High-quality crystals of monoclinic KLu(WO4)2, shortly KLuW, were grown with sizes sufficient for its characterization and substantial progress was achieved in the field of spectroscopy and laser operation with Yb 3+ -and Tm 3+ -doping. We review the growth methodology for bulk KLuW and epitaxial layers, its structural, thermo-mechanical, and optical properties, the Yb 3+ and Tm 3+ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using InGaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous-wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈ 57 and ≈ 66%, respectively. Output powers as high as 3.28 W were obtained with diode pumping in a simple two-mirror cavity where the slope efficiency with respect to the incident pump power reached ≈ 78%. Passively Q-switched laser operation of bulk Yb:KLuW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈ 1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈ 1138 nm with maximum energies of 32.4 and 14.4 µJ, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode-locking by a semiconductor saturable absorber mirror (SESAM) produced bandwidth-limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Projection of the KLu(WO4)2 structure parallel to the b crystallographic direction [010].Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diodepumped Tm:KLuW laser. The slope efficiency reached with an epitaxial Tm:KLuW laser under Ti:sapphire laser pumping was 64 %. The tunability achieved with bulk and epitaxial Tm:KLuW lasers extended from 1800 to 1987 nm and from 1894 to 2039 nm, respectively.

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.

We present our recent achievements in the growing and optical characterization of KYb(WO 4 ) 2 ͑hereafter KYbW͒ crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies ͑absorption and emission measurements at room and low temperature͒ have been carried out in the spectral region near 1 m characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with 41% slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort ͑ps/fs͒ pulse laser systems.

The crystal structure of monoclinic KYb(WO4)2 (KYbW) crystals has been refined (in space group C2/c) at room temperature by using single‐crystal X‐ray diffraction data. KYbW undoped crystals were grown by the TSSG (top‐seeded‐solution growth) slow‐cooling method. The crystals show {110}, {11}, {010} and {310} faces, which basically define their habit. The linear thermal expansion tensor has been determined and the principal axis with maximum thermal expansion ( = 16.68 × 10−6 K−1), , was located 12° from the c axis. Its principal , and axes are [302], [010] and [106] directions, respectively, in the crystallographic system. The optical tensor has been studied at λ = 632.8 nm at room temperature; two principal axes, Ng and Nm, are located in the ac plane, while the other, Np, is parallel to [010]. The principal axis with maximum refractive index (ng = 2.45), Ng, was located 19° from the c axis.

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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.

The crystal structure of monoclinic KGd(WO4)2 (KGW) has been refined at room temperature by using single‐crystal X‐ray diffraction data. The unit‐cell parameters are a = 10.652 (4), b = 10.374 (6), c = 7.582 (2) Å, β = 130.80 (2)°, with Z = 4, in space group C2/c. The linear thermal expansion tensor has been determined and the principal axes are [302], [010] and [106]. The principal axis with maximum thermal expansion ( = 23.44 × 10−6 K−1), , was located 12° from the c axis. Undoped crystals of KGW and crystals that were partially doped by Pr, Nd, Ho, Er, Tm and Yb were grown by the top‐seeding‐solution growth slow‐cooling method. The effect of doping on the KGW structure was observed in the cell parameters and in morphological changes. The changes in parameters follow the changes in lanthanide ionic radii. The doped crystals show {021} and {21} faces in addition to the {110}, {11}, {010}, {130} and {310} faces which basically follow the habit of the undoped KGW crystals. The development of the faces is related to the number of the most important periodic bond chains parallel to them.

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