Rare earth doped nanocrystals in sol-gel planar waveguides

Thin films of controlled refractive index may be conveniently prepared by sol-gd processing and thick multilayer deposits may be used as planar waveguides for integrated optics (IO). Doping of these films with rare-earth elements such as Nd or Er enables the fabrication of IO devices such as lasers or amplifiers; incorporation of these active elements into nanocrystallites may increase their fluorescence efficiency. This paper describes the controlled growth of nanocrystallites in SiO2-TiO2 sol-gel films through suitable heat treatments. Both undoped and Er-doped (0.5-2.0 mol%) thin films were deposited by spin-coating onto vitreous silica or single crystal silicon substrates and they were densified in the form of amorphous multilayers. These were then subjected to controlled heat treatments at temperatures between 900 degrees C and 1100 degrees C, for periods of time up to 2 hours. The growth of nanocrystallites was followed by grazing incidence X-ray diffraction (XRD), Raman spectrometry and Transmission Electron Microscopy (TEM). The major crystalline phases found were anatase, rutile and Er2Ti2O7 and the relationship between the crystallization of Er2Ti2O7 and the formation of the rutile or anatase phases of titania was investigated The experimental results show that in the absence of Er, the transformation is mostly to anatase;for low erbium concentrations (0-0.5 mol% Er), anatase is the main crystalline phase formed, whereas rutile is a minor one. Almost no anatase is found in systems with Er concentrations higher than 1 mol%, for which the transformation to nanocrystalline rutile and Er2Ti2O7 is preferred, for high enough temperature and sufficiently long treatment times. A good qualitative agreement was found between the XRD and Raman results. The crystallite sizes, observed in the TEM micrographs, were found to be in the range of 50-100 nm. Through heal treatment of the Er-doped SiO2-TiO2 films, the Er fluorescence lifetime of the 1.55 mu m transition could be increased by up to 100%. These values may be further improved by proper dehydration of the sol-gel films.