Orange Persistent Luminescence and Photodarkening Related to Paramagnetic Defects of Nondoped CaO-Ga2O3-GeO2 Glass

Orange persistent luminescence and photodarkening phenomena were observed after ultraviolet (UV) excitation in calcium-gallium-germanate (CGG) glass with a composition of 3CaO-1Ga2O3-3GeO2 prepared by a melt-quenching method. The CGG glass was characterized by using X-ray diffraction, differential scanning calorimetry, and Raman scattering analyses. Also, the optical and electronic properties of the CGG glass were investigated. Based on the electron paramagnetic resonance (EPR) spectra, it is found that new defects, NBOHC (nonbridging oxygen hole center) and E'(Ge) (unpaired electron at Ge ion bonding three oxygen), were created after UV excitation. From the good agreement between the time dependence of the photodarkening absorption intensity and EPR intensity of the NBOHC, the origin of the photodarkening phenomenon is found to be caused by NBOHC. Also, from the relationship between the time dependence of persistent luminescence intensity and the time-derivative EPR intensities of NBOHC and E'(Ge) centers, the persistent luminescence is found to be caused by the recombination process between these two different defects.