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 [4]. In 2011, it is an impressive 670 W output power that was obtained in CW regime also using thin disk technology [5]. 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 [6] 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 % (=Gd_1.72Yb_0.28O₃, 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 [9]), 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 [10]. 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 Gd_1.72Yb_0.28O₃ single crystal grown by our new flux method.

[1]    A. Yoshikawa and V. Chani, MRS Bulletin, 34 (2009) 266.

[2]    A. Fukabori, V. Chani,  K. Kamada, F. Moretti and A. Yoshikawa, Cryst. Growth & Des., 11 (2011) 2404.

[3]     R. Peters, C. Kränkel, K. Petermann and G. Huber, J. Cryst. Growth, 310 (2008) 1934.

[4]     C. Baer, C. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, T. Südmeyer, G. Huber and U. Keller, Opt. Lett., 35 (13) (2010) 2302.

[5]     B. Weichelt, K. Wentsch, A. Voss, M. Abdou Ahmed, Th. Graf, Laser Phys. Lett., (2012) 110–115.

[6]     Ph. Veber, M. Velázquez, V. Jubera, S. Pechev and O. Viraphong, Cryst. Eng. Comm. 13 (16) (2011) 5220.

[7]     C. McMillen, D. Thomson, T. Tritt and J. Kolis, Cryst. Growth & Des., 11 (10) (2011) 4386.

[8]     A. Fukabori, V. Chani, K. Kamada, A. Yoshikawa, J. Cryst. Growth, 352 (2012) 124.

[9]     V. Peters, Growth and spectroscopy of Ytterbium-doped sesquioxides, PhD thesis, University of Hamburg, Germany, (2001).

[10] L. Laversenne, Y. Guyot, C. Goutaudier, M.-Th. Cohen-Adad, G. Boulon, Opt. Mater., 16 (2001) 475.

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