Optical and EPR properties of BaY2F8 single crystals doped with Yb

Yb doped materials have turned out to be relevant for efficient high-power continuous-wave lasers, up to the kilowatt class. In the present paper we analyze optical and EPR properties of BaY2F8:Yb (5wt.%) single crystals obtained using Czochralski method. The Yb energy levels for the crystal are as follows: F7/2 (0, 216, 430, 530 cm), F5/2 (10277, 10395, 10735 cm ). BaY2F8 highly doped with Yb reveals complex spectral properties due to possible double structure being partly monoclinic and partly orthorhombic as in case of BaYb2F8. Thus, applied to laser host, it shows some limitations connected with quenching processes by energy transfers to defects and other impurities. Yb centers are not observed in the absorption spectrum of “as-grown” crystal but small amount of the centers we observed just after irradiation of the crystal with γ-quanta (bands with maxima at: 213, 257, 345 and 520 nm). The shape of the additional absorption suggests the Yb centers are formed as a result of recharging of one of Yb ions from a pair. So, among different kinds of the defects one can observe also Yb pairs formation, and, as a consequence Yb pairs emission. When we excited BaY2F8:Yb (5wt.%) single crystal with 980 nm, we have observed emissions in the VIS and NIR range as bands centered at 478, 560, 649 and 789 nm. The pairs were also observed in the EPR spectra, as satellite lines near central Yb line but only for specific angles (15-65 in ZY plane) with respect to magnetic field direction (see Fig. 1). We have analyzed also anisotropy of EPR lines to describe possible symmetries of Yb ions in BaY2F8 lattice and temperature dependencies of the Hamiltonian parameters to show the kind and strange of magnetic interactions. It is shown that when pairs are not observed in the EPR spectrum as satellite lines (see Fig. 2), three temperature ranges one can observe. For temperatures lower than 6 K, g-factor is growing up from 3,15 to 3,24, magnetic moment is slowing down. Between 6 K and 30 K g-factor stays constant and equal to 3,24 (a value characteristic for isolated Yb ions), magnetic moment is constant, while integral intensity in the range 3-30 K obeys Curie-Weiss low, with Tc=1,11 K (Fig. 3). Above 30 K g-factor is again growing up and magnetic moment slowing down. When pairs are observed, two temperature ranges one can recognize. Below 30 K, g-factor stays constant and equals to 2,045, the integral intensity of the EPR line obeys Curie-Weiss relation with Tc=-3,5 K suggesting low antiferromagnetic interactions (Fig. 4.). Above 30 K, g-factor is growing up and interactions change to ferromagnetic type. In both cases the linewidth is slowing down when temperature increase from 3 K up to 30 K and next it is growing up. EPR signal vanishes above 50 K (Fig. 5, 6). * corresponding author: [email protected] b). Wytworzony w tym laboratorium kolejny monokryształ, CaF2 domieszkowany iterbem, został u nas zbadany pod względem właściwości optycznych i magnetycznych, a wynik tych badań opublikowano we wspólnej pracy: S.M. Kaczmarek, G. Leniec, G. Boulon, "EPR results and Raman spectroscopy as a complementary characterization of isolated Yb ions and Yb pairs in CaF2:Yb single crystals", J. All. Comp., 451 (1/2) (2008) 116-121