Monoclinic KGd Research Articles

Red Sm:KGd(WO4)2 laser at 649 nm.

We report on the first laser operation of a Sm3+-doped monoclinic KGd(WO4)2 double tungstate crystal in the red spectral range. Pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) at 479.1 nm, the 0.8 at.% Sm:KGd(WO4)2 laser generated an output power up to 17.6 mW at 649.1 nm (the 4G5/2 → 6H9/2 transition) with a slope efficiency of 16.9%, a laser threshold down to 29 mW and a linear polarization. The laser exhibited a self-pulsing behavior, delivering µs-long pulses with a repetition rate of a few kHz. The polarized spectroscopic properties of Sm3+ ions were determined as well.

Polarized spectroscopy of Sm3+ ions in monoclinic KGd(WO4)2 crystals

We report on a polarization-resolved spectroscopic study of Sm3+-doped monoclinic KGd(WO4)2 crystals. The transition probabilities for Sm3+ ions were calculated using a modified Judd-Ofelt theory. For the 4G5/2 → 6H9/2 transition in the red spectral range, the stimulated-emission cross-section is 5.59×10-21 cm² at 649.0 nm (for light polarization E || Np) and the luminescence lifetime of the 4G5/2 state is 719 μs (0.4 at.% Sm3+-doping). Sm:KGd(WO4)2 is promising for orange and red lasers.

Potassium gadolinium tungstate nanocrystals doped with holmium ions as candidates for optical imaging

Holmium-doped monoclinic KGd(WO4)2 nanoparticles were synthesized by the mechano-chemical method. The structure and morphology of KGW:Ho 10 at.% nanopowders were investigated with the use of X-ray Powder Diffraction, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Emission and excitation spectra of KGW nanocrystals were also measured using a fluorometer with a xenon lamp and OPO laser source. The influence of nanoparticles of KGW doped with 10 at.% of holmium on the viability of J774.1 mouse BALB/c monocyte macrophage cells was tested by the WST8 assay. Preliminary immunity cell studies demonstrated a high viability of KGW:Ho 10 at%. Hence, these nanoparticles are promising for imaging applications. To characterize and quantify the physical stability of the nanoparticles of KGW:Ho 10 at.% suitable for biomedical applications, nanoparticles of KGW:Ho in the range of 80–200 nm were prepared.

Revealing modes from controlling an off-optical axis conical diffraction laser

Thanks to controlling the off-optical axis propagation in the monoclinic KGd(WO4)2:Nd biaxial crystal used as a laser gain medium, we exhibited remarkable crescent laser modes and mode switching, demonstrating the preservation of the conical diffraction. We revealed an optical singularity in addition to the optical axis: the directions of polarization changed abruptly to the highest emission cross-section mode, which leads to un-polarized lasing. The key explanation is a severe mode selection due to the threshold behavior of lasing. This is confirmed by a theoretical model taking into account a two-axes crystal rotation and including the conical diffraction behavior. The rotation around the two-fold b crystallographic axis of the frame, which makes the imaginary part of the dielectric susceptibility tensor diagonal, is exhibited.

Synthesis, structural and vibrational studies on mixed alkali metal gadolinium double tungstate, K1−xNaxGd(WO4)2

Mixed alkali double tungstates K1−xNaxGd(WO4)2 (KNGW) (0 ⩽ x ⩽ 1) were synthesized by solid state reaction using sodium doped monoclinic KGd(WO4)2 (KGW). Synthesized KNGW powders were characterized using powder X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM) and Raman analysis. DTA analysis confirms that the melting point of the KGW matrix increases from 1063°C to 1255°C with increasing sodium content. The Powder XRD analyses reveal that mixed phases were observed up to 40 wt.% of Na in the KGW matrix above that percentage there is domination of scheelite structure in the synthesized powder. Polyhedral type, bi-pyramidal shape and spheroid shape morphology was observed for KGW, NKGW and NGW powders respectively. The Raman analysis was carried out to understand the vibrational characteristic changes with mixing of sodium ions in the KGW matrix.

Synthesis and characterization of monoclinic KGd(WO4)2 particles for non-cubic transparent ceramics

Monoclinic KGd(WO4)2 (KGW) particles were synthesized using polymeric metal complex sol–gel synthesis method. The derived particles were made as colloidal suspension form for better rotation of particles to have optimum response for applied magnetic field. The KGW fine particle suspension was prepared using electrostatic repulsion by electrolyte polymer. Prepared suspension was kept at 2T and particles were settled using electrophoretic sedimentation using pH adjustment. The derived sediment was dried at 100°C and the properties were characterized using XRD.

Synthesis of monoclinic KGd(WO4)2 nanocrystals by two preparation methods

The monoclinic crystalline phase of KGd(WO4)2 is a well-known solid-state laser host. The present article describes how monoclinic nanocrystals of KGd(WO4)2 are synthesized by two different methodologies: the sol–gel modified Pechini method and femtosecond pulsed laser ablation of bulk material in deionized water. Transmission Electron Microscopy images show KGdW nanocrystals between 15 and 100 nm for both methods. The nanocrystals are studied by X-ray powder diffraction (XRD) in order to identify the crystalline phase and to refine the unit cell parameters. For the sol–gel modified Pechini method, differential thermal analyses and XRD techniques were also used to optimize the method of preparation.

