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Flux growth of highly Yb3+-doped cubic Gd2O3 laser crystals
|Matias Velazquez 1, Philippe Veber 1, Gabriel Buse 1, Emmanuel Véron 2, Daniel Rytz 3, Frédéric Druon 4, Sylvie Janicot 4, Stanislav Péchev 1, Oudomsack Viraphong 1, Marwan Abdou-Ahmed 5, Thomas Graf 5, Patrick Georges 4|
1. Institut de Chimie de la Matiere Condensée de Bordeaux, ICMCB - CNRS, 87 avenue du Dr Albert Schweitzer, Pessac 33608, France
Developing large laser grade cubic rare-earth sesquioxides (RE2O3, RE=Sc,Y,Lu) single crystals doped with Yb3+ ions stands as one of the most challenging endeavours of today’s crystal growth [1,2], essentially because of the high melting point of these compounds (~2400-2500 °C) and, in some of them, a series of structural phase transitions occurring upon cooling. Recent studies on cubic RE2O3 single crystals have demonstrated the laser potential of these materials and highlighted the extreme thermodynamic conditions in which their growth takes place [1-3]. In 2010, a successful power scaling of a passively mode-locked femtosecond thin-disk laser was achieved with Lu2O3:Yb3+ crystals which delivered an average power of 141 W under diode pumping at 976 nm . In 2011, it is an impressive 670 W output power that was obtained in CW regime also using thin disk technology . Several methods have been put forward these last three years to grow pure and RE’3+-doped (RE’=Tm, Er or Yb) cubic RE2O3 (RE=Y, Lu or Sc) single crystals [2,6-8]. In this work, we will present Yb3+-doped Gd2O3 single crystals of the cubic phase, with dimensions 6X5X1.4 mm3, which were recently grown by a newly designed high-temperature solution growth method  and characterized by means of X-ray diffraction, Fourier transformed infrared (FTIR) spectroscopy, electron probe microanalysis (EPMA) coupled with wavelength dispersive spectroscopy (WDS). Our method uses an original and nontoxic solvent with a growth setup design operating in air and at half the melting temperature of rare-earth sesquioxides. The high-temperature solution growth conditions will be discussed. Because of the closeness of the sursaturation temperature of the new solvent used for the growth and that of the monoclinic-cubic phase transition temperature, the latter was completely revisited by means of high-temperature powder X-Ray diffraction. Our data evidence the temperature spreading from ~1200 to 1300°C as well as the strongly hysteretic nature of this first-order phase transition. They illustrate the benefit of using the Li6Gd(BO3)3-based solvent in order to stabilize directly the cubic polymorph of Gd2O3. Indeed, cubic Gd2O3 contracts isotropically and less than its metastable monoclinic polymorph, the thermal contraction of which remains anisotropic from 1250°C to room temperature. The case of cubic Gd2O3:Yb3+ is particularly interesting since its basic spectroscopic and laser properties have never been detailed and we were capable of achieving doping levels as high as 14 % (=Gd1.72Yb0.28O3, that is, an Yb3+ concentration of 3.6 1021 cm-3). Such a concentration proves to be much more off the experimental lifetime optimum resulting from the antagonistic effects of self-trapping and concentration quenching but it will permit to reduce the thickness of the crystal down to a few hundred microns, allowing for an efficient cooling while maintaining a high absorption yield under thin-disk laser operation. In this presentation, we will emphasize that this flux growth process allows for achieving optimal doping for high-power laser applications, impedes the dissolution of OH- groups in the crystals, avoids the reduction of Yb3+ ions into Yb2+ ones (and its resulting absorption bands around 600, 520 and 480 nm ), favours broader absorption and emission bands. Such an inhomogeneous broadening, as well as anti-Stokes emission spectra, will be discussed within the scope of an exhaustive and complete chemical analysis of the samples by GDMS. We will highlight the fact that the 5-4 transition peaks at 1074 nm in the cubic form of Gd2O3, hence reducing the quantum defect with respect to the classical monoclinic Gd2O3 crystals, in which this transition is shifted at ~1105 nm . Finally, these uncoated new Yb3+-doped cubic Gd2O3 crystals have been succesfully tested in different laser cavities under Ti:sapphire and diode pumping. Under diode pumping at 977 nm, the maximum achieved laser power is 1.4 W in Qcw regime and 0.27 W in cw regime. For this high doping level, the laser emission was at 1076 nm.
Figure 1 : a typical cubic Gd1.72Yb0.28O3 single crystal grown by our new flux method.
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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Matias Velazquez
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-13 16:36 Revised: 2013-04-15 11:35