Photoluminescence and up-conversion of Er3+ in tetragonal NaBi(XO4)(2), X=Mo or W, scheelites

Er doped NaBi(XO4)(2), X = Mo or W, single crystals with tetragonal structu re have been grown with optical quality by the Czochralski method. Er conce ntrations up to 1.4x10(20) cm(-3) and 1.3X10(20) cm(-3) have been attained in Mo and W compounds respectively. S-4(3/2)--> I-4(15/2) emission of Er3has been achieved at room temperature by up-conversion exciting the I-4(11/ 2) or I-4(9/2) multiplets. This green emission is strongly polarised parall el to the crystallographic c-axis. The efficiency of the up-conversion proc ess in tetragonal NaBi(MoO4)(2) and NaBi(WO4)(2) crystals are similar and a lso similar to that found in monoclinic KGd(WO4)(2) crystal. The slope of t he logarithmic dependence between up-converted and pump intensities suggest s a two-photon excitation process. A schematic model of the up-conversion p rocess is outlined. (C) 2001 Elsevier Science B.V. All rights reserved.