Growth and characterization of new laser materials is a very important research field. In the last time an important effort has been devoted to research of new Yb-doped laser crystals. Although RE ions are most easily stabilized as trivalent ions in CaF₂ crystals, the divalent state has certain distinct advantages for laser applications [1,2]. The change of valence has been attained by baking the crystals in a suitable atmosphere [3], by electrolytic coloration or exposing them to ionizing radiation [3,4].

In this work we report the growth and some optical properties of YbF₃- doped CaF₂ crystals with high divalent Yb ions concentration obtained without any other treatment. Pure and various concentration YbF₃-doped calcium fluoride crystals have been grown by vertical Bridgman method [5] using a special procedure. Transparent colorless, 1cm in diameter, 7cm long crystals were obtained. The optical absorption spectra of the crystals exhibit intense UV absorption bands, characteristic for divalent Yb ions: 230, 263, 275 and 365nm [1,3] with absorption coefficient ten times higher than for the IR bands. The optical properties of the crystals depend on the Yb and F ions positions in the lattice. The trivalent Yb ions substitute for Ca ions in the lattice. The excess charge on the RE ion is compensated by an interstitial fluoride ion located in various positions giving rise to a rich multisite structure, which leads to broad absorption and emission bands. The divalent ions substitute for Ca ions, do not need charge compensation and posses cubic symmetry. The emission spectra of the CaF₂-Yb⁺² crystals have been investigated by other authors only by exciting by 365nm absorption band [1,2,3].The influence of the excitation by UV bands on the IR emission bands has been investigated by [3]. We studied the emission spectra of the obtained crystals exciting by 230nm absorption band and we obtained, for the first time, a high intensity, broad emission bands with 313, 335, 380 and 395nm peaks. The emission intensity strongly depends on the YbF₃ concentration.

References

[1] A. A. Kaplyanskii et al., Opt. Spectrosc. 41(1976)615,

[2] S. Lizzo et al. J. Lumin. 63(1995)223,

[3] S. M. Kaczmarek et al. J. Phys.: Condens. Mater. 17(2005)3771,

[4] D. S. McClure and Z. Kiss, J. Chem. Phys. 39(1963)3251,

[5] D. Nicoara and I. Nicoara, Mater. Science and Eng. A 102, L1 (1988).

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