PLD-grown Thin Film Saturable Absorbers

The subject of this thesis is the preparation and characterization of thin films made of oxidic dielectrics which may find their application as saturable absorber in passively Q-switched lasers. The solely process applied for fabrication of the thin films was the pulsed laser deposition (PLD) which stands out against other processes by its flexibility considering the composition of the systems to be investigated. Within the scope of this thesis the applied saturable absorbers can be divided into two fundamentally different kinds of functional principles:On the one hand, saturable absorption can be achieved by ions embedded in a host medium. Most commonly applied bulk crystals are certain garnets like YAG (Y3Al5O12) or the spinel forsterite (Mg2SiO4), in each case with chromium as dopant. Either of these media was investigated in terms of their behavior as PLD-grown saturable absorber. Moreover, experiments with Mg2GeO4, Ca2GeO4, Sc2O3, and further garnets like YSAG or GSGG took place. The absorption coefficients of the grown films of Cr4+ :YAG were determined by spectroscopic investigations to be one to two orders of magnitude higher compared to commercially available saturable absorbers. For the first time, passive Q-switching of a Nd:YAG laser at 1064 nm with Cr4+ :YAG thin films could be realized as well as with Cr:Sc2O3 thin films.On the other hand, the desirable effect of saturable absorption can also be generated by quantum well structures. For this purpose, several layer system like YAG/LuAG, Cu2O/MgO, and ZnO/corumdum were investigated. It turned out that layer systems with indium oxide (In2O3) did not only grew in an excellent way but also showed up a behavior regarding their photo luminescence which cannot be explained by classical considerations. The observed luminescence at roughly 3 eV (410 nm) was assumed to be of excitonic nature and its increasing oscillator strength against thinner films was assumed to be due to confining.The grown thin films were structurally investigated with Reflection High-Energy Electron Diffraction (RHEED). It could be observed that the applied cubic systems like Sc2O3, In2O3, or YAG kept the orientation of the substrate, thus they are said to grow in an epitaxial way. Considering thin films of Cr:Sc2O3, one could observe epitaxial Frank-van der Merwe growth (layer-by-layer) even after the growth of films with thicknesses in a macroscopic scale.The appearance of particulates is a major drawback when preparing thin films by pulsed laser deposition. Within this thesis, the deposition process could considerably be improved by the implementation of a velocity filter. Despite the filter's installation one could still make use of the measurement equipment like pyrometer, reflectometer, and the RHEED system. Furthermore, the existing facility was improved in such a way that the growth of multilayered systems was possible with only little effort which significantly simplified the growth of the mentioned quantum well structures.