Laser-induced darkening in thin films of fluorescein-doped boric acid glass.

We report laser-induced permanent darkening in thin films of fluorescein-doped boric acid glass. The photodarkening is accompanied by a substantial increase in the absorption of the irradiated samples. The photoinduced absorption appears to be permanent. Optical and electron paramagnetic resonance measurements suggest the creation of a new photochemical species on irradiation within the absorption band of the fluorescein molecule. A possible mechanism for the photochemical change responsible for the darkening is proposed. Optical storage of information through the formation of permanent holograms is demonstrated.