On February 16th, 2011, Lukasz Olejniczak successfully defended his PhD thesis and was awarded the degree of Doctor in Engineering. His PhD research focused on "Polarization properties and nonlinear dynamics of quantum dot lasers" and was done in a joint PhD co-tutelle project between VUB and Supelec. The promotors of his PhD were prof. dr. sc. Krassimir Panajotov, B-Phot-VUB, Brussels, Belgium and prof. dr. ir. Marc Sciamanna, Supelec, Metz, France.
The goal of this work was twofold: First, to study experimentally polarization properties of quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs with submonolayer (SML) and Stranski-Krastanov (SK) QDs and second, to study theoretically the nonlinear dynamics of QD edge emitting lasers induced by optical injection.
His investigations showed that SML QD VCSELs exhibit very stable and reproducible polarization switching accompanied by polarization mode hopping between non-orthogonal, elliptically polarized states. Close to the lasing threshold the emission is linearly polarized. At some current it changes to stable elliptical polarization. The ellipticity and polarization angle increase with the current. Next, a region of polarization mode hopping between elliptically polarized, non-orthogonal modes starts. This mode hopping is symmetric in a large current region with average dwell time decreases by 8 orders of magnitude, from seconds to nanoseconds, without any external manipulation to the inherent anisotropies. This is the first observation of polarization switching between non-orthogonal elliptically polarized states and polarization mode hoping with unusual dwell time scaling.
Optically injected QD EELs are studied by an extended model that accounts for the QD excited states. Bifurcation maps in the plane detuning vs. injection strength show that relaxation (capture) time has a strong (weak) impact on the dynamics. In close proximity of the codimension-two saddle-node Hopf bifurcation point for negative detuning he has unveiled a region of complex dynamics including bistability, period-one and period-two time-periodic, and chaotic oscillations. Close to the saddle-node on limit cycle bifurcation bordering the locking region he has identified regions of deterministic self-pulsations with interpulse time depending on detuning in a inverse square root scaling law. Dynamics of the ground-state mode are limited to single-frequency locking (excited-state mode suppressed), double-frequency locking (excited-state mode unsuppressed), and unlocked time-periodic intensity oscillations. We have unveiled regime of picosecond pulse generation caused by a gain switching mechanism due to modulation of the relaxation time by the injection-induced oscillations in the occupation of the energy states.