The main advantages of the monoclinic potassium double tungstates as laser hosts are the very high values of the absorption and emission cross sections of the rare-earth dopants, partly due to the strong anisotropy of these biaxial crystals, and the possibility to dope them with high concentration of the active ions without substantial fluorescence quenching. The monoclinic KT(WO4)2 crystals where T=Gd, Y or Lu, were all known as Nd-hosts for efficient flash-lamp pumped lasers but in the last decade they played an essential role in the development of diode pumped Yb-laser systems operating in the same 1-µm spectral range and more recently showed their high potential also in the 2 µm spectral range with Tm- and Ho-ion doping. Tetragonal double tungstates with the same chemical formula but mainly Na or Li as alkali ion and T=La, Gd, Y, Lu, or Bi as trivalent passive ions (as well as the related molybdates) are higher symmetry uniaxial crystals which show local disorder, a property related to inhomogeneous broadening of the dopant spectral lines which is attractive for tunable or short pulse laser operation. In the last decade, the interest in such rare-earth laser hosts has experienced a revival, related also mainly to the Yb-, Tm-, Ho-, and Er-doping, in addition to the well known laser operation with Nd-doping. Both monoclinic and tetragonal double tungstates can be considered to be multifunctional materials since they are attractive also as Raman active crystals, not only in the nanosecond but also in the picosecond regime.

© 2011 Optical Society of America

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