Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic and monoclinic Gd2O3☆
Introduction
Yb3+ ion spectroscopy have received considerable attention over the past years; because of its potential laser emission around , it could substitute Nd3+ ion for doped laser crystals. The main interest of Yb3+ ion lies in its very simple energy level diagram: it presents only one excited state which is separated from the ground state by approximately . There are no additional manyfold, contrary to other trivalent rare earth ions, and owing to this unique configuration, Yb3+ presents remarkable properties:
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complications in laser medium like up-conversion or excited state absorption are avoided and so not supposed to affect laser performance;
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laser diode pumping is ideally suitable and that is an advantage for systems' miniaturization.
Materials likely to substitute the much used Nd3+: YAG (Y3Al5O12) laser emitting at are a significant economic stake and many spectroscopic studies have still been led in a wide range of materials from glasses [1] to crystals [2]. In particular with Yb3+:YAG, Lacovara et al. [3] performed an InGaAs diode pumped laser with 31% slope efficiency at room temperature. Very recently Peters and his co-workers have published some important results concerning Yb3+-doped sesquioxides grown by Czochralski method from rhenium crucibles. They have demonstrated efficient diode-pumped laser oscillation with a 56% slope efficiency in Yb: Sc2O3 [4].
Rare earth sesquioxides are refractory matrices (they all melt above 2350°C) and they have been selected as hosts first because they present high and better thermomechanical properties than in YAG; for e.g., thermal conductivity at room temperature is double in Y2O3 with K=27 W/m K versus K=13 W/m K in YAG [5]. Moreover they are chemically related with the dopant that insures a very good substitution. Low phonon energy insures good laser efficiency by minimizing energy loss due to non-radiative processes; it is already low in YAG () but it is weaker in rare earth oxides with and for Gd2O3 [6] and Y2O3 [7].
As Yb2O3, both Y2O3 and Lu2O3 present a cubic structure, so they are isotropic, and a total solid solution is expected in the diagram with Yb2O3. In the case of monoclinic Gd2O3 we have previously demonstrated that there exists a partial monoclinic solid solution in the Gd2O3 rich range separated from a partial cubic solid solution (resting at the other end) by a two-phase domain.
Whereas the previous and recent crystal samples have been grown by using the Czochralski technique with a rhenium crucible, in the present paper we report results from samples grown by the Laser Heated Pedestral Growth (LHPG) method which is very convenient for refractory oxide crystal growth since there is no need of crucible. In addition, we show for the first time results given by a new analysis method from only one sample containing concentration gradient so that main spectroscopic properties can be observed by a simple translation of the sample.
Section snippets
Materials
Yttrium and lutetium sesquioxide are very close structurally: they belong to the bixbyite type which is body-centered cubic with space group Th7 (or Ia3). The elementary cell contains 16 Y2O3 formula units with 32 cation (six-fold coordinated) sites for which the trivalent rare earth dopants can substitute: eight centro-symmetric sites with symmetry C3i (or S6) and 24 noncentro-symmetric sites with symmetry C2. Lattice constants are equal to 10.60 and 10.39 Å, respectively [8].
Experimental techniques
Composition measurements have been achieved on a CAMECA SX-100 with a 20 kV–40 nA accelerated beam and a LiF crystal receiving the response of K-lines of the RE atoms. In order to increase the resolution of these measurements we have calibrated the results using standards that are pure oxides synthesized in our laboratory.
Phonon energy of the rate earth oxide matrices has been studied from undoped single crystals on a Dylor XY spectrometer using the 514.5 nm line of an ionized argon laser.
Raman spectroscopy
We have recorded Raman shift for each of the matrices studied at room temperature. Concerning Gd2O3 and Y2O3 (Fig. 1, Fig. 2), results obtained on single crystals correspond to those already existing in literature: for Gd2O3 the main band is [6] and for Y2O3 the highest energy transition is located at [7].
We report, for the first time to our knowledge, the Raman spectrum of single crystal Lu2O3 (Fig. 3) and its assignment determined by comparing it with the Y2O3 one.
Yb3+ luminescence
Conclusion
We have studied Yb3+ spectroscopy around in these matrices and have shown that lutetium and yttrium oxides give very similar results: absorption and emission spectra are very close due to the identical structure. We report the results obtained by a new method to optimize spectroscopic properties as decay times versus the dopant content using “concentration gradient samples”. As an example, we have shown that both C2 and C3i may be active sites for optical transitions and that energy
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Presented at the Fourth French–Israeli Workshop on Optical Properties of Inorganic Materials, Villeurbanne, France, December 5–8, 1999 (Optical Mater. 16, Nos. 1/2 (2001)).