This work is devoted to investigate light confinement in rolled-up microtubular cavities and their optofluidic applications. The microcavities are fabricated by a roll-up mechanism based on releasing pre-strained silicon-oxide nanomembranes. By defining the shape and thickness of the nanomembranes, the geometrical tube structure is well controlled. Micro-photoluminescence spectroscopy at room temperature is employed to study the optical modes and their dependence on the structural characteristics of the microtubes. Finite-difference-time-domain simulations are performed to elucidate the experimental results. In addition, a theoretical model (based on a wave description) is applied to describe the optical modes in the tubular microcavities, supporting quantitatively and qualitatively the experimental findings. Precise spectral tuning of the optical modes is achieved by two post-fabrication methods. One method employs conformal coating of the tube wall with Al2O3 monolayers by atomic-layer-deposition, which permits a mode tuning over a wide spectral range (larger than one free-spectral-range). An average mode tuning to longer wavelengths of 0.11nm/ Al2O3-monolayer is obtained. The other method consists in asymmetric material deposition onto the tube surface. Besides the possibility of mode tuning, this method permits to detect small shape deformations (at the nanometer scale) of an optical microcavity. Controlled confinement of resonant light is demonstrated by using an asymmetric cone-like microtube, which is fabricated by unevenly rolling-up circular-shaped nanomembranes. Localized three-dimensional optical modes are obtained due to an axial confinement mechanism that is defined by the variation of the tube radius and wall windings along the tube axis. Optofluidic functions of the rolled-up microtubes are explored by immersing the tubes or filling their core with a liquid medium. Refractive index sensing of liquids is demonstrated by correlating spectral shift of the optical modes when a liquid interacts with the resonant light of the microtube. In addition, a novel sensing methodology is proposed by monitoring axial mode spacing changes. Lab-on-a-chip methods are employed to fabricate an optofluidic chip device, allowing a high degree of liquid handling. A maximum sensitivity of 880 nm/refractive-index-unit is achieved. The developed optofluidic sensors show high potential for lab-on-a-chip applications, such as real-time bio/chemical analytic systems.

Im Rahmen dieser Arbeit wurden die optischen Eigenschaften von auf ZnO-basierenden Heterostrukturen untersucht. Besonderes Augenmerk lag hierbei auf ihrer Eignung als aktives Material in Laserdioden für den ultravioletten und sichtbaren Spektralbereich. Es wurde gezeigt, dass ZnO und seine ternären Mischkristalle ZnCdO und ZnMgO erstaunlich vielfältige Anwendungen ermöglichen. Mit diesem Materialsystem lässt sich sowohl ein sehr großer Spektralbereich für Lasertätigkeit abdecken als auch eine Vielzahl von Laseranordnungen realisieren. Im Detail wurde demonstriert, dass sich die Lasertätigkeit von ZnCdO/ZnO Quantengraben-Strukturen vom violetten bis in den grünen Spektralbereich verschieben lässt. Obwohl diese Strukturen starke interne elektrische Felder aufweisen, konnte optisch gepumpte Lasertätigkeit bei Zimmertemperatur bis zu einer Wellenlänge von 510 nm gezeigt werden. Die für die Lasertätigkeit nötige optische Rückkopplung wird durch makroskopische Defekte der Probe verursacht und die Proben fungieren somit als Zufallslaser. Die Herstellung von Mikroresonatoren ermöglichte die Untersuchung des Zusammenspiels von Fabry-Perot- und Zufalls-Rückkopplung. Die experimentellen und theoretischen Ergebnisse zeigen, dass der Schwellengewinn eines Zufallslasers in der Regel größer ist als der des Fabry-Perot-Lasers. Des Weiteren wurde gezeigt, dass hoch reflektierende Braggreflektoren für den ultravioletten und blau/grünen Spektralbereich aus ZnO- und ZnMgO-Schichten hergestellt werden können. Ferner wurden die teils unbekannten Brechungsindexverläufe der verwendeten ternären Materialen erarbeitet und Mikrokavitäten mit ZnO/ZnMgO Quantengraben Strukturen als aktive Schichten realisiert. An diesen Kavitäten konnte bei Temperaturen bis zu 150 K starke Kopplung zwischen Exzitonen und Photonen nachgewiesen werden. Bei Zimmertemperatur konnte vertikal-emittierende Lasertätigkeit im nahen ultravioletten Spektralbereich demonstriert werden. / In the framework of this thesis, the optical properties of ZnO-based heterostructures fabricated by molecular beam epitaxy have been investigated, particularly with regard to their suitability for semiconductor laser devices operating in the ultraviolet and visible spectral range. It turned out that ZnO and its ternary alloys ZnMgO and ZnCdO are extremely versatile. They allow to tune the laser emission in a wide spectral range as well as to realize various laser geometries. In detail, it was shown that the laser emission of ZnCdO/ZnO multiple quantum wells can cover a spectral range from violet to green wavelengths. Although these structures suffer from large built-in electric fields, room temperature laser action under optical pumping was demonstrated up to a wavelength of 510. The optical feedback for lasing is provided by growth imperfections on a macroscopic length scale turning these structures into random lasers. The fabrication of micro-resonators allowed to study the interplay between random and Fabry-Perot feedback. The experimental and theoretical analysis shows that random feedback generally requires a larger gain than under Fabry-Perot feedback. Further, this work demonstrates that ZnO- and ZnMgO-layers can be used to fabricate highly reflective distributed Bragg reflectors for applications in the ultraviolet and blue/green spectral range. The partly unknown dispersion curves of the index of refraction of the employed ternary alloys have been elaborated. This enabled the realization of all monolithic microcavities with ZnO/ZnMgO quantum wells as active zone. For temperatures below 150 K strong exciton-photon coupling is observed in such microcavities. At room temperature, vertical cavity surface emitting laser action in the near UV spectral range is demonstrated for appropriately designed microcavities.