ИСТИНА

Monoclinic wolframite-type monotungstate crystals M2+WO4 (where M = Mg, Zn, Mn, Ni, etc.) are the promising laser hosts for doping with trivalent rare-earth ions (RE3+ ). They possess good thermo-mechanical properties, strong Raman activity and provide broad and strong absorption and emission bands for the RE3+ ions with polarized light. Recently efficient laser operation in the near-infrared was demonstrated at RE3+ -doped MgWO4 crystals. In the present work, we report on Czochralski (Cz) growth, structure, vibronic properties and polarized room- and low-temperature spectroscopic studies of an Yb 3+ -doped zinc monotungstate crystal. ZnWO4 :5 at.% Yb3+ :5 at.% Li+ : crystal (nominal composition) was grown by the Cz method along the [100] direction from melt in air from a platinum–rhodium crucible. Pulling rate was 1 mm/h, rotation rate 6 rpm. After growth the crystal was cooled to 300K with the speed of 8 K/h. The initial charge was prepared from a mixture of ZnO, WO3, Yb2O3 and Li2CO3 of high purity grade. Lithium ions were added to ensure the change compensation. The solid-phase powder synthesis of the preliminarily carefully mixed charge was performed at 900°C for 12 h. The structure (monoclinic, sp. gr. C42h - P2/c) and the phase purity of the crystals were confirmed by X-ray diffraction. The crystals exhibited a cleavage along the (010) plane. Polarized Raman spectra were measured for the crystal. The most intense Raman band is at ~904 cm-1 . ZnWO4 is an optically biaxial crystal: only one of the optical indicatrix axes (X) coincides with the 2-fold axis (b) and the other two ones (Y and Z) are located in the a-c plane. The Yb3+,Li:ZnWO4 crystals for the spectroscopic studies were oriented in the frame of the optical indicatrix. Absorption and luminescence spectra of Yb3+ ions corresponding to the 2 F 7/2 ↔ 2 F 5/2 transition were measured for light polarizations E || X, Y, Z. The spectra are strongly polarized. The maximum stimulated-emission cross-section is 2.82×10 -20 cm2 at 1055.5 nm for light polarization E || X and the emission bandwidth is ~12 nm. The lifetime of Yb 3+ ions is 367 μs; the luminescence decay is single-exponential. The Stark splitting is resolved using low-temperature (6 K) spectroscopy. The Yb 3+ :ZnWO 4 crystal is promising for CW and mode-locked lasers at ~1 μm.