Developments in Ceramic Materials Research

Optical Fluoride and Oxysulfide Ceramics: Preparation and Characterization 85
At higher concentration of Nd 3+ (0.5 wt%) the fluorescence kinetics shows
nonexponential decay independent on the temperature. This can be a result of concentration
quenching or presence of different types of optical centers (Figure 29a). To check these ideas
time-resolved fluorescence spectra are measured using standard gated Boxcar averager (PAR
162/164) with variable time gate tgate and gate delay td. Time-resolved fluorescent spectra
measured at 77 K under 893.5 nm pulsed laser excitation at different time gates and gate
delays do not show any significant changes for this sample (Figure 30). This indicates the
absence of different types of optical centers in the Gd2O2S:Nd 3+ optical ceramics for the
concentrations of Nd 3+ up to 0.5 wt% and confirms that nonexponential fluorescence decay
deals with concentration quenching. The energy transfer kinetics for the Gd2O2S:Nd 3+ (0.5
wt%) (Figure 29b) was obtained by subtracting the intracenter decay using the lifetime
measured for the Gd2O2S: Nd 3+ (0.1 wt%) ceramic sample with minimal concentration of
Nd 3+ (τmeas=107.0 μs) (curve 1 in Figure 29a). The energy transfer kinetics in the
Gd2O2S:Nd 3+ (0.5 wt%) ceramic sample can be analyzed in terms of the model of Eqs. (4),
(5).
By plotting the energy transfer kinetics −lg(−ln I(t)) as a function of lg(t) for
Gd2O2S:Nd 3+ (0.5 wt%) (Figure 31a), we can determine from the slope of this graph the
exponent of the time parameter tg =3/s [see Eq. (4)], and to determine the s parameter. We
found that tg eventually tends to , which corresponds to s=6 and the dipole–dipole
quenching interaction between the excited and unexcited Nd 3+ ions.
Fluorescence, a.u.
3
2
1
2
1
λ, nm
Gd
2O2S:Nd 3+ (0.5 %)
T=77 K
t gate = 200 μs τd = 350 μs
t gate = 1 μs τd = 20 μs
0
1060 1070 1080 1090 1100 1110 1120
Figure 30. Time- resolved fluorescence spectra measured at the 4 F 3/2→ 4 I 11/2 transition of Nd 3+ in the
Gd 2O 2S:Nd 3+ (0.5 wt%) optical ceramics at 77 K under 893.5 nm pulsed laser excitation with 300 μm
slits width, gate width tgate=1 μs, and gate delay t d=20 μs—1; and t gate=200 μs and t d=350 μs—2.