Energy transfer in erbium doped optical waveguides based on silicon

Abstract

Energy transfer in erbium doped optical waveguides based on silicon This thesis describes the energy transfer processes occurring in materials that can be used for the fabrication of silicon compatible optical integrated circuits, operating at 1.54 mm.The thesis consists of three parts: Part I discusses energy transfer in optical waveguide amplifiers on silicon. The waveguide amplifiers use an intra-4f transition of the rare earth ion erbium occurring at 1.54 mm. To achieve significant amplification, high Er concentrations (> 10 20 cm -3 ) are required. At these concentrations Er ions can interact, resulting in co-operative upconversion, in which one excited Er ion is excited into a higher energy state, while a neighboring Er ion returns to the ground state. In Part I this and other gain limiting effects are investigated. In particular, it is shown that the atomic-scale Er distribution is of crucial importance for the performance of Er doped waveguide amplifiers. Part II discusses the energy transfer processes occurring in Er doped silicon. It is shown that Er in silicon can be excited by electron-hole pair recombination in an impurity-Auger effect. This effect may be used to fabricate an electrically pumped Er doped waveguide amplifier. The reverse process can also occur: an excited Er ion can de-excite and generate an electron-hole pair in the silicon, due to a non-radiative backtransfer process. This process has an efficiency of 70% at room temperature. This observation enables the fabrication of an Er doped photo-detector in a Si waveguide operating at 1.54 mm. The performance of such a detector is discussed. Part III discusses the interaction between Er and silicon nanocrystals embedded in SiO2. Si nanocrystals efficiently absorb visible light, resulting in the generation of quantum confined electron-hole pairs. The electron-hole pairs efficiently transfer energy to Er. Due to the energy transfer, the effective Er absorption cross section is five orders of magnitude larger than for direct optical excitation. Silicon nanocrystals may thus be used to increase the pumping efficiency of an Er doped waveguide amplifier. It is shown that a single nanocrystal can excite ~1 Er ion at a time and that an optical gain of 0.6 dB/cm may be achieved in a typical nanocrystal sensitized waveguide amplifier. Due to the broad absorption band of Si nanocrystals, a broad band light source can be used as pump source in such a nanocrystal sensitized Er doped waveguide amplifier.