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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NaBi(WO 4 ) 2 (NBW), NaBi(MoO 4 ) 2 (NBMo) and LiBi(MoO 4 ) 2 (LBMo) single crystals grown by the Czochralski technique have been doped up to a praseodymium concentration of [Pr] ≈ 1 × 10 20 cm −3 in the crystal. 10 K polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 32, 39 and 36 Pr 3+ Stark levels in NBW, NBMo and LBMo crystals, respectively. These energy levels were labelled with the appropriate irreducible representations corresponding to a C 2 local symmetry of an average optical centre. Single-electron Hamiltonians including free-ion and crystal field interactions have been used in the fitting of experimental energy levels and in the simulation of the full sequence of the 4f 2 Pr 3+ configuration. 300 K absorption spectra of different 2S+1 L J Pr 3+ multiplets were determined and used in the context of the Judd-Ofelt theory and for the calculation of the 1 D 2 -related emission cross sections of this average Pr 3+ centre. Non-radiative electron relaxation from the 3 P 0 level feeds the 1 D 2 multiplet. This latter level efficiently decays radiatively to the ground 3 H 4 multiplet but still there is a significant rate of radiative decay to the 1 D 2 → 3 F 3 praseodymium laser channel. For [Pr] 2 × 10 19 cm −3 , non-radiative electric dipole-dipole Pr pair energy transfer limits the radiative yield.

The polarized optical absorption (OA) and photoluminescence of Pr3+ doped KGd(WO4)2 (KGW) single crystals have been measured at selected temperatures between 7 and 300 K. For the studied Pr concentrations, [Pr] = 0.03×1020-1.9×1020 cm-3, a unique site is observed. 74 energy levels were experimentally determined for this site and labelled with the appropriate A or B irreducible representations corresponding to the C2 symmetry of the Gd point site in KGW. The set was fitted to a Hamiltonian of adjustable parameters including free-ion as well as real Bqk and complex Sqk crystal-field parameters. A good simulation of the experimental crystal field energies was achieved with a root mean square deviation σ = 15.3 cm-1. Distortions in the oxygen bonds to Pr3+ are found to contribute to the broadening of some OA bands, particularly those related to the 1D2 multiplet. The OA edge is determined by the interconfigurational 4f→4f15d1 Pr3+ transition peaking at 34 200 cm-1. From the average 300 K OA cross sections the radiative lifetimes of the Pr3+ multiplets have been calculated considering the standard and modified Judd-Ofelt (JO) theories. A better agreement with the experimental results is obtained by the standard theory: the average JO parameters obtained at ω̄2 = 12.0×10-20 cm2, ω̄4 = 8.15×10-20 cm2 and ω̄6 = 2.64×10-20 cm2. Electrons excited to the 3P0 multiplet decay very efficiently to the 1D2 multiplet even at 15 K. In samples with [Pr]⩾0.3×1020 cm-3 the excitation of the 1D2 multiplet decays non-radiatively by an electric dipole-dipole transfer between Pr neighbours.

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Lasing of Yb3+ in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both sigma- and pi-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter.

A thorough study of the RbTiOPO4 (RTP) crystallization in its self-flux and WO3-containing fluxes (10, 20, and 30 mol % WO3) has been performed. The composition regions and isotherms of crystallization were obtained, and most of the crystallized neighboring phases were identified. Afterward, the possibilities of doping and codoping RTP crystals with Er3+ and Nb5+ were studied. Adding Nb2O5 substituting TiO2 in the solution increases the distribution coefficient of Er3+ but changes the crystal morphology toward thin plates significantly. This means it is difficult to grow crystals of sufficient quality and size for research and applications. To optimize the crystal growth process, the conditions for growing doped and codoped RTP single crystals with Er3+ and Nb3+ by the top seeded solution growth technique (TSSG) were studied. For crystal growth from self-flux, stirring the solution with an immersed platinum turbine significantly increased the efficiency of the crystal growth process. These conditions allow achieving 0.65 × 1020 atom·cm-3 as an Er3+ dopant concentration in the crystal. The Judd−Ofelt parameters for Er3+ in RTP:Nb determined from the 300 K optical absorption spectra are Ω2 = 5.99 × 10-20, Ω4 = 0.54 × 10-20, and Ω6 = 0.37 × 10-20 cm2. Finally, the second harmonic generation (SHG) efficiency of RTP:Nb single crystals increased as the concentration of Nb increased up to a 4 atom % of Ti4+ substitution, after which the SHG efficiency decreased.