Ultrafast polarization dynamics in QD spin VCSELs

• Κύριος συγγραφέας: Τσέλιος, Χρήστος
• Άλλοι συγγραφείς: Tselios, Christos
• Γλώσσα: Greek
• Έκδοση: 2022
• Θέματα: Polarization dynamics; Nonlinear dynamics; Spintronics; Optical communication systems; Δυναμικά πόλωσης; Μη γραμμικά δυναμικά

Ultrafast polarization dynamics in spin- Vertical-Cavity Surface-Emitting Lasers (VCSELs) have become an active area of research motivated by potential applications. This thesis explores the effect of materials used in the active region, cavity anisotropies and optical pumping on the polarization control and polarization oscillations in the case of Quantum-Dot (QD) spin- VCSEL. We have focused on a comprehensive theoretical analysis of QD spin- VCSEL by means of Spin Flip Model (SFM) and both Largest Lyapunov Exponent (LLE) and bifurcation diagrams to determine regions of stability. The dependence of the shape and the size of stability regions on additional parameters for the case of QD spin- VCSEL is investigated and the results are demonstrated with Three-Dimensional (3D) stability maps in the plane of pump ellipticity and normalized pump intensity. This research is extended to investigate the dependence of the output polarization ellipticity on that of the pump. The combination of Poincaré sphere and Stokes vectors reveals rich nonlinear dynamics, such as periodic oscillations and chaos. More importantly, Two-Dimensional (2D) contour maps for output polarization ellipticity shows that polarization switching and polarization control are evident inside the stability region. We then investigate the sustained polarization oscillations, which are attributed to Hopf Bifurcation (HB). Power Spectral Density (PSD) plots demonstrate that output polarization ellipticity oscillates with a frequency determined by birefringence, leading to oscillations in the tens of Gigahertz for high values of birefringence. The polarization response at these high values of birefringence is studied in terms of 2D contour maps for Stokes vectors in the plane of birefringence rate and pump ellipticity. The results show polarization behaviours ranging from spin amplification to less circular output polarization. In order to investigate the capability of the optical pumping to control the output polarization time varying injection with polarized carriers with different spin is applied. We also implement a new metric system to find the optimum values of device parameters for faster temporal switching between output polarization degree. By encoding information in the polarization of the injected carriers faster operation is feasible. Using eye diagrams we highlight that QD spin VCSELs under polarization modulation can achieve data transfer at high bit rates, a crucial feature for ultrafast communication systems.