Abstract
The work in this thesis is concerned with novel routes towards producing low-index optical coatings for laser optics via sol-gels. Optical coatings have been prepared traditionally by CVD or sputtering. Recently sol-gel methods have been employed. Here coatings have been prepared for potential applications in high-power lasers as optical components (HR or AR). Two systems have been investigated: organically modified silica and magnesium fluoride. A standard TEOS-based silica sol-gel was modified with varying amounts of the organic tether (1,6-bis(trimethoxysilyl)hexane) in both acidic and basic conditions in ethanolic solvent. The base-catalysed route was investigated thoroughly. In such materials shrinkage due to drying was minimised. This resulted in materials that were highly porous, of high surface areas (similar to an aerogel but dried at ambient conditions) and with some degree of flexibility and hydrophobicity. The derived coatings were highly transparent in the UV region with very low refractive indices. The preparation of magnesium fluoride investigated was alkoxide-based. Products were of very high surface area and porosity. The derived thin-films exhibited -99% transmission and low refractive index. Such films were found to be far superior to films produced by CVD, but inferior to those of sol-gel silica. Both the optimised organically modified silica and the MgF2 sols consisted of highly networked structures and average particle sizes between 5-7nm. Both materials were also very resistant to laser induced damage (which is uncharacteristic in materials of such high porosity). Highly porous silica is a material known to adsorb considerable amounts of water that eventually results in collapse of pores due to capillary pressure. BIS-modifieation proved to reduce the amount of adsorption by water as shown by TG/DTA analysis. The highly porous magnesium fluoride was also found to be particularly susceptible to adsorption by water.