Structural, spectroscopic, and tunable laser properties of Yb3+-doped NaGd(WO4)2

Abstract

Single crystals of Yb3+-doped NaGd(WO4)2 with up to 20 mol % ytterbium content have been grown by the Czochralski technique in air or in N-2+O-2 atmosphere and cooled to room temperature at different rates (4-250 degrees C/h). Only the noncentrosymmetric tetragonal space group I-4 accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group I4(1)/a is much lower for crystals cooled at a fast rate. Na+, Gd3+, and Yb3+ ions share the two nonequivalent 2b and 2d sites of the I-4 structure, but Yb3+ (and Gd3+) ions are found preferentially in the 2b site. Optical spectroscopy at low (5 K) temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the F-2(7/2)(n) and F-2(5/2)(n(')) Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb3+, NaGd(WO4)2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb3+-laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the pi polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014-1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at approximate to 97 MHz repetition rate. Using uncoated samples of Yb:NaGd(WO4)2 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.