New Materials for Ultrafast Switching and Optical Regeneration

Abstract

This thesis consists of an experimental study of materials which are promising for the development of integrated optical devices on polymer based platforms. Such devices should be able to perform different functions, including optical amplification and optical switching. Materials for electro-optical switching are studied in the first part of this work, while the second part is dedicated to nanocrystals for optical amplification.

2nd order nonlinear optical chromophores have a large figure of merit compared to more traditional materials, but suffer photodegradation after extended irradiation. An in depth study was performed of the different parameters affecting the photostability and of strategies leading to an improvement of the stability. The addition of excited state quenchers or the removal of oxygen led to dramatic improvements of the photostability. The intensity dependence of the photodegradation quantum efficiency was related to oxygen depletion in the polymer films at high intensities. Efforts were made to obtain 2nd order nonlinear inorganic-organic hybrid materials by incorporating nanocrystals, which exhibit 2nd order nonlinear effects, into polymers. Therefore ferroelectric nanoparticles (BaTiO3 and BaMgF4) were synthesized and incorporated into polymer films. Poling of these composite films was attempted using an electric or magnetic field, but only a limited degree of orientation was obtained due to clustering of the particles.

Fluoride nanocrystals such as LaF3, KMgF3 and BaLiF3 doped with rare earth and transition metal ions are promising materials for polymer optical amplifiers. The structural and optical properties of these nanocrystals were analyzed. In particular Eu3+ doped LaF3 nanocrystals possess a high internal quantum efficiency. The influence of the capping ligand on the optical properties was studied and by selecting certain ligands it was possible to obtain energy transfer from the capping ligand to the rare earth ions following excitation of the ligand. In this way the absorption cross section of the rare earth ions was able to be increased by more than two orders of magnitude. It is demonstrated that rare earth ions located at the surface experience a different crystal field compared to ions located at the center of the particles.

It is also possible to use quantum dots as the active material for optical amplifiers. Therefore, the optical properties of transparent alloy CdSe1-xSx quantum dot/polymer composites which emit in the visible were investigated. When the selenium concentration was increased a red shift and an increase in the quantum yield of the photoluminescence emission was found. Efficient energy transfer between the quantum dots in these polymer composites was observed due to clustering. No sign of dark excitons was detected at low temperatures indicating a spatial separation of the electron and hole which is beneficial for optical amplification.