Rare earth-doped oxyfluoride glass of the 50SiO2 -50PbF2 -5ErF3 composition in molar ratio was developed. When the oxyfluoride glass is heat-treated at the first cystallization temperature, the glass gives the glass-ceramic in which rare earth-containing fluorite-type nanocrystals of about 20 nm in diameter uniformly precipitate in the glass matrices. The glass-ceramic is trasnparent to the naked eye like no heat-treated oxyfluoride glass. The glass-ceramic exhibits highly efficient upconversion luminescence under 800 and/or 980 nm laser light excitation.
On the other hand, the oxyfluoride glass can be locally changed to glass-ceramic in the forms of dot, line, plane, letter, etc. by thermal energies generated from light absorption of various lasers. In the case of CO2 laser irradiation the formation of such glass-ceramic occurs near the surface of glass. In the case of 800 or 980 nm irradiation by Ti:sapphire laser or laser diodes, on the other hand, the formation of such glass-ceramic occurs near the surface and/or inside of glass. The glass-ceramic parts can be easily read by upconversion luminescence under laser excitation. Therefore, the presently developed rare earth-doped oxyfluoride glass can be utilized as optical devices of the writing and reading memory, which can be utilized as specific devices for security information.