedding self-assembled quantum dots as gain material. The implementation of cavity electrodynamics effects increases the large signal modulation bandwidth significantly, with measured modulation speeds of the order of 10 GHz while keeping an extinction ratio of 19 dB. A linear transient wavelength shift is reported, corresponding to a chirp of less than 100 pm for a 35 ps laser pulse. We observe that the chirp characteristics are independent of the repetition rate of the laser up to 10 GHz.

Homoclinic snaking in a semiconductor-based optical system.

We report on experimental observations of homoclinic snaking in a vertical-cavity semiconductor optical amplifier. Our observations in a quasi-one-dimensional and two-dimensional configurations agree qualitatively well with what is expected from recent theoretical and numerical studies. In particular, we show the bifurcation sequence leading to a snaking bifurcation diagram linking single localized states to "localized patterns" or clusters of localized states and demonstrate a parameter region where cluster states are inhibited.

Transverse spatial structure of a high Fresnel number Vertical External Cavity Surface Emitting Laser.

The transverse spatial structure of an optically-pumped, Vertical External Cavity Surface Emitting Laser is investigated experimentally. The Fresnel number of the laser cavity is controlled with an intracavity lens. We show how the emission profile changes when passing from a low to a high Fresnel number configuration and analyze the RF spectrum of the total laser intensity. Though the laser operates in a multi-longitudinal mode configuration, the transverse profile of the laser emission shows well organized patterns.

Modeling pattern formation and cavity solitons in quantum dot optical microresonators in absorbing and amplifying regimes.

We present a complete overview of our investigation past and present of the modelization and study of the spatiotemporal dynamics of a coherent field emitted by a semiconductor microcavity based on self-assembled quantum dots. The modelistic approach is discussed in relation to prospective growth and experimental research, and the model is then applied to resonators for which the medium is either passive (coherent photogeneration of carriers) or active (carrier pumping by current bias). The optical response of the system is investigated, especially in what concerns the linewidth enhancement factor, which turns out to be critical for the onset of self-organized patterns. The regimes in which one can expect bistable response, modulational instabilities, pattern formation, and cavity soliton formation are investigated. The pattern scenario is described, and experimentally achievable conditions are predicted for the occurrence of stable cavity solitons.

Physical model for the incoherent writing/erasure of cavity solitons in semiconductor optical amplifiers.

We present a physical mechanism that explains the recent observations of incoherent writing and erasure of Cavity Solitons in a semiconductor optical amplifier [S. Barbay et al, Opt. Lett. 31, 1504-1506 (2006)]. This mechanism allows to understand the main observations of the experiment. In particular it perfectly explains why writing and erasure are possible as a result of a local perturbation in the carrier density, and why a delay is observed along with the writing process. Numerical simulations in 1D are performed and show very good qualitative agreement with the experimental observations.

Incoherent and coherent writing and erasure of cavity solitons in an optically pumped semiconductor amplifier.

Cavity solitons in semiconductor amplifiers were, from the beginning of their study and observation, obtained either spontaneously or in a controlled manner by local coherent excitation. We describe two experiments that demonstrate coherent and, we believe for the first time, incoherent writing and erasure of cavity solitons in an optically pumped vertical-cavity semiconductor amplifier by short optical pulses.

Reorganization of remote cortical regions after ischemic brain injury: a potential substrate for stroke recovery.

Although recent neurological research has shed light on the brain's mechanisms of self-repair after stroke, the role that intact tissue plays in recovery is still obscure. To explore these mechanisms further, we used microelectrode stimulation techniques to examine functional remodeling in cerebral cortex after an ischemic infarct in the hand representation of primary motor cortex in five adult squirrel monkeys. Hand preference and the motor skill of both hands were assessed periodically on a pellet retrieval task for 3 mo postinfarct. Initial postinfarct motor impairment of the contralateral hand was evident in each animal, followed by a gradual improvement in performance over 1-3 mo. Intracortical microstimulation mapping at 12 wk after infarct revealed substantial enlargements of the hand representation in a remote cortical area, the ventral premotor cortex. Increases ranged from 7.2 to 53.8% relative to the preinfarct ventral premotor hand area, with a mean increase of 36.0 +/- 20.8%. This enlargement was proportional to the amount of hand representation destroyed in primary motor cortex. That is, greater sparing of the M1 hand area resulted in less expansion of the ventral premotor cortex hand area. These results suggest that neurophysiologic reorganization of remote cortical areas occurs in response to cortical injury and that the greater the damage to reciprocal intracortical pathways, the greater the plasticity in intact areas. Reorganization in intact tissue may provide a neural substrate for adaptive motor behavior and play a critical role in postinjury recovery of function.

Noise-assisted transmission of binary information: theory and experiment.

