The photochemical reactions which were induced by irradiation with excimer lasers (KrF, 5.0eV; XeCl, 4.0eV) or a Hg lamp (4.9eV) were studied in 90:10 SiO2-GeO2 glass. It was found that irradiation with excimer lasers generated 2 types of paramagnetic defect. These were an electron-trapped center which was associated with 4-fold coordinated Ge ions, and a self-trapped hole center on bridging O. By taking account of the optical band-gap (7.1eV) and the power density of the laser pulses (10 to 90mJ/cm2, 20ns), it was concluded that these centers were created by band-to-band excitation via 2-photon absorption. On the other hand, lamp illumination (16mW/cm2) caused the formation of Ge E’ centers from pre-existing O-vacancy type defects via a 1-photon absorption process. These 2 types of reaction proceeded independently, depending upon the power densities of the ultra-violet beams.

J.Nishii, N.Kitamura, H.Yamanaka, H.Hosono, H.Kawazoe: Optics Letters, 1995, 20[10], 1184-6