Starke Kopplung, Lasing und Manipulation der Lichtauskopplung durch Mikrokavitäten verschiedener Geometrien

Abstract

During the last decades semiconductor microcavities have gained a lot of interest for technological applications as well as for fundamental research. This includes the use as basic component in vertical-cavity surface-emitting lasers (VCSELs), which are well established in data-communication. In addition they present a system for studying cavity quantum electrodynamic phenomena. Semiconductor quantum wells or quantum dots can be integrated as active media into the microcavity. The integration of quantum wells is interesting for laser applications and for entering the strong-coupling regime. The latter one beeing promising for a new kind of lasers with reduced laser threshold. By integrating quantum dots into pillar microcavities, their outcoupling efficiency can be enhanced via the Purcell effect making this system a very promising approach for the realization of efficient solid-state single-photon sources. These single-photon sources are a key element for applications in the field of quantum-information processing like, e.g., quantum cryptography. In this thesis, microcavities with different geometries and different active media (quantum wells or quantum dots) are studied by microphotoluminescence (µPL) experiments in real and k-space. By means of focused ion beam (FIB) milling the investigated structures are processed out of ZnSe- and GaN-based planar microcavities. For the first time, an optical characterization of monolithically grown microcavities with three embedded ZnSe quantum wells is presented. Excitation density dependent measurements reveal a superlinear increase of the PL intensity. A laser threshold of 5 pJ is determined. Reflectivity measurements in real and k-space show clear indications of the lower and upper polariton branches demonstrating the strong coupling regime. For the Rabi splitting, a value of 19 meV is determined. It is an efficient way of generating polarized single photons if the emission of a quantum dot is brought into spectral resonance with a polarized cavity mode. Therefore, the use of pillar microcavities with elliptical geometry is shown as an efficient way to lift the polarization degeneracy of the cavity modes. The efficiency of this method is studied by investigating the polarization degree of the emitted light. The emission of a single quantum dot is tuned into a polarized mode and out again by temperature variation allowing for a switching of the polarization state of the emitted light. Connected microcavities with different pillar diameters were processed by FIB etching. Within these structures, the individual optical fields of the two cavities couple to each other leading to the evolution of new optical states. The optical field distribution is studied by means of an investigation of the far field emission. In this way, the two-dimensional field distribution of a structure with different pillar diameters was experimentally obtained for the first time, revealing the coexistence of localized as well as delocalized modes in the same structure. Furthermore, the internal mode distribution of elliptically shaped microcavities is investigated and an analogy to the coupled pillar microcavities is revealed. This analogy allows the treatment of elliptical microcavities as a system of coupled cavities with a rather large coupling constant. The emission properties of monolithic GaN-based VCSEL-structures with an active layer of InGaN quantum dots are investigated. For these structures, an enhancement of the outcoupling efficiency of the quantum dot emission due to a coupling to the cavity modes was observed for the first time. These results demonstrate the huge potential of quantum cavity electrodynamics and the possibility to tailor the emission properties of semiconductor quantum wells and dots by use of microcavities with different geometries.