Influence of cerium on the pulsed UV nanosecond laser processing of photostructurable glass ceramic materials

Photostructurable glass ceramic (PSGC) materials contain a sensitizer that is used to facilitate the optical exposure process. The primary role of the sensitizer is to absorb incident radiation and generate photoelectrons. With thermal treatment, these photoelectrons can then interact with nascent metal ions to induce the formation of metallic clusters and the precipitation of a soluble crystalline phase in the glass matrix. The photo-ionization efficiency of the sensitizer species is strongly dependent on its spectral absorption and oxidation state in the base glass. Stabilizing compounds are typically added to the glass matrix to maintain the photo-active oxidation state and promote efficient exposure. To investigate the effectiveness of the photo-initiator, we have conducted experiments in which sample coupons of a commercial PSGC material (Foturan™, Schott Corp., Germany) were carefully exposed to various photon doses by pulsed UV nanosecond lasers at λ = 266 nm and 355 nm. Foturan is a lithium aluminosilicate glass that contains trace amounts of cerium as the photosensitive agent (0.01-0.04 wt.% admixture Ce2O3). The photo-initiator efficiency was investigated by using samples with cerium and without cerium. The irradiation wavelengths were selected because they lie above and below the primary absorption band of the cerium photo-initiator. Optical transmission spectroscopy (OTS) was employed to identify and monitor the population density of the photo-induced trapped electron state as a function of incident laser irradiance. The irradiated samples were thermally processed and then analyzed again with OTS to measure the quenching of the trapped electron state and the concurrent growth of a spectral band associated with the formation of nanometer-scale metallic clusters. The growth of metallic clusters signifies the “fixing” of the exposure and permanent image formation in the glass. The OTS results reveal that for λ = 266 nm laser irradiation, at least two photoelectron donors (including cerium) contribute to the formation of the trapped electron (defect state) density and fixing of the exposure. For λ = 355 nm laser irradiation, however, the photo-initiator is critical to the exposure process and nearly all of the trapped electron density is formed via cerium photoelectron donors.