Point Defects in Ge-doped SiO_2 Glass-Their Structures and Roles in Nonlinear Optical Effects

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(1) The generation mechanism of the absorption changes, which cause a photorefractive change through the Kramers-Kronig relation in Ge-doped SiO_2 glass, has not been clarified yet. In the present paper, we examined the laser-power dependence of the absorption changes around 5 eV,induced by a KrF excimer laser. The induced absorption around 5 eV is composed of three different components, centering at 4.50,5.08, and 5.80 eV.The increasing behavior of each absorption component depends strongly on the energy density. The three absorption components reach different saturation levels, depending on the energy density. Furthermore the absorption induced by a high-power KrF excimer laser is bleached by a laser, the energy density of which is about one-twentieth of the inducing laser. Combining the results of mathematical analysis, it was found that a two-photon process and a one-photon process are, respectively, involved with the induction and the bleach of each absorption. It was also found t … More hat the precursor defects, which causes the absorption change, is of an oxygen-deficient type.(2) Optical absorption change in the microsecond order in oxygen-deficient Ge-doped silica glass was measured as a function of time just after photon irradiation from a KrF excimer laser. The absorption above 3 eV was found to decay with the same time constant as that of the luminescence at 3.1 eV.From this, it is confirmed that the observed absorption change is due to the excitation of electrons from the lowest excited triplet state to an upper state.(3) Defects in buried SiO_2 films in Si formed by implantation of oxygen ions were characterized by photoluminescence (PL) excited by KrF (5.0 eV) excimer laser and synchrotron radiation. Two PL bands were observed at 4.3 and 2.7 eV.The 4.3 eV band has two PL excitation bands at 5.0 and 7.4 eV,and its decay time is 4.0 ns for the 5.0 eV excitation and 2.4 ns for the 7.4 eV excitation. The decay time of the 2.7 eV PL band is found to be 9.7 ms. These results are very similar to those for the 4.3 eV and the 2.7 eV PL bands, which are observed in bulk silica glass of an oxygen-deficient type and attributed to the oxygen vacancy. Through the change in the PL intensity with the film thickness, the buried SiO_2 film is considered to contain the oxygen vacancy defects in a high amount throughout the oxide. Less