Abstract. KGd(WO 4 ) 2 single crystals doped with Er 3+ have been grown by the flux top-seeded-solution growth method. The crystallographic structure of the lattice has been refined, being the lattice constants a = 10.652(4), b = 10.374(6), c = 7.582(2) Å, β = 130.80(2)• . The refractive index dispersion of the host has been measured in the 350-1500 nm range. The optical absorption and photoluminescence properties of Er The technological interest in the development of solidstate lasers for application in long-distance optical communications has promoted the study of laser ions with an emission close to the minimum of the optical losses in silica fibers, namely 1.5 µm. The present development of a new laser generation requires us to find crystals with low excitation threshold and suited to be excited by the emission of diode lasers. Er 3+ only has weak absorption bands in the 600-1000 nm region, but its photoluminescence can be sensitised by energy transfer from Yb 3+ , which shows a strong optical absorption in the 900-1000 nm range. This region overlaps the emission of InGaAs diode lasers. As a matter of fact, InGaAs diodepumped room-temperature laser operation has been recently demonstrated in KGd(WO 4 ) 2 :Yb:Er crystals [7] (hereafter KGd(WO 4 ) 2 is abbreviated as KGW), however the efficiency of the process was weak and the physical processes involved were poorly understood. Moreover, Er has been used to sensitise the Tm 3+ emission in KGW crystals at liquid nitrogen temperature [8].Despite the relevance of the optical properties of Er 3+ in KGW crystals, its spectroscopic properties have been reported at 77 K only for the 4 S 3/2 or lower energy levels [9,10]. The present work reports a spectroscopic study of the Er 3+ ions incorporated in KGW crystals grown by the flux top-seeded-solution growth (TSSG) technique.KGW crystals have been also used as a laser host for Nd 3+ ions because of the high efficiency of the 4 F 3/2 → 4 I 11/2 transition [11,12] as well as a host for other rare-earth laser ions [8]. Recently, some research has focused attention on crystals with relevant cubic nonlinearity χ (3) because with these materials it is possible to obtain unconventional lasers, such as lasers with stimulated-Raman-scattering (SRS) frequency self-conversion. The KGW:Nd possesses an effective cubic nonlinearity of about 10 −13 esu and presents a good efficiency in the process of SRS self-conversion [13].In view of the relevance of the KGW lattice host, we have also performed a refinement of the crystal structure, in order to improve the currently known lattice constants and to help in the discussion of the local lattice site symmetry when required. Further, we discuss the orientation of the indicatrix of the crystal with regards to the crystallographic axes and we have obtained the value of the refractive indices in a wide spectral region.

Single crystals of Yb 3+ -doped NaGd͑WO 4 ͒ 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°C/h͒. Only the noncentrosymmetric tetragonal space group I4 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 + , Gd 3+ , and Yb 3+ ions share the two nonequivalent 2b and 2d sites of the I4 structure, but Yb 3+ ͑and Gd 3+ ͒ ions are found preferentially in the 2b site. Optical spectroscopy at low ͑5 K͒ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F 7/2 ͑n͒ and 2 F 5/2 ͑nЈ͒ 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 Yb 3+ , NaGd͑WO 4 ͒ 2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -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 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 Ϸ97 MHz repetition rate. Using uncoated samples of Yb: NaGd͑WO 4 ͒ 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.

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