Nonlinear and Nonequilibrium Dynamics of Quantum-Dot Optoelectronic Devices

Abstract

In this thesis the dynamics and performance of optoelectronic devices based on semiconductor quantum-dots are investigated.In the first part, the dynamics of quantum-dot lasers under external perturbations is discussed. Using a microscopically based balance equation model that incorporates detailed charge-carrier scattering dynamics and the possibility to describe nonequilibrium between intra-band electronic states, the relaxation oscillations of the quantum-dot laser are investigated. Three qualitatively different dynamic regimes are identified in dependence of the scattering rates -the "constantreservoir" regime for slow scattering, the "overdamped" regime, and the "synchronized" regime for high scattering -characterized by a varying degree of nonequilibrium between the quantum-dot and reservoir states.Important differences to conventional lasers are found in the modulation response and the dynamics in optical injection and feedback setups. Common theoretical models and approaches used to describe these applications are shown to yield inaccurate predictions, especially in the "constant-reservoir" and "overdamped" dynamic regimes. An important consequence is that the amplitude-phase coupling in quantum-dot lasers, commonly described by the α-factor, differs from conventional descriptions due to the desynchronization of gain and refractive index. While the α-factor describes bifurcations of fixed points accurately, it fails in describing dynamic solutions and overestimates the extent of complex dynamics. The observed low sensitivity to optical perturbations in quantum-dot lasers can therefore be attributed partly to the charge-carrier nonequilibrium. Three quantum-dot laser models on different levels of sophistication are presented that can accurately describe the quantum-dot nonequilibrium dynamics.In the second part of the thesis, the performance of quantum-dot semiconductor optical amplifiers is investigated, and two types of applications unique to quantum-dots as active medium are discussed. The ground and excited states of the quantum-dots allow an ultra-broad-band amplification of optical data streams. Amplified signals on the ground-state frequencies are shown to generally exhibit higher quality than on the excited state, due to a lower sensitivity of the groundstate to carrier-density variations. Nevertheless the quantum-dot amplifier is found to allow effective amplification on both frequency ranges. Furthermore, a parameter range is identified that allows for a simultaneous amplification of data signals on the ground and excited state in a counter-propagating setup.The long microscopically polarization dephasing times in quantum-dots are found to enable quantum-coherent interactions on a macroscopic scale at room-temperature. By comparison with experiments, the occurrence of Rabi-oscillations by amplification of ultra-short pulses is demonstrated. Quantum-dot based devices could therefore be used for future applications based on quantum-coherent effects.

Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser

“…Our experimental observations are in excellent agreement with the predictions of a numerical model of the pulsed, driven laser system obtained in the framework of the well-known semiconductor Bloch equations for lasers [21] [22] with microscopically motivated carrier dynamics [21]. A stochastic spontaneous emission noise source was introduced to simulate the effects of dephasing [21] [22].…”

“…A stochastic spontaneous emission noise source was introduced to simulate the effects of dephasing1718. The Bloch equations are extended by an additional equation describing the time-dependent carrier occupation in the reservoir, defined by the electronic states addressed by the excitation pulses tuned to an energy of 1.59 eV, and the resulting incoherent scattering processes into the lasing level at 1.51 eV16.…”

“…The spontaneous emission factor ( β ) was chosen to be β =0.01. For the numerical integration, the noise is implemented as a stochastic Gaussian white noise source ξ with noise strength refs 17, 18. The parameter g m is the coupling constant that determines the gain of the light–matter interaction.…”

Class-C semiconductor lasers with time-delayed optical feedback

“…We note that more complex formulations of the amplitude-phase coupling, as they appear, e.g. in semiconductor quantum-dot lasers [32][33][34][35], are not considered here, as they can usually be locally approximated by a constant, effective α-parameter [36]. We now proceed by analysing the steady-state solutions of the class-C laser with respect to their linear stability and eigenvalues.…”

Mode-switching induced super-thermal bunching in quantum-dot microlasers

“…The model used in this paper is based on semiclassical rate equations [27] taking into account the electron scattering mechanisms into the QDs as derived in our previous works [28,29] extended to the contribution of two optical modes. This is crucial as due to the cylindrical symmetry the microlaser exhibits two polarization modes, see sketch in figure 1(a).…”