Pump Dependence Of The Dynamics Of Quantum Dot Based Waveguide Absorbers

The nonlinear two stage recovery of quantum dot based reverse-biased waveguide absorbers is investigated experimentally and analytically as a function of the initial ground state occupation probability of the dot. The latter is controlled experimentally by the pump pulse power. The slow stage of the recovery is exponential and its basic timescale is independent of pump power. The fast stage of the recovery is a logistic function which we analyze in detail. The relative strength of slow to fast components is highlighted and the importance of higher order absorption processes at the highest pump level is demonstrated

Phase Dynamics Of Inas/Gaas Quantum Dot Semiconductor Optical Amplifiers

The gain and phase dynamics of InAs∕GaAs quantum dot amplifiers are studied using single and two-color heterodyne pump probe spectroscopy. The relaxation of the wetting layer carrier density is shown to have a strong effect on the phase dynamics of both ground and excited state transients, while having a much weaker effect on the gain dynamics. In addition, the dynamical alpha factor may also display a constant value after an initial transient. Such behavior is strongly encouraging for reduced pattern effect operation in high speed optical networks

The Fast Recovery Dynamics Of A Quantum Dot Semiconductor Optical Amplifier

We consider a rate equation model of a quantum dot semiconductor optical amplifier that takes into account carrier capture, escape, and Pauli blocking processes. We evaluate possible differences between phonon-assisted or Auger processes being dominant for recovery. An analytical solution which corresponds to phonon-assisted interaction is then used to accurately fit experimental recovery curves and allows an estimation of both the carrier capture and escape rates

Mixed State Effects In Waveguide Electro-Absorbers Based On Quantum Dots

Multi-pulse heterodyne pump-probe measurements are used to investigate the reverse bias dynamics of InAs/GaAs quantum dots in a waveguide structure. Using a femtosecond pulse, we simultaneously populate high energy ground states and low energy excited states and measure the resulting gain and phase dynamics over the bandwidth of the pulse. We identify a ∼5 ps timescale in the phase dynamics which can be associated with low energy ground states outside the pulse bandwidth and may provide an explanation for the deterioration of monolithic mode locked laser performance at high reverse voltages

Induced Absorption Dynamics In Quantum Dot Based Waveguide Electroabsorbers

Two-color pump-probe measurements are used to study the carrier dynamics of InAs/GaAs quantum dots in a waveguide structure under reverse bias conditions. For the case of initially populating the ground state (GS), we find relaxation dynamics that include both absorptive and bleaching components in the excited state (ES) wavelength range. We reproduce the main features of this induced absorption dynamics using a simple model with an additional term for induced absorption at the ES due to carriers injected at the GS. The induced absorption dynamics includes multiple recovery timescales which can be attributed to phonon-assisted processes of GS/ES interaction

Intradot Dynamics Of Inas Quantum Dot Based Electroabsorbers

The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots’ ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively

Recovery Time Scales In A Reversed-Biased Quantum Dot Absorber

The nonlinear recovery of quantum dot based reverse-biased waveguide absorbers is investigated both experimentally and analytically. We show that the recovery dynamics consists of a fast initial layer followed by a relatively slow decay. The fast recovery stage is completely determined by the intradot properties, while the slow stage depends on the escape from the dot to the wetting layer

Refractive Index Dynamics Of Quantum Dot Based Waveguide Electroabsorbers

The refractive index dynamics of InAs/GaAs quantum dot based waveguide absorbers is studied using heterodyne pump-probe measurements. Absorption reduction due to the pump can be accompanied by either positive or negative refractive index changes depending on the wavelength used. This change in sign of the phase amplitude coupling can be understood by considering the atomlike nature of the quantum dot transitions involved

Porting marine ecosystem model spin-up using transport matrices to GPUs

We have ported an implementation of the spin-up for marine ecosystem models based on transport matrices to graphics processing units (GPUs). The original implementation was designed for distributed-memory architectures and uses the Portable, Extensible Toolkit for Scientific Computation (PETSc) library that is based on the Message Passing Interface (MPI) standard. The spin-up computes a steady seasonal cycle of ecosystem tracers with climatological ocean circulation data as forcing. Since the transport is linear with respect to the tracers, the resulting operator is represented by matrices. Each iteration of the spin-up involves two matrix-vector multiplications and the evaluation of the used biogeochemical model. The original code was written in C and Fortran. On the GPU, we use the Compute Unified Device Architecture (CUDA) standard, a customized version of PETSc and a commercial CUDA Fortran compiler. We describe the extensions to PETSc and the modifications of the original C and Fortran codes that had to be done. Here we make use of freely available libraries for the GPU. We analyze the computational effort of the main parts of the spin-up for two exemplar ecosystem models and compare the overall computational time to those necessary on different CPUs. The results show that a consumer GPU can compete with a significant number of cluster CPUs without further code optimization

Delay Induced Excitability

We analyse the stochastic dynamics of a bistable system under the influence of time-delayed feedback. Assuming an asymmetric potential, we show the existence of a regime in which the systems dynamic displays excitability by calculating the relevant residence time distributions and correlation times. Experimentally we then observe this behaviour in the polarization dynamics of a vertical cavity surface emitting laser with opto-electronic feedback. Extending these observations to two-dimensional systems with dispersive coupling we finally show numerically that delay induced excitability can lead to the appearance of propagating wave-fronts and spirals.Comment: 5 pages, 6 figure