Photochemical process of divalent germanium responsible for photorefractive index change in GeO2–SiO2 glasses

Abstract The photoluminescence spectra of the divalent Ge (Ge2+) center in GeO2–SiO2 glasses with different photosensitivities were investigated by means of excitation-emission energy mapping to correlate the ultraviolet light induced photorefractivity with the local structure around the Ge2+ centers. The glasses with a larger photorefractivity tended to exhibit a greater band broadening of the singlet–singlet transition to higher excitation energies accompanied by an increase in the Stokes shifts. This strongly suggests the existence of a highly photosensitive Ge2+ centers with higher excitation energies. It was also found that the introduction of a hydroxyl group or boron species modifies the local environment of Ge2+ leading to an enhanced photorefractivity.