Developments in Ceramic Materials Research

68
Fluorescence, a.u.
1
T. T. Basiev, V. A. Demidenko, K. V. Dykel’skii et al.
2 Powdered ceramics
λ, nm
1 Crystal
CaF 2 :Er 3+ (5%)
4 I11/2 --> 4 I 15/2
T=300 K
λ exc =810 nm
0
950 960 970 980 990 1000 1010 1020 1030 1040
Figure 13. Normalized fluorescence spectra at the 4 I 11/2 → 4 I 15/2 transition measured in the CaF 2 : Er 3+
crystal – 1 and in the powdered ceramics of same composition – 2 under CW laser excitation at 810 nm
wavelength at room temperature.
Normalized to the unity at each fluorescence maximum the spectra of three ceramic
samples (Figure 14) practically do not differ from each other that denotes to the independence
of the fluorescence spectra form – factor on the concentration. Absence of narrow spectral
lines of the transitions between crystal – field (Stark) levels of the different manifolds ( 4 I11/2
and 4 I15/2) in comparison with ErF3 powder fluorescence spectrum (Figure 15) points to
inhomogeneous splitting, i.e. to the multi - site structure of the samples.
Fluorescence kinetics decay of the 4 I11/2 manifold is measured in the CaF2:Er 3+ (5 %) and
CaF2:Er 3+ (10 %) crystals under 980 nm pulsed LiF: F2 → F2 + laser excitation and 987 nm
fluorescence detection and in the CaF2:Er 3+ (5 mol. %) ceramic samples, and in the ErF3
precursors for the ceramics at 974 nm laser excitation and 987.5 nm fluorescence detection
(Figure 16). It is necessary to note that the 4 I11/2 level is the initial laser level for lasing in the
spectral range of 3 µm, therefore, its fluorescence quenching because of ion – ion interaction
have to be minimized. Strong acceleration of fluorescence decay (Figure 16, curves 2 and 6)
is observed for different ErF3 precursors in comparison with the ceramic samples (Figure 16,
curves 1 and 5) and especially with the crystals (Figure 16, curves 3 and 4). There is no
difference in kinetics decay found between ceramic sample and the powdered ceramics that
confirms the absence of fluorescence trapping for the large concentration of the Er 3+ ions.
Acceleration of fluorescence kinetics decay can be explained by the fluorescence quenching
by uncontrolled impurities as a result of direct energy transfer and/or migration over the
donors (Er 3+ ions). Analysis of the disordered stage of energy transfer kinetics for the CaF2:
Er 3+ (5 mol. %) ceramics (В(2)39-05 sample) and the ErF3 precursor (Eqs. (4) and (5)) gives
quadrupole – quadrupole interaction (multipolarity of interaction s = 10) between the energy
donors (excited erbium ions) and the acceptors (uncontrolled impurities).