Oxyfluoride Transparent Glass Ceramics - Functionalized Inorganic Fluorides - page 293

9.6 Optical Properties of Glass-Ceramics

We know that, in the best cases, Ln 3 þ ions are segregated in the crystallites. In this section
we will discuss and compare the optical properties (absorption, emission, lifetime) of
glass-ceramics, glasses and single-crystals. Throughout the section, we will see the con-
sequences of the segregation of Ln 3 þ in fluoride crystallites on the glass-ceramic optical
properties.

Influence of the Devitrification on the Spectroscopic Properties of Ln 3 þ 9.6.1

As was mentioned earlier, the devitrification process results in a change of the environ-
ment of the lanthanide ions (or, at least, some of them), from being in an oxyfluoride glass
to being segregated in fluoride crystallites. These structural and chemical changes bring
about changes in the spectroscopic properties of Ln 3 þ . In the following section, we will
compare the optical properties of glasses and their corresponding glass-ceramics.

9.6.1.1 Reduction of the Inhomogeneous Optical Linewidth and Increase of the
Absorption/Emission Cross-Section
In Ln 3 þ -doped glasses, Ln 3 þ ions are randomly distributed and occupy many different
sites. The doping ions are subject to various environments and thus, different crystal-field
perturbations. As a result, they have slightly different energy-level splittings and their
optical transitions occur at slightly different wavelengths, leading to rather broad absorp-
tion/emission bands. On the contrary, in single-crystals, Ln 3 þ ions have well defined sites
and exhibit absorption/emission with narrow inhomogeneous linewidth.
In the glass-ceramic systems where the Ln 3 þ ions are well incorporated into the crystal-
line nanoparticles, the linewidth of an optical transition is narrower than it is in glasses. It
results from the segregation of Ln 3 þ in the nanoparticles which provide them a crystalline
environment. This is illustrated here with the 50GeO 2 :40PbO:10PbF 2 system doped with
ErF 3 , where it was proved that all Er 3 þ were incorporated into the PbF 2 nanocrystals [25].
As shown on Figure 9.19, the absorption band centred at 540 nm, corresponding to
the 4 I 15/2 ! 4 S 3/2 transition of Er 3 þ is much narrower in the glass-ceramic than in the
as-melted glass.
In this particular example (Figure 9.19), the reduction of the inhomogeneous linewidth
induces an increase of the maximum cross-sections in the glass-ceramic. Similarly for
the emission, in the SiO 2 :Al 2 O 3 :CdF 2 :PbF 2 system doped with NdF 3 , an increase of the
stimulated emission cross-section by a factor of 1.5 in the glass-ceramic compared to the
glass is reported for the 4 F 3/2 ! 4 I 11/2 transition [54]. Moreover, Quimby et al. experi-
mentally demonstrated that this change of environment from amorphous to crystalline
increases the quantum efficiency of Pr 3 þ at 1.3 mm [55]. It suggests that Ln 3 þ -doped glass-
ceramics have real potential for optical amplification.
However, an increase of the cross-section or quantum efficiency after the devitrification
process is not always observed as the change from an oxide environment to a fluoride
environment tends to decrease the oscillator strength [56]. Also, crystal-field strength is
weaker than Stark level splitting in a fluoride environment compared to an oxide one.