34 research outputs found
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Computational Simulations of the Lateral-Photovoltage-Scanning-Method
The major task for the Lateral-Photovoltage-Scanning-Method is to detect doping striations and the shape of the solid-liquid-interface of an indirect semiconductor crystal. This method is sensitive to the gradient of the charge carrier density. Attempting to simulate the signal generation of the LPS-Method, we are using a three dimensional Finite Volume approach for solving the van Roosbroeck equations with COMSOL Multiphysics in a silicon sample. We show that the simulated LPS-voltage is directly proportional to the gradient of a given doping distribution, which is also the case for the measured LPS-voltage
Deep Time-Delay Reservoir Computing: Dynamics and Memory Capacity
The Deep Time-Delay Reservoir Computing concept utilizes unidirectionally
connected systems with time-delays for supervised learning. We present how the
dynamical properties of a deep Ikeda-based reservoir are related to its memory
capacity (MC) and how that can be used for optimization. In particular, we
analyze bifurcations of the corresponding autonomous system and compute
conditional Lyapunov exponents, which measure the generalized synchronization
between the input and the layer dynamics. We show how the MC is related to the
systems distance to bifurcations or magnitude of the conditional Lyapunov
exponent. The interplay of different dynamical regimes leads to a adjustable
distribution between linear and nonlinear MC. Furthermore, numerical
simulations show resonances between clock cycle and delays of the layers in all
degrees of the MC. Contrary to MC losses in a single-layer reservoirs, these
resonances can boost separate degrees of the MC and can be used, e.g., to
design a system with maximum linear MC. Accordingly, we present two
configurations that empower either high nonlinear MC or long time linear MC
Delay-induced dynamics and jitter reduction of passively mode-locked semiconductor lasers subject to optical feedback
We study a passively mode-locked semiconductor ring laser subject to optical feedback from an external mirror. Using a delay differential equation model for the mode-locked laser, we are able to systematically investigate the resonance effects of the inter-spike interval time of the laser and the roundtrip time of the light in the external cavity (delay time) for intermediate and long delay times. We observe synchronization plateaus following the ordering of the well-known Farey sequence. Our results show that in agreement with the experimental results a reduction of the timing jitter is possible if the delay time is chosen close to an integer multiple of the inter-spike interval time of the laser without external feedback. Outside the main resonant regimes the timing jitter is drastically increased by the feedback.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 87159868, GRK 1558: Kollektive Dynamik im Nichtgleichgewicht: in kondensierter Materie und biologischen SystemenEC/FP7/264687/EU/Postgraduate Research on Photonics as an Enabling Technology/PROPHE
Dynamics of a stochastic excitable system with slowly adapting feedback
We study an excitable active rotator with slowly adapting nonlinear feedback
and noise. Depending on the adaptation and the noise level, this system may
display noise-induced spiking, noise-perturbed oscillations, or stochastic
busting. We show how the system exhibits transitions between these dynamical
regimes, as well as how one can enhance or suppress the coherence resonance, or
effectively control the features of the stochastic bursting. The setup can be
considered as a paradigmatic model for a neuron with a slow recovery variable
or, more generally, as an excitable system under the influence of a nonlinear
control mechanism. We employ a multiple timescale approach that combines the
classical adiabatic elimination with averaging of rapid oscillations and
stochastic averaging of noise-induced fluctuations by a corresponding
stationary Fokker-Planck equation. This allows us to perform a numerical
bifurcation analysis of a reduced slow system and to determine the parameter
regions associated with different types of dynamics. In particular, we
demonstrate the existence of a region of bistability, where the noise-induced
switching between a stationary and an oscillatory regime gives rise to
stochastic bursting
Broadband semiconductor light sources operating at 1060 nm based on InAs:Sb/GaAs submonolayer quantum dots
In this paper, we investigate the potential of submonolayer-grown InAs:Sb/GaAs quantum dots as active medium for opto-electronic devices emitting in the 1060 nm spectral range. Grown as multiple sheets of InAs in a GaAs matrix, submonolayer quantum dots yield light-emitting devices with large material gain and fast recovery dynamics. Alloying these structures with antimony enhances the carrier localization and red shifts the emission, whereas dramatically broadening the optical bandwidth. In a combined experimental and numerical study, we trace this effect to an Sb-induced bimodal distribution of localized and delocalized exciton states. While the former do not participate in the lasing process, they give rise to a bandwidth broadening at superluminescence operation and optical amplification. Above threshold laser properties like gain and slope efficiency are mainly determined by the delocalized fraction of carriers
Nonlinear dynamics of doped semiconductor quantum dot lasers
We discuss the influence of wetting layer doping on the
turn-on dynamics of a quantum dot (QD) laser by using a
microscopically based rate equation model which separately treats
the dynamics of electrons and holes. As the carrier-carrier
scattering rates depend nonlinearly on the wetting layer carrier
densities we observe drastic changes of relaxation oscillation
frequency and damping if the wetting layer is doped. We gain insight
into the nonlinear dynamics of the QD laser by a detailed analysis
of various sections of the five-dimensional phase space focusing on
changes in the coupling between QD electron and holes dynamics