The nature of optically active defect centers in vitreous silicon dioxide.

Point defects are detrimental to the most applications which make use of the otherwise excellent optical and dielectric properties of vitreous SiO2. One notable exception to this rule is the writing of photoinduced gratings in SiO2:GeO2 glass fibers. There is ample evidence that the photosensitivity is connected to the presence of point defects in the glass, particularly, to the presence of the so called "oxygen deficiency centers" (ODC’s). While significant advances in optimizing the photoinduced grating writing have been made in recent years, the basic understanding of the underlying microscopic defect processes is still insufficient. The nature of a number of defect-related optical bands is still a matter of discussions. Most unsatisfactorily, the structure of the "key defect" to the fiber writing, the ODC, remains controversial.