Semiconductor Nanostructures: From Spin Lasers to Nodal Ground States
MetadataShow full item record
This thesis consists of two parts. In the first part, characteristics of spin lasers are studied. Considering circular polarization, an optical analog of electron spin, semiconductor lasers with spin-polarized carriers can open up unexplored possibilities for spin-controlled devices. Once spin-polarized carriers are introduced in the gain region of lasers, by circularly polarized light or electrical spin injection, the operation of such spin lasers should be revisited to incorporate their novel properties. Spin-polarized carriers can enhance the performance of lasers for communication and signal processing. In the steady state, such spin lasers have already demonstrated a lower threshold current for the lasing operation compared to their conventional (spin-unpolarized) counterparts. However, the most exciting opportunities come from their dynamical operation. We reveal that the spin modulation in lasers can lead to an improvement in the two key figures of merit: enhanced bandwidth and reduced parasitic frequency modulation--chirp. Analyses are carried out under generalized modulation regimes we propose. Different characteristics of quantum dots and quantum wells as a gain medium are also discussed and we provide a mapping between the quantum-dot and quantum-well based lasers. In the second part, electronic states in semiconductor quantum dots are investigated. Unlike the common expectation, theoretical calculations in quantum wires and quantum dots have predicted hole ground state wave functions with a node that are often associated with the formation of dark excitons. The inversion of the energy-level ordering between nodeless (S-like) and nodal (P-like) wave-function states occurs due to various factors, e.g., confinement size and strength, choice of material, and spin-orbit interaction. However, the existence of nodal ground states has been debated and even viewed merely as an artifact of a k·p model. Using complementary approaches of both k·p and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that the nodal ground states in quantum dots are not limited to a specific theoretical model. Remarkably, the emergence of the nodal ground states can be attributed to the formation of the orbital vortex textures that minimizes "divergence''. We suggest an experimental test for our predictions of the reversed-energy ordering and the existence of nodal ground states. We discuss how our findings and the studies of orbital textures could be also relevant for other materials systems.