Thermophysical and optical investigations of carbon dioxide laser beam interaction with silica glass and PMMA

Silica glass and PMMA polymer are two materials widely applied to engineering problems in optics, electronics and biotechnologies, among other fields. As typical of amorphous materials, some of their physical and chemical properties strongly depend on their thermal history. A unique thermal history for most conventional glass or polymer processes can be produced by a high intensity laser delivering concentrated energy in a short period of time. The high heating and cooling rates imposed by this rapid thermal cycle can induce modified material structure, and thus material properties, in a local region confined within micrometers of the surface. Investigating the laser interaction with silica glass and PMMA not only enhances fundamental understanding of the formation of glassy materials under short-time scales, but also suggests promising methods for fabricating micro-optical devices and microscale structures with processing conditions that are inaccessible to other techniques.; In this dissertation, the laser beam interaction with silica glass is investigated first. The thermophysical nature of the silica glass reaction to a rapid laser scan is explored experimentally and theoretically. A numerical three-dimensional heat transfer model is developed to investigate the variation of glass thermodynamic temperature and fictive temperature during laser processing. The glass Si-O-Si bridge bond angle variation induced by laser processing is analyzed based on the fictive temperature found from the thermophysical model. To validate the thermophysical model results, a wet chemical etching method is developed to spatially investigate the laser-affected zone over which the silica structure is modified. The relationship between etch rate and fictive temperature is determined, which allows the fictive temperature to be experimentally measured. The experimentally determined fictive temperature in the laser-affected zone agrees very well with the thermophysical model. The optical property change in the laser-affected zone is further studied. The change of the optical properties of silica glass induced by the laser is experimentally measured with a prism coupler and a surface reflectivity method. Results are compared between the experimental methods, and theoretically with the Lorenz-Lorenz model. Methodologies that were developed to investigate the laser interaction with silica glass are used to investigate PMMA polymer processing with a CO2 laser. The PMMA surface topography is characterized for different laser processing conditions, and changes in optical properties and chemical resistance of PMMA are experimentally studied. Based on the success of these studies, several possible directions for further research are proposed and possible applications are discussed.