We study the response of a bistable vertical cavity surface emitting laser to an aperiodic binary signal, by adding a variable amount of noise. The resulting behavior is an example of aperiodic stochastic resonance, and in this work we give a detailed comparison between analytical and numerical results and accurate experimental measurements. We characterize the phenomenon by using different appropriate indicators, which also allow us to quantify the binary information transmission. We show that the quality of the transmission is enhanced by a suitable amount of noise, and we give a physical picture of the phenomenon.

Experimental evidence of binary aperiodic stochastic resonance

A novel kind of aperiodic stochastic resonance is experimentally studied in a vertical cavity surface emitting laser. We characterize the response of the system to a random, binary signal as a function of an applied external noise. A maximum in the input-output correlation is found for a nonzero added noise. We present analytic results with a good agreement with the measurements. We also discuss the physical meaning of the phenomenon using simple arguments, and we compare it to stochastic resonance.

Large cortical lesions produce enduring forelimb placing deficits in un-treated rats and treatment with NMDA antagonists or anti-oxidant drugs induces behavioral recovery.

Previous studies have utilized a lesion model of cortical injury that produces transient behavioral impairments to investigate the recovery of function process. To better understand the recovery process, it would be beneficial to use a lesion model that produces more severe, enduring, behavioral impairments. The purpose of experiment 1 was to validate whether large lesions of the sensorimotor cortex (SMC), which included the rostral forelimb and caudal forelimb regions, produced enduring behavioral deficits. Rats were given large unilateral electrolytic lesions of the SMC, administered either the N-methyl-D-aspartate (NMDA) antagonist, MK-801 or saline 16 h after injury, and tested on a battery of behavioral tests. Enduring behavioral deficits were observed, for at least 6 months, on two tests of forelimb placing while transient deficits were observed on the foot-fault and somatosensory neutralization tests. Administration of MK-801 facilitated recovery on the somatosensory neutralization test; however, it did not induce recovery on either forelimb placing test. A second experiment was performed to determine if earlier administration of MK-801, the NMDA antagonist magnesium chloride (MgCl(2)), or the anti-oxidant N-tert-butyl-alpha-phenylnitrone (PBN) could induce behavioral recovery in this chronic model. Treatment with these drugs induced behavioral recovery on the forelimb placing tests, whereas, the saline-treated rats did not show any signs of behavioral recovery for at least 3 months. Anatomical analysis of the striatum showed that MK-801 and MgCl(2) but not PBN reduced the extent of lesion-induced striatal atrophy. These results suggest that administration of MK-801, MgCl(2), or PBN shortly after cortical injury can induce recovery of function when recovery is otherwise not expected in un-treated rats.

Stochastic resonance in vertical cavity surface emitting lasers

We report a detailed experimental investigation of stochastic resonance (SR) in the polarized emission of a pump-modulated vertical cavity surface emitting laser. We characterize SR in the time and frequency domains, with a quantitative agreement with existing theories. We further report a statistical analysis of SR in terms of residence-time probability distributions exhibiting alternative features which are fully explained here. By using an accurate choice of the indicator, we are also able to give clear evidence of bona fide resonance.

Factors contributing to motor impairment and recovery after stroke.

The goal of the present study was to examine factors affecting motor impairment and recovery in a primate model of cortical infarction. Microelectrode stimulation techniques were used to delineate the hand representation in the primary motor cortex (M1). Microinfarcts affecting approximately 30% of the hand representation were made by electrocoagulation of surface vessels. Electrophysiologic procedures were repeated at 1 month after the infarct to examine changes in motor map topography. Before the infarct, and at approximately 1 week (early period) and 1 month (late period) after the infarct, manual performance was assessed on a reach-and-retrieval task that required skilled use of the digits. Contrary to the expected outcome, early impairment was inversely related to the amount of digit representation destroyed by the infarct. That is, animals with less involvement of the M1 digit area demonstrated the greatest motor deficit in the early postinfarct period. In addition, improvement in motor performance between early and late postinfarct periods was directly related to a decrease in the extent of the digit + wrist/forearm area in the final postinfarct map. These results suggest that specific aspects of motor-map remodeling are expressions of adaptive mechanisms that underlie functional recovery after stroke. Further, they suggest that the adaptive mechanisms underlying postinjury recovery differ in detail from those that operate in normal motor learning. The potential role of compensatory mechanisms in these phenomena is discussed.

Noise-assisted binary information transmission in vertical cavity surface emitting lasers.

The response of a polarization-bistable vertical cavity surface emitting laser to a small amplitude, pseudorandom binary signal is experimentally studied. A high-contrast output is obtained when the optimum amount of noise is added to the system. The quality of the transmission, measured by the bit-error rate, exhibits a resonantlike behavior as a function of the input noise. We analyze possible applications of this transmission scheme to optical communications and compare it with those used in a standard amplitude modulation scheme.