Studies of some properties of hydroxide-catalysis bonds

Jointing materials is an inevitable step in the fabrication of many high performance optical devices. Today there is an increasing demand, both from industry and academic research, for reliable techniques for jointing of optical components which can be performed simply and inexpensively, where the bonds possess high strength and precision with low mechanical loss. Several methods of bonding have been defined and employed over time. Currently, the most common techniques of bonding are optical contacting, diffusion, epoxy and glass frit. Each of these techniques has positive aspects which make them appropriate for a range of applications, but not ideal in cases where a thin optically transparent and mechanically strong joint between optical compounds is required. Hydroxide-catalysis bonding is an interesting candidate in such cases: it has already been demonstrated to have excellent performance with respect to mechanical stability, precision and strength in numerous applications, such as in space systems, optics and gravitational wave detectors. At present, there are not many papers that talk exhaustively about the optical properties of hydroxide-catalysis bonding. Most of the time, the published results are closely dependent on requirements imposed by the experiment that is being conducted. Since there are a lot of interesting potential applications and increasing industrial interest in usage of hydroxide-catalysis bonding, it is important to develop techniques to characterise the optical properties of these bonds. In this thesis, a non-destructive technique for measuring the optical properties of hydroxide-catalysis bonding is reported. More specifically, the bond refractive index and thickness are determined from reflectivity measurements. By applying this method, it will be possible to understand how the optical properties and chemistry of a hydroxide-catalysis bond vary when different bonding solutions and substrate materials are used and, consequently, to tailor better the bonds to various utilisations of interest. Sodium silicate solution at different concentrations with water and potassium hydroxide solution are used to bond fused silica and sapphire substrates. Curing at room temperature and at 100 °C for eight hours is chosen to study the influence of temperature on the properties of a hydroxide-catalysis bond. It was found that the bond optical reflectivities are less than 1% for fused silica samples and less than 10% for sapphire samples and they decrease over time. Bond refractive indices start from a value close to the refractive index of water and approach the refractive index of fused silica as the cure proceeds. Bond thicknesses cured at room temperature decrease over time plateauing at a more or less constant value, different for each case studied (about a few hundred nanometres), whereas bond thicknesses cured at high temperature seem to increase over time (less than about 400 nanometres)