Coupled Networks, Patterns and Complexity - Abstract
Lüdge, Kathy
Quantum-dot (QD) laser devices offer a variety of advantages over conventional quantum-well (QW) lasers due to their low threshold currents and their high temperature stability. In general, QD lasers show strongly damped relaxation oscillations (RO) after an electric pump pulse, which decreases their sensitivity to optical feedback. On the other hand, the possibility for fast dynamic response to an electrically modulated pump current is limited by the small RO frequency and the strong damping of the RO oscillations. The resulting comparably low modulation bandwidth of QD lasers is often attributed to the slow charge carrier capture into the QDs. However, we show that this is in fact not the limiting process, as the impact of the electron-electron scattering between QD and QW on the modulation properties is strongly nonlinear, opening the possibility to optimize the device performance by band structure engineering. Asymptotic analysis of the complex rate equation system reveals a decrease of the damping of the ROs with decreasing electron scattering rates, which allows to improve the modulation properties (compare solid and dashed line in Fig.1a). Nonetheless the dependency between RO damping and lifetime is reversed as soon as the scattering lifetimes between QD and QW carriers become larger than the QW carrier lifetimes. Beyond this point a further increase of the scattering rates leads again to a drastic reduction of the RO damping rate. Additionally we investigated the QD laser under an external optical signal and optical feedback. We found that changes in the carrier scattering lifetimes and hereby in the damping of the ROs lead to variations in the dynamics of the injected QD laser. Especially the locking behavior as well as the bifurcation scenarios are sensitive to the RO damping rate.