  Transparent glass-ceramics are nanocomposite materials which offer specific
characteristics of capital importance in photonics. This kind of two-phase
materials is constituted by nanocrystals embedded in a glass matrix and the
respective composition and volume fractions of crystalline and amorphous phase
determine the properties of the glass-ceramic. Among these properties
transparency is crucial, in particular when confined structures, such as
dielectric optical waveguides and optical fibers, are considered, and the
number of papers devoted to this topic is continuously increasing. Another
important point is the role of the nanocrystals when activated by luminescent
species, as rare earth ions, and their effect on the spectroscopic properties
of the glass-ceramic. The presence of the crystalline environment around the
rare earth ion allows high absorption and emission cross sections, reduction of
the non-radiative relaxation thanks to the lower phonon cutoff energy, and
tailoring of the ion-ion interaction by the control of the rare earth ion
partition. This last point is crucial and still object of intense experimental
and theoretical studies. The composition of the glass matrix also impacts the
properties of the rare earth ions located in nanoparticles. Moreover, some
kinds of nanocrystals can play as effective rare earth sensitizers.
Fabrication, assessment and application of glass-ceramic photonic systems,
especially waveguides, deserve an appropriate discussion which is the aim of
this paper, focused on luminescent glass-ceramics. In this work, a brief
historical review, consolidated results and recent advances in this important
scientific and technological area will be presented, and some perspectives will
be outlined.
