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.
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 single crystal of NaGd(WO 4) 2 with disordered structure was grown by the Czochralski method with 5 mol % of Tm 3+-doping in the melt. This crystal host belongs to the I 4 tetragonal space group. The Tm 3+ optical absorption and emission spectroscopy revealed strong polarization dependence in accordance with the S 4 local site symmetry of the uniaxial host. The Stark energy levels of Tm 3+ in this host were determined from 5 K optical absorption and photoluminescence measurements. The 300 K lifetime of the upper laser level for the 3 F 4 → 3 H 6 transition amounts to 1.35±0.20 ms. The maximum ground state absorption cross sections for the 3 H 4 level of interest for diode pumping near 795 nm are 2.9×10-20 and 1.18×10-20 cm 2 for the σ and π polarization, respectively. The maximum emission cross sections amount to 10.6×10-21 cm 2 at 1796 nm and 9.5×10-21 cm 2 at 1847 nm, for the σ and π polarization, respectively. Laser operation in the cw regime has been achieved at room temperature under Ti:sapphire and diode laser pumping using an uncoated sample. A maximum output power of ≈300 mW was obtained at 1925 nm for the σ polarization under Ti:sapphire laser pumping. Slightly higher powers were obtained with diode pumping but the threshold pump power also increased. In this case the laser naturally selected the σ polarization. The optical bands of Tm 3+ in this crystal have inhomogeneous contribution and their FWHM reaches, e.g. for the 3 H 6 → 3 F 4 transition at 5 K, about 60 cm-1. This is related to Na and Gd (or Tm) local disorder between the two nonequivalent lattice sites of the host. The full cw laser tunability range at 300 K, from 1813 to 2025 nm, corresponds to ≈17 THz, and is one of the broadest achieved with a Tm 3+-doped crystalline material, despite the relatively low efficiency obtained with this first sample of Tm:NaGd(WO 4) 2 .
We report, for the first time to our knowledge, femtosecond-pulse operation of a Tm; Ho:NaYðWO 4 Þ 2 laser at around 2060 nm. Transform-limited 191 fs pulses are produced with an average output power of 82 mW at a 144 MHz pulse repetition frequency. Maximum output power of up to 155 mW is generated with a corresponding pulse duration of 258 fs. An ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror is used for passive mode-locking maintenance.
Double tungstate and molybdate compounds with the general formulae MT(WO 4 ) 2 and MT(MoO 4 ) 2 where M is a monovalent alkali cation (Li-Cs) and T is a trivalent cation (Y, La or rare earth Ln) exhibit ordered phases with separate sites for M and T cations and disordered phases where M and T cations are randomly distributed over the same cationic sublattice [1]. Some of the optically passive ordered phases like the monoclinic KGd(WO 4 ) 2 , KY(WO 4 ) 2 , and KLu(WO 4 ) 2 are established laser hosts with very large absorption and emission cross sections of the active dopant. The sodium compounds NaT(WO 4 ) 2 and NaT(MoO 4 ) 2 represent disordered phases with tetragonal structure at room temperature. For T = Y, La, Ce-Er, they exhibit also a congruent melting character. Hence their growth with active Ln-dopants by the Czochralski method is very attractive for the synthesis of novel crystalline laser materials.The revived interest in tetragonal sodium double tungstates and molybdates is due to their potential to ensure larger tunability and bandwidths in mode-locked diodepumped solid-state lasers in comparison to ordered crystals. This is especially true for doping with Yb 3+ which, due to the stronger electron-phonon coupling to the lattice, exhibits intrinsically broader linewidths than the Nd 3+ ion. Hence, the requirements to the pump laser diodes are reduced and Yb-doped disordered hosts hold a greater promise for the generation of mode-locked pulses shorter than 100 fs. It should be added that Yb 3+ possesses longer energy-storage lifetime and smaller quantum defect than Nd 3+ , and that it can be pumped by the optically more robust InGaAs laser diodes operating in the 900 -1000 nm spectral range [2]. Finally, the relatively simple two-manifold structure of Yb 3+ prohibits excited state absorption, up-conversion and cross-relaxation processes.The sodium lanthanum crystals NaLa(WO 4 ) 2 (NaLaW) and NaLa(MoO 4 ) 2 (NaLaMo) were studied in the past as room temperature hosts only for Nd 3+ lasers. Lamp-pumped pulsed laser operation of Nd: NaLaW was reported for the 4F 3/2 -4I 11/2 transition at 1063.5 nm [3,4] and for the 4F 3/2 -4I 13/2 transition at 1335.5 nm [5]. Lamp-pumped operation of Nd:NaLaMo was demonstrated not only in the pulsed regime at 1059.5 -1065.3 nm [6, 7] and 1338 -1344 nm [5] but also in the continuous-wave (cw) regime at 1065.3 nm [8]. In addition NaLaMo, which is an efficient Raman active medium [9], when doped with Nd 3+ , was shown to be an efficient self-converting Raman crystal both in the picosecond [10] and in the nanosecond [11] regime.Here we report on the cw laser performance of Yb :NaLaW and Yb:NaLaMo crystals at room temperature. Note that the isostructural NaGd(WO 4 ) 2 or NaGdW, was the first and so far only disordered laser crystal of this type for which room-temperature cw laser operation could be demonstrated with Yb 3+ doping [12].Single crystals of disordered NaLa(WO 4 ) 2 and NaLa(MoO 4 ) 2 doped with Yb 3+ are grown by the Czochralski method from the melt. Continuous-wave...
The simultaneous correlation of liquid−liquid (LL), liquid−solid (LS), and LLS equilibrium data for four water + organic solvent (alcohol or acetone) + inorganic salt (NaCl) ternary systems at 25 °C has been carried out. First, the NRTL equation was used to formulate the excess Gibbs energy (G E), assuming nondissociation of the dissolved salt in the liquid phase. Afterward, a modification of the NRTL equation taking into account the ionic behavior of the solid (electrolyte-NRTL) was introduced into the simultaneous correlation of the equilibrium regions for the same systems. The binary parameters and the comparison of the experimental and calculated equilibrium data obtained with both the molecular and electrolyte models are presented in this work. The stability analysis based on the Gibbs energy function of mixing, together with the common tangent criteria have been used to validate the parameters calculated.
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