Photochemical reactions in GeO₂-SiO₂ glasses induced by ultraviolet irradiation: Comparison between Hg lamp and excimer laser

GeO_2-SiO₂ glasses prepared by vapor-phase axial deposition were exposed to ultraviolet (uv) radiation from a Hg discharge lamp (4.9 eV) and excimer lasers (KrF laser: 5.0 eV, XeCl laser: 4.0 eV). Two photochemical reaction channels were ascertained: (1) The exposure of the glasses to the Hg lamp radiation (∼16 mW/cm²) induced Ge E’ centers accompanied by bleaching of the absorption band due to oxygen-deficient defects near 5 eV (5-eV band) and the emergence of an intense band near 6.4 eV. (2) The irradiation with KrF and XeCl lasers (power densities of 10 and 90 mJ/cm²/pulse, respectively, pulse duration of 20 ns) generated two types of paramagnetic defects, electron trapped centers associated with fourfold coordinated Ge ions (GEC) and a self-trapped hole center (STH: bridging oxygen trapping a hole). The former and the latter were considered to be caused via one-photon and two-photon absorption processes, respectively. These alternative reactions proceeded independently depending on the power densities of uv photons. The formation of GEC’s was saturated easily by irradiation with KrF laser pulses, and then the conversion of GEC to Ge E’ centers was caused by prolonged irradiation.