High-quality crystals of monoclinic KLu(WO 4 ) 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 power 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 incidert 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 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 diode-pumped 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.

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Growth and properties of KLu(WO4)2, and novel ytterbium and thulium lasers based on this monoclinic crystalline host

Conical refraction was produced by a transparent biaxial crystal of KGd(WO4)2 illuminated by a laser beam. The ring patterns at different distances from the crystal were magnified and projected onto a screen, giving rings whose diameter was 265 mm. Comparison with theory revealed all predicted geometrical and diffraction features: close to the crystal, there are two bright rings of internal conical refraction, separated by the Poggendorff dark ring; secondary diffraction rings decorate the inner bright ring; as the distance from the crystal increases, the inner bright ring condenses onto an axial spot surrounded by diffraction rings. The scales of these features were measured and agreed well with paraxial theory; this involves a single dimensionless parameter r0, defined as the radius of the rings emerging from the crystal divided by the width of the incident beam. The different features emerge clearly in the asymptotic limit r0[1; in these experiments, r0Z60.

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Conical diffraction: observations and theory

http://lib.fibopt.ru/Articles/PAPER/GPI/JCrGrv175%20p135-140.pdf

Highly Yb-doped oxides for thin-disc lasers

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 5K temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F7/2n and 2 F5/2n 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+ , NaGdWO42 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 97 MHz repetition rate. Using uncoated samples of Yb: NaGdWO42 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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Structural, spectroscopic, and tunable laser properties of Yb3+-doped NaGd(WO4)2

By utilizing the proportional relationship between the excitation and absorption spectra for some special excited multiplets of rare-earth (RE) ions that are followed by a very fast nonradiative relaxation to the monitored level, we propose a new approach to determine the Judd-Ofelt (JO) intensity parameters that are crucial to the evaluation of laser and luminescent materials via excitation spectra. To validate this approach, the JO parameters of NaGd(WO(4))(2) : Er(3+) and YLiF(4) : Nd(3+) crystals are calculated and compared through both the excitation and absorption spectra. The JO parameters derived from this approach are in good agreement with that determined from the conventional method (absorption spectra). Furthermore, the JO intensity parameters of Y(2)O(3) : Er(3+) nanocrystals are derived from the excitation spectra by taking into account the nano-size effects, which are comparable to the values of the crystal counterpart. The proposed approach is of particular importance for those powders or nanophosphors with low RE doping concentration that their quantitative absorption spectra are difficult to measure.

Determination of Judd-Ofelt intensity parameters from the excitation spectra for rare-earth doped luminescent materials.

We present our recent achievements in the growing and optical characterization of $\mathrm{KYb}({\mathrm{WO}}_{4}{)}_{2}$ (hereafter KYbW) 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 temperature) have been carried out in the spectral region near $1 \ensuremath{\mu}\mathrm{m}$ 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/fs) pulse laser systems.

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Growth, optical characterization, and laser operation of a stoichiometric crystal KYb(WO 4 ) 2

We demonstrate for the first time laser operation with the monoclinic stoichiometric crystal KYb(WO4)2 (hereafter KYbW). Lasing on the 2
F

 5/2
 ?2
F

 7/2
 transition of Yb3+ at room temperature has been achieved near 1074 nm with >41% slope efficiency (>26% maximum conversion efficiency) using a 0.5-mm-thick plate of KYbW. This new laser material holds great promise for diode-pumped high-power lasers, thin-disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

P. Klopp

Papers

Growth, optical characterization, and laser operation of a stoichiometric crystal KYb(WO 4 ) 2

Laser operation of the new stoichiometric crystal KYb(WO4)2