Photosensitivity of rare-earth-doped ZBLAN fluoride glasses

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Abstract
We have performed experiments to elucidate the mechanism of photosensitivity in rare-earth-doped fluorozirconate (ZBLAN) glasses. Glasses doped with Ce, Tb, Tm, and Pr were studied. Permanent holographic gratings were written in bulk samples using 248 nm light, with the strongest gratings observed in Ce:ZBLAN. Ultraviolet (UV) induced changes in both 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 results indicate that the mechanism of photosensitivity involves color center(s) creation through a stepwise two photon excitation of a Ce ion. The color center(s) can be subsequently bleached by a single photon at 248 nm.