Kathy Lüdge

Quantum-Dot Lasers—Desynchronized Nonlinear Dynamics of Electrons and Holes

We analyze the complex turn-on behavior of semiconductor quantum-dot (QD) lasers in terms of a nonlinear rate equation model for the electron and hole densities in the QDs and the wetting layer, and the photons. A basic ingredient of the model is the nonlinearity of the microscopic carrier-carrier scattering rates. With the framework of detailed balance, we analytically relate the microscopic in- and out-scattering rates. We gain insight into the anomalous nonlinear dynamics of QD lasers by a detailed analysis of various sections of the 5-D phase space, accounting for density-dependent carrier scattering times. We show that the strongly damped relaxation oscillations are characterized by a desynchronization of electron and hole dynamics in the dots. Analytic approximations for the steady-state characteristics are also derived.

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.

Optically injected quantum dot lasers: impact of nonlinear carrier lifetimes on frequency-locking dynamics

Carrier scattering is known to crucially affect the dynamics of quantum dot (QD) laser devices. We show that the dynamic properties of a QD laser under optical injection are also affected by Coulomb scattering processes and can be optimized by band structure engineering. The nonlinear dynamics of optically injected QD lasers is numerically analyzed as a function of microscopically calculated scattering lifetimes. These lifetimes alter the turn-on damping of the solitary QD laser as well as the complex bifurcation scenarios of the laser under optical injection. Furthermore, we find a pump current sensitivity of the frequency-locking range, which is directly related to the nonlinearity of the carrier lifetimes.

Amplitude-phase coupling drives chimera states in globally coupled laser networks.

For a globally coupled network of semiconductor lasers with delayed optical feedback, we demonstrate the existence of chimera states. The domains of coherence and incoherence that are typical for chimera states are found to exist for the amplitude, phase, and inversion of the coupled lasers. These chimera states defy several of the previously established existence criteria. While chimera states in phase oscillators generally demand nonlocal coupling, large system sizes, and specially prepared initial conditions, we find chimera states that are stable for global coupling in a network of only four coupled lasers for random initial conditions. The existence is linked to a regime of multistability between the synchronous steady state and asynchronous periodic solutions. We show that amplitude-phase coupling, a concept common in different fields, is necessary for the formation of the chimera states.

Impact of carrier-carrier scattering and carrier heating on pulse train dynamics of quantum dot semiconductor optical amplifiers

We investigate the impact of carrier-carrier scattering on the gain recovery dynamics of a quantum dot (QD) semiconductor optical amplifier. Simulations, based on semiconductor Bloch equations with microscopically calculated Coulomb scattering rates between the carrier reservoir and the QDs, show a very good agreement with experimentally obtained pump-probe dynamics over a range of injection currents. With the microscopically obtained scattering rates at hand, we can conclude that fast cascading relaxation processes between the two-dimensional carrier reservoir and the QDs in combination with carrier heating enhancing the scattering efficiency drives the ultrafast gain recovery observed in QD based semiconductor devices.

Failure of the α factor in describing dynamical instabilities and chaos in quantum-dot lasers

We show that the long-established concept of a linewidth-enhancement factor α to describe carrier-induced refractive index changes in semiconductor lasers breaks down in quantum dot (QD) lasers when describing complex dynamic scenarios, found for example under high-excitation or optical injection. By comparing laser simulations using a constant α-factor with results from a more complex non-equilibrium model that separately treats gain and refractive index dynamics, we examine the conditions under which an approximation of the amplitudephase coupling by an α-factor becomes invalid. The investigations show that while a quasi-equilibrium approach for conventional quantum well lasers is valid over a reasonable parameter range, allowing one to introduce an α-factor as a constant parameter, the concept is in general not applicable to predict QD laser dynamics due to the different timescales of the involved scattering processes.

Feedback and injection locking instabilities in quantum-dot lasers: a microscopically based bifurcation analysis

We employ a nonequilibrium energy balance and carrier rate equation model based on microscopic semiconductor theory to describe the quantum-dot (QD) laser dynamics under optical injection and time-delayed feedback. The model goes beyond typical phenomenological approximations of rate equations, such as the ?-factor, yet allows for a thorough numerical bifurcation analysis, which would not be possible with the computationally demanding microscopic equations. We find that with QD lasers, independent amplitude and phase dynamics may lead to less complicated scenarios under optical perturbations than predicted by conventional models using the ?-factor to describe the carrier-induced refractive index change. For instance, in the short external cavity feedback regime, higher critical feedback strength is actually required to induce instabilities. Generally, the ?-factor should only be used when the carrier distribution can follow the QD laser dynamics adiabatically.

Complex dynamics of semiconductor quantum dot lasers subject to delayed optical feedback

We study a five-variable electron-hole model for a quantum-dot (QD) laser subject to optical feedback. The model includes microscopically computed Coulomb scattering rates. We consider the case of a low linewidth enhancement factor and a short external cavity. We determine the bifurcation diagram of the first three external cavity modes and analyze their bifurcations. The first Hopf bifurcation marks the critical feedback rate below which the laser is stable. We derive an analytical approximation for this critical feedback rate that is proportional to the damping rate of the relaxation oscillations (ROs) and inversely proportional to the linewidth enhancement factor. The damping rate is described in terms of the carrier lifetimes. They depend on the specific band structure of the QD device and they are computed numerically.

Quantum coherence induces pulse shape modification in a semiconductor optical amplifier at room temperature

Coherence in light–matter interaction is a necessary ingredient if light is used to control the quantum state of a material system. Coherent effects are firmly associated with isolated systems kept at low temperature. The exceedingly fast dephasing in condensed matter environments, in particular at elevated temperatures, may well erase all coherent information in the material at timescales shorter than a laser excitation pulse. Here we show for an ensemble of semiconductor quantum dots that even in the presence of ultrafast dephasing, for suitably designed condensed matter systems quantum-coherent effects are robust enough to be observable at room temperature. Our conclusions are based on an analysis of the reshaping an ultrafast laser pulse undergoes on propagation through a semiconductor quantum dot amplifier. We show that this pulse modification contains the signature of coherent light–matter interaction and can be controlled by adjusting the population of the quantum dots via electrical injection.

ErAs interlayers for limiting interfacial reactions in Fe/GaAs(100) heterostructures

In situ scanning tunneling microscopy and x-ray photoelectron spectroscopy were combined to examine the formation of the Fe/GaAs interface for Fe films grown on GaAs(100) As-rich surfaces by molecular beam epitaxy. Scanning tunneling microscopy images acquired following the growth of ultrathin layers of Fe on GaAs (2×4)/c(2×8)β2 surfaces show the initial growth of Fe results in little disruption of the As-dimer rows located directly adjacent to the deposited Fe clusters for growth temperatures between −15 and 175 °C. X-ray photoemission spectra show the interfacial Fe–Ga–As reactions depend on the growth temperature and can be minimized by growing at temperatures below 95 °C. However, approximately 0.7 ML of As was found to segregate to the Fe surface during growth, independent of the growth temperature. Atomic layer-by-layer calculations of the normalized intensity curves obtained from x-ray photoemission were used to quantify the extent of the interfacial reactions as a function of growth temperature. A...