Negative thermo-optical coefficients and athermal directions in monoclinic KGd(WO4)2 and KY(WO4)2 laser host crystals in the visible region

The temperature dependence of three principal refractive indices of the biaxial KGd(WO4)2 and KY(WO4)2 crystals in the temperature range of 288–673 K was measured at wavelengths of 438.5 and 632.8 nm. Their thermo-optical coefficients (TOC) of the first and second orders were determined on the basis of the experimental data obtained. TOC were found to be negative for the ng and np refractive indices of the KY(WO4)2 crystal and for the ng refractive index of the KGd(WO4)2 crystal. It was established that in a fairly wide temperature range contributions of the positive and negative TOC to the refractive index of a light wave propagating in these crystals in certain directions compensate each other. Such athermal directions were determined for the index ellipsoid principal planes of these crystals on the basis of experimental data.

1.84 μ m emission of Tm3+ sensitized by Yb3+ ions in monoclinic KGd(WO4)2 single crystals

By exciting at 940nm, we have characterized the 1.84μm near infrared emission of trivalent thulium ions in Yb3+, Tm3+:KGd(WO4)2 single crystals as a function of the dopant concentration and temperature, from 10K to room temperature. An overall H63 Stark splitting of ∼470cm−1 for the Tm3+ ions in the Yb3+, Tm3+:KGd(WO4)2 was obtained. We also studied the blue emission at 476nm (Tm3+) and the near infrared emissions at 1.48μm (Tm3+) and 1μm (Yb3+) as a function of the dopant concentration. Experimental decay times of the G41, H43, and F43 (Tm3+) and F5∕22 (Yb3+) excited states have been measured as a function of Yb3+ and Tm3+ ion concentrations. For the F43→H63 transition of Tm3+ ions, we used the reciprocity method to calculate the maximum emission cross section of 3.07×10−20cm2 at 1.84μm for the polarization parallel to the Nm principal optical direction.

Upconversion luminescence of Tm3+ sensitized by Yb3+ ions in monoclinic KGd(WO4)2 single crystals

We observed a significant blue emission at 476nm in Tm3+ and Yb3+-codoped KGd(WO4)2 monoclinic single crystals by pumping at 940nm. The single crystals were grown using the top-seeded-solution-growth slow-cooling method (TSSG). We also report thulium emissions at 650nm, 802nm and 1.48μm. The emission spectra are characterized as a function of the Tm3+ and Yb3+ ions concentration and temperature from 6K to room temperature. The energy upconversion excitation mechanism for thulium emitting levels is assigned to a cooperative energy transfer process by the ytterbium-sensitizer ions.

Crystal growth and spectroscopic analysis of codoped (Ho,Tm):KGd(WO 4 ) 2

We present the crystal growth of monoclinic KGd(WO4)2 codoped with holmium and thulium by a top-seeded solution growth and slow cooling method. Macrodefect-free crystals with sizes about 8.5×5×12 (\(a^{\ast}\times{b}\times{c}\), dimensions in mm) were obtained. A complete study of the spectroscopic parameters of the holmium emission \({} 5 I_7\to{} 5 I_8\), around 2 microns, sensitized by the thulium ion, was made. Polarized absorption measurements were performed to characterize the optimum pumping wavelength in the system. The maximum value of the cross section is 8.0×1020 cm2 with E//Np direction at a wavelength value of 794 nm. The emission cross section of the 2-micron holmium transition has been calculated by the reciprocity method. The experimental lifetime of the 5I7 level has been measured in relation to holmium and thulium concentrations.

1.48 and 1.84 μm thulium emissions in monoclinic KGd(WO4)2 single crystals

By exciting at 788 nm, we have characterized the near infrared emissions of trivalent thulium ions in monoclinic KGd(WO4)2 single crystals at 1.48 and 1.84 μm as a function of dopant concentration from 0.1% to 10% and temperature from 10 K to room temperature. We used the reciprocity method to calculate the maximum emission cross-section of 3.0×10−20 cm2 at 1.838 μm for the polarization parallel to the Nm principal optical direction. These results agrees well with the experimental data. Experimental decay times of the H43→3F4 and F43→3H6 transitions have been measured as a function of thulium concentration.

Blue luminescence in Tm 3+-doped KGd(WO 4) 2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO 4) 2 single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO 4) 2 single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the 3F 2+ 3F 3 energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the 3H 6 ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.

Photoluminescence and up-conversion of Er 3+ in tetragonal NaBi(XO 4) 2, X=Mo or W, scheelites

Er doped NaBi(XO 4) 2, X=Mo or W, single crystals with tetragonal structure have been grown with optical quality by the Czochralski method. Er concentrations up to 1.4×10 20 cm −3 and 1.3×10 20 cm −3 have been attained in Mo and W compounds respectively. 4S 3/2→ 4I 15/2 emission of Er 3+ has been achieved at room temperature by up-conversion exciting the 4I 11/2 or 4I 9/2 multiplets. This green emission is strongly polarised parallel to the crystallographic c-axis. The efficiency of the up-conversion process in tetragonal NaBi(MoO 4) 2 and NaBi(WO 4) 2 crystals are similar and also similar to that found in monoclinic KGd(WO 4) 2 crystal. The slope of the logarithmic dependence between up-converted and pump intensities suggests a two-photon excitation process. A schematic model of the up-conversion process is outlined.

Structural study of monoclinic KGd(WO4)2and effects of lanthanide substitution

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 area= 10.652 (4),b= 10.374 (6),c= 7.582 (2) Å, β = 130.80 (2)°, withZ= 4, in space groupC2/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 (\boldalpha'_{33} = 23.44 × 10−6 K−1), {\bf X}'_{3}, was located 12° from thecaxis. 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 {\bar{2}21} faces in addition to the {110}, {\bar{1}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.