André Röhm

Understanding Ground-State Quenching in Quantum-Dot Lasers

Quantum-dot lasers can exhibit simultaneous ground- and excited-state lasing. With increasing pump current, a quenching of the ground-state lasing intensity is sometimes observed. The causes for this are investigated, and its dependence on temperature, gain, and electron-hole asymmetry is studied via an analytical approach. A numerical model based on the semiconductor Bloch equations with a set of rate equations for electrons and holes is used for validation. We also investigate the influence of doping and different cavity lengths on the two-state lasing dynamics. We find that ground-state quenching is more common in p-doped, short cavity devices with low gain.

Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation

The impact of ground state amplification on the laser emission of In(Ga)As quantum dot excited state lasers is studied in time-resolved experiments. We find that a depopulation of the quantum dot ground state is followed by a drop in excited state lasing intensity. The magnitude of the drop is strongly dependent on the wavelength of the depletion pulse and the applied injection current. Numerical simulations based on laser rate equations reproduce the experimental results and explain the wavelength dependence by the different dynamics in lasing and non-lasing sub-ensembles within the inhomogeneously broadened quantum dots. At high injection levels, the observed response even upon perturbation of the lasing sub-ensemble is small and followed by a fast recovery, thus supporting the capacity of fast modulation in dual-state devices.

Small chimera states without multistability in a globally delay-coupled network of four lasers.

We present results obtained for a network of four delay-coupled lasers modelled by Lang-Kobayashi-type equations. We find small chimera states consisting of a pair of synchronized lasers and two unsynchronized lasers. One class of these small chimera states can be understood as intermediate steps on the route from synchronization to desynchronization and we present the entire chain of bifurcations giving birth to them. This class of small chimeras can exhibit limit-cycle or quasiperiodic dynamics. A second type of small chimera states exists apparently disconnected from any region of synchronization, arising from pair synchronization inside the chaotic desynchronized regime. In contrast to previously reported chimera states in globally coupled networks, we find that the small chimera state is the only stable solution of the system for certain parameter regions, i.e. we do not need to specially prepare initial conditions.

Ground-state modulation-enhancement by two-state lasing in quantum-dot laser devices

We predict a significant increase of the 3 dB-cutoff-frequency on the ground-state lasing wavelength for two-state-lasing quantum-dot lasers using a microscopically motivated multi-level rate-equation model. After the onset of the second lasing line, the excited state acts as a high-pass filter, improving the ground-state response to faster modulation frequencies. We present both numerically simulated small-signal and large-signal modulation results and compare the performance of single and two-state lasing devices. Furthermore, we give dynamical arguments for the advantages of two-state lasing on data-transmission capabilities.

Bistability in two simple symmetrically coupled oscillators with symmetry-broken amplitude- and phase-locking

In the model system of two instantaneously and symmetrically coupled identical Stuart-Landau oscillators, we demonstrate that there exist stable solutions with symmetry-broken amplitude- and phase-locking. These states are characterized by a non-trivial fixed phase or amplitude relationship between both oscillators, while simultaneously maintaining perfectly harmonic oscillations of the same frequency. While some of the surrounding bifurcations have been previously described, we present the first detailed analytical and numerical description of these states and present analytically and numerically how they are embedded in the bifurcation structure of the system, arising both from the in-phase and the anti-phase solutions, as well as through a saddle-node bifurcation. The dependence of both the amplitude and the phase on parameters can be expressed explicitly with analytic formulas. As opposed to the previous reports, we find that these symmetry-broken states are stable, which can even be shown analytically. As an example of symmetry-breaking solutions in a simple and symmetric system, these states have potential applications as bistable states for switches in a wide array of coupled oscillatory systems.

Bistability in optically injected two-state quantum dot lasers

Semiconductor lasers based upon self-assembled quantum dots (QDs) are a promising source for applications in optical networks. Their charge-carrier confinement in all three spatial dimensions results in a range of special attributes such as ultra low threshold current, high material gain, small line width and weak temperature dependence when compared to conventional quantum well lasers [1]. When operated under proper conditions, QD based laser can not only emit light from the QD ground state (GS) but also from the first excited state (ES). The relatively slow QD carrier dynamics give rise to an incomplete gain clamping of the ES population, which consequently allows for simultaneous two-state lasing. This phenomenon, not to be confused with multi-mode lasing, is so far unique among all lasers. Moreover, some material systems exhibit a decline or even complete quenching of the GS intensity after the onset of ES lasing [2]. All optical switching of GS and ES lasing and an optically induced hysteresis via injection into the GS of such quenched devices have been experimentally observed and recently reported [3]. We numerically investigate the dynamics of such devices under external optical injection into the ground state using a model, that is based upon semiconductor-Bloch-equations and includes microscopically calculated charge-carrier scattering rates.

Understanding QD Laser Regimes of Operation

Understanding GS-quenching is made difficult, by the high dimensionality of the system and the many experimentally not accessible variables. When described in a non-excitonic picture, one needs to at least include four different carrier reservoirs and two electric field amplitudes. Korenev et al. [KOR13a] have analytically solved a somewhat reduced representation of this system, by assuming a common hole GS level and neglecting charge conservation. They are able to derive GS-quenching light-current characteristics by assuming that holes are less likely to enter the QD than electrons. Yet they lack an explicit modelling of the current dependence.

Theoretical Concepts of Lasers

Lasers are light sources with very narrow bandwidths, high output power and long coherence lengths [HAK86]. They are, in some regards, completely different from other light sources, that surround us every day. As opposed to the thermal radiation of light bulbs, stars and the sun, or the fluorescence used in neon tubes, the photons of a laser are mainly emitted through stimulated emission.

Pump-Probe Experiments

In this chapter two sets of experiments with a two-state QD device shall be described and their respective behaviour will be reproduced by the numerical QD model. The experiments were carried out by the work group of Prof. Woggon (TU Berlin), in particular Yuecel Kaptan and Bastian Herzog.

Modes of Operation of QD Lasers

Before delving into the dual-colour or two-state lasing aspects of the QD laser model introduced in the previous section, this section will shortly review the single-colour dynamics of a QD laser. This is done by setting the ES gain gES = 0, so that only GS lasing is achieved. Then, the ES simply acts as an intermediate reservoir for the carriers.