PHOTOSENSITIVITY OF RARE-EARTH-DOPED ZBLAN FLUORIDE GLASSES

We have performed experiments to elucidate the mechanism of photosensi tivity in rare-earth-doped fluorozirconate (ZBLAN) glasses. Glasses do ped with Ce, Tb, Tm, and Pr were studied. Permanent holographic gratin gs were written in bulk samples using 248 nm light, with the strongest gratings observed in Ce:ZBLAN. Ultraviolet (UV)-induced changes in bo th absorption and electron spin resonance (ESR) spectra were observed. In the Ce-doped glasses, the grating formation dynamics were recorded as a function of write beam intensity and Ce concentration. The resul ts indicate that the mechanism of photosensitivity involves color cent er(s) creation through a stepwise two photon excitation of a Ce ion. T he color center(s) can be subsequently bleached by one photon at 248 n n.