Developments in Ceramic Materials Research

90
T. T. Basiev, V. A. Demidenko, K. V. Dykel’skii et al.
CONCLUSION
Thus, the possibility of preparation of transparent fluoride ceramics with low optical
losses at the level of 10 -2 - 10 -3 cm -1 is demonstrated. Such a loss factor achieved indicates
that the prepared materials are appropriate to use for the development of the photonics
elements. The measurements are carried out for the ceramics at 1 μm wavelength. At this
wavelength the transparency of the ceramics is much worse than that for the single crystal
(optical absorption of the single crystal is lower than 10 -4 cm -1 ). The difference increases in
passing from IR to UV range. The CaF2 single crystal has more far absorption edge in the UV
range than the optical ceramics. However, in passing from near to mid IR range the difference
between the single crystal and the optical ceramics goes down and practically disappears.
Elucidation of the nature of such behavior requires additional studies. Evidently, this is not
concerned with the cation contamination. The results show that right now fluoride optical
ceramics is a material for the near and mid IR ranges (1 – 8 μm).
Like an oxide laser ceramics the fluoride optical ceramics reveals improved mechanical
properties comparing to single crystals. The fluoride ceramics keeps very important property
of single crystals – high thermal conductivity that makes it attractive for high power laser
systems.
The spectroscopic properties of fluoride ceramics are not differ strongly from the single
crystals of similar composition.
It is demonstrated for Gd and La oxysulfide optical ceramics that in spite of anisotropy of
raw materials preparation of optically transparent material by hot pressing assisted method is
possible.
High emission rate A about 10 4 s −1 is measured for the 4 F3/2 laser level in Gd2O2S:Nd 3+
due to the large refractive index (n=2.2) of this oxysulfide. That is together with narrow
fluorescence spectral lines provides high emission cross- section at 1.08 μm wavelength,
which is one of the most IR shifted among neodymium doped laser media. Rather short
depletion time of the order of hundred nanoseconds for the 4 I11/2 low laser level is estimated
as well.
For the 4 F3/2 laser level in the Gd2O2S:Nd 3+ and La2O2S: Nd 3+ ceramic samples the
detailed fluorescence kinetics decay measurements and analysis are provided. Analysis of the
kinetics decay and concentration behavior for the Gd2O2S:Nd 3+ (0.1; 0.5 wt%) ceramic
samples in the temperature range from T = 77 to 300 K leads to the conclusion that the
transfer of energy from the 4 F3/2 excited state of the Nd 3+ ion occurs directly to the acceptors
(unexcited Nd 3+ ions) in accordance with the dipole–dipole mechanism (s=6). A high value of
the quenching microparameter CDA, i.e. 3.8×10 −39 cm 6 /s, because of large emission and
absorption cross-sections of electronic transitions is found. In the La2O2S:Nd 3+ (1 w%)
ceramic sample at T=300 K together with direct energy transfer (DET) with
CDA=2.2×10 −39 cm 6 /s a stationary migration-controlled stage is observed starting from
tbound2=54.6 μs with microefficiency of donor–donor Nd–Nd interaction CDD=6.2×10 −38 cm 6 /s
calculated in hopping model of energy migration. It can be concluded that moderate and small
concentrations of Nd 3+ ions should be used for synthesis of effective oxysulfide transparent
optical ceramics for 1 μm lasing.