Between glass and crystal: glass-ceramics, a new way for optical materials

We report results of controlled devitrification of rare-earth-doped glasses made in order to improve the initial spectroscopic properties of the as-melted glass to obtain crystal-like properties. This study is made to allow the design of reduced-size photonic devices owing to the use of more efficient active materials. The main possibilities of devitrification of glasses are evaluated in two glassy systems and compared. The key point is the choice of the glass composition allowing partial crystallization during thermal treatments adapted to the thermal stability of the material. A strong improvement in the spectroscopic properties of the rare-earth ions has been obtained in a heterogeneously nucleated glass (GeO2-PbO-PbF2,). The final glass-ceramic is ultratransparent with weak additional losses mainly in the short-wavelength region. Another possibility has been evaluated through the spinodal decomposition of a fluorozirconate glass (ZrF4-LaF3-AlF3-GaF3-ErF3). Its spectroscopic properties are also mostly crystal like with only a weak scattering-induced reduction of transparency.