Developments in Ceramic Materials Research

Fluorescence, a.u.
Optical Fluoride and Oxysulfide Ceramics: Preparation and Characterization 79
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1071.1
1074.5
1076.2
1080.2
1082.4
1092.8
1090.1
La 2 O 2 S:Nd 3+ (1%)
4 F3/2 --> 4 I 11/2
T=300 K
T=77 K
0
1060 1070 1080 1090 1100 1110
b
Figure 24. Fluorescence spectra of the 4 F 3/2→ 4 I 11/2 transition in Gd 2O 2S:Nd 3+ (0.1 wt%)—a and
La2O 2S:Nd 3+ (1 wt%)—b under 812 nm laser excitation at 77 and 300 K.
The fluorescence spectrum at the 4 F3/2→ 4 I11/2 transition is measured with high spectral
resolution in Gd2O2S:Nd 3+ (0.1 wt%) (Figure 24a) and La2O2S:Nd 3+ (1 wt%) (Figure 24b) at
77 and 300 K using CW diode laser excitation. The positions of the three lowest CF levels of
the 4 I11/2 manifold are found in the Gd2O2S:Nd 3+ ceramics (Figure 22). The fluorescence
spectral lines measured at the 4 F3/2→ 4 I11/2 CF level transitions in the Gd2O2S:Nd 3+ (0.1 wt%)
optical ceramics at 77 and 300 K are much more narrow (Figure 24a) comparing to those in
the La2O2S:Nd 3+ (1 wt%) optical ceramics (Figure 24b), because of the same reasons as in the
case of absorption spectra.
MULTIPHONON RELAXATION IN OXYSULFIDE CERAMICS [49]
The 4 I11/2 state can be potentially a bottleneck during 1-μm lasing and gives rise to
transient absorption at laser wavelength. So, the faster the rate of multiphonon relaxation
(MR), faster the depletion of the 4 I11/2 level and the smaller the losses [50].
It is a well-known fact that the energy gaps of the 4 G7/2→ 4 G5/2; 2 G7/2 and the 4 I11/2→ 4 I9/2
transitions of Nd 3+ are nearly the same in oxide (ħωmax = 800 cm -1 ) and fluoride (ħωmax = 400
cm -1 ) crystals and their relaxation rates are mostly determined by MR. The cut-off frequencies
about 440 and 400 cm −1 , respectively, are found from Raman spectra of the Gd2O2S and
La2O2S optical ceramics doped by Nd 3+ (Figure 25). Highly sensitive Raman spectrometer
with photon counting is used for measurements. The value ωmax for the La2O2S optical
ceramics is in a good agreement with that measured in the La2O2S crystal [51]. Therefore, one
can estimate the rate of the 4 I11/2→ 4 I9/2 transition from the rate of the 4 G7/2→ 4 G5/2; 2 G7/2
b)