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22LS-P-4 , ALT'22
LASER SYSTEMS AND MATERIALS
New Ca3(VO4)2 :Cr crystal: synthesis, doping technique, optical properties
I.S. Voronina, E.E. Dunaeva, M.E. Doroshenko, L.I. Ivleva
Prokhorov General Physics Institute, RAS, 38 Vavilova str., Moscow 119991, Russia
The calcium orthovanadate Ca3(VO4)2 (CVO) single crystals are considered as a laser host material with high optical damage threshold and nonlinear optical coefficients compared with KDP. They can be potentially useful for frequency conversion of tunable laser sources. In periodic scientific literature many papers are devoted to growth and investigations of CVO single crystals doped with RE and TM metal ions. CVO is multifunctional material whose properties are defined by disordered whitlokite-like acentric crystal structure. At present work CVO crystals doped with chromium ions were obtained for the first time and their spectroscopic parameters depending on crystallization conditions and doping technique were studied.
CVO crystals doped with chromium ions were obtained by two different doping techniques: growth from the melt by Czohralski method and high temperature diffusion technique. Cz-crystals were grown from the melt with adding Cr2O3 in concentration from 0.01 to 0.1 wt.% over stoichiometry, along [100] crystallographic direction, pulling rate was 6mm/h, rotation velocity - 30 rpm. CVO:Cr crystals (20x70mm) of optical quality suitable for preparing active elements were obtained.
Another CVO samples doped with chromium ions were fabricated by high-temperature diffusion doping. Nominally pure CVO plates prepared from the Cz-grown single crystal were used as an effective matrix for impurity ions diffusion from solid phase (Cr2O3 powder) to solid phase (CVO single crystal). The high-temperature diffusion conditions were optimized (temperature 1300 C, time 24 h) to obtain the doped crystals of optical quality using special annealing zone. In diffusion doped CVO crystals estimated Cr ions concentration of 0.12 at.% was 3.5 times higher in comparison with optically homogeneous Cz-CVO:Cr (0.035 at.% Cr, 2.5*1019 cm-3). Some of Cz-grown CVO:Cr samples were annealed in the air at 1000 C during 6 h. In this case typical green coloration of Cr-doped crystals was varied to yellow one.
Green and yellow CVO:Cr samples were investigated by the methods of optical spectroscopy. In CVO:Cr crystals the presence of Cr3+, Cr4+ and Cr5+ ions was detected. The position of absorption and fluorescence lines for chromium ions in all valence states seems not to differ much from that observed previously in forsterite crystal. At the same time lifetimes for chromium ions in all valence states (1.3 ms, 0.3 ms, 9 ms) were measured to be several times shorter than that in forsterite. Annealing of the Ca3(VO4)2:Cr crystal in the air was observed to result in increase of chromium ions in 4+ valence state. Some Cr5+ to Cr3+ ions charge exchange process (Cr5+^Cr4++e-; Cr3++e-^Cr4+) resulting in formation of two Cr4+ ions is suggested to explain the decrease of both Cr5+ and Cr3+ ions concentration while annealing.
