Two-photon processes in defect formation by excimer lasers in synthetic silica glass

Defect formation in dehydrated silica glasses was investigated using various excimer lasers with different energies. The ArF laser (6.4 eV) generates the E' center more effectively than the KrF laser (5.0 eV), while the XeCl laser (4.0 eV) generated no centers. Defect generation was found to be proportional to the square of the laser photon density, indicating that it occurs dominantly due to a two-photon process which makes band-to-band excitation possible. The E' center probably originated from oxygen-deficient centers. Contributions to the E' center formation from a process involving direct absorption at the sites of intrinsic defects in SiO₂ glass were discussed on the basis of the excitation energy dependence and a comparison with the effect of a low-pressure mercury lamp.