Epitaxially grown Yb:KLu(WO4)2 composites for continuous-wave and mode-locked lasers in the 1 μm spectral range

Epitaxial layers of up to 50% Yb-doped monoclinic KLu(WO₄)₂ could be successfully grown on passive KLu(WO₄)₂ substrates. These composite samples were characterized and continuous-wave and mode-locked laser operation was achieved with Ti:sapphire and diode-laser pumping. A 10% Yb-doped epitaxy provided an output power exceeding 500 mW at 1030 nm and a maximum slope efficiency of 66% with Ti:sapphire laser pumping. A 50% Yb-doped epitaxy exhibited serious thermal problems without special cooling and rather limited cw performance. Quasi-cw operation provided in this case an average output power of 43 mW at 1032 nm for a 10% duty cycle. More than 100 mW cw could be generated at 1030 nm also with diode-pumping of the 10% Yb-doped KLu(WO₄)₂ epitaxy. Pulses as short as 114 fs were generated at 1030 nm with this same sample under Ti:sapphire laser pumping in a laser mode-locked by a saturable absorber mirror.     

Glasses in the SbPO4–WO3 system

Glasses in the binary system (100 − x)SbPO₄–xWO₃ (20 ⩽ x ⩽ 60, x in mol%) have been prepared and characterized. Differential thermal analysis (DTA) shows that the glass transition temperature, T_g, increases from 412 °C, for samples containing 20 mol% of WO₃, to 481 °C observed for glass containing 60 mol%. Sample containing 40 mol% in WO₃ were observed to be the most stable against devitrification. The structural organization of the glasses has been studied by using Fourier transform infra-red (FTIR), Raman, ³¹P Magic angle spinning (MAS) and spin echo nuclear magnetic resonance (NMR) spectroscopies. Results suggest two distinct networks comprising the glass structure, one with high SbPO₄ content and the other characteristic of the highest WO₃ content samples. The glasses present photochromic properties. Colour changes are observed for samples after exposure to ultraviolet or visible laser light. XANES, at L₁ absorption edge of tungsten, suggests partial reduction from W⁶⁺ to W⁵⁺ species during the laser irradiation. The photochromic effects and the colour changes, promoted by laser excitation, are reversible and easily removed by heat for during 1 h at 150 °C. Subsequent ‘write/erase’ cycles can be done without degradation of the glasses.     

Production and properties of high purity TeO2− WO3−(La2O3, Bi2O3) and TeO2− ZnO−Na2O−Bi2O3 glasses

TeO₂–WO₃ (TW), TeO₂–WO₃–La₂O₃ (TWL), TeO₂–WO₃–La₂O₃–Bi₂O₃ (TWLB) and TeO₂–ZnO–Na₂O–Bi₂O₃ (TZNB) glasses were produced by high-purity oxide mixtures melting in platinum or gold crucible at 800 °C in the atmosphere of purified oxygen. The total content of Cu, Mn, Fe, Co and Ni impurities was not more than 0.1–0.5 ppm wt in the initial oxides and glasses. The stability of TZNB glasses to crystallization, characterized by (T_x ? T_g) value more than 150 °C, was demonstrated by DSC measurements at heating rate 10 K/min. In the case of La₂O₃-containing glasses the thermal effects of both crystallization and fusion of the crystallized phases were not observed. The hydroxyl groups absorption coefficients of pure tellurite glasses at the maximum of the absorption band (λ ~ 3 μm) were in the region of 0.012–0.001 cm^?1. The optical absorption losses, measured by the laser calorimetry method at λ = 1.56 and 1.97 μm, did not exceed 100 dB/km.     

Thermal and optical properties of TeO2–BaO–SrO–Nb2O5 based glasses: New broadband Raman gain media

We systematically added WO₃ (up to 10 mol%) and P₂O₅ (up to 16 mol%) in TeO₂–BaO–SrO–Nb₂O₅ (TBSN) glass system and studied thermal and optical properties of the resultant glasses. The dependences of the additive concentration on glass transition (T_g) and crystallization (T_x) temperatures are presented. The TBSN glass added with ⩾4 mol% WO₃ and P₂O₅ showed high stability against crystallization. The changes in optical band gap energy due to WO₃ and P₂O₅ addition was studied using UV–VIS–NIR absorption spectrometry. The WO₃ addition shifted the optical band gap to longer wavelengths, whereas P₂O₅ addition shifted that to shorter wavelengths. Effects of the WO₃ and P₂O₅ addition on the Raman spectra of TBSN glass are clarified. New Raman bands due to WO₄ and PO₄ tetrahedra formed in the resultant glasses broadened their Raman spectra. Present glasses are characterized by higher thermal stability and wider Raman spectra, therefore, they are promising candidates for fiber Raman amplifiers in photonics systems.     

Local order around tungsten atoms in tungstate fluorophosphate glasses by X-ray absorption spectroscopy

X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO₃–BaF₂–WO₃ glasses and the results were compared with crystalline references Na₂WO₄ and WO₃. XANES measurements at the W-L₁ edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO₃. Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO₃ concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO₃. With increasing WO₃ concentration, it was found that R₂ decreases from 1.96 to 1.92 Å whereas R₃ increases from 2.07 to 2.12 Å.     

On the optical absorption and photoluminescence of Er-doped Ge–S–Ga glasses

We have studied the absorption and photoluminescence (PL) of (GeS₂)₈₀(Ga₂S₃)₂₀ glasses doped with 0.17, 0.35 and 1.05 at.% Er. The sharp bands centered at around 660, 810, 980 and 1540 nm in the absorption spectra can be associated with intra 4f-shell transitions in Er³⁺ ions from ⁴I_15/2 level to ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 levels, respectively. It has been observed that the absorption edge shifts towards lower energies with increasing Er concentration. A decrease in the absorption coefficient in the range of weak absorption, as well as the host luminescence in more heavily doped samples has been established, which may be associated with less native defects in the glassy structure. The role of excitation wavelength (λ_ex) on the PL emission band at 1540 nm using different Er³⁺-doping level has been evaluated. It has been found that the total PL band remains almost the same under direct excitation of Er³⁺ ions (at λ_ex = 644, 770 and 982 nm), while it becomes narrower under the host excitation (at λ_ex = 532 nm).     

Characterization of the reversible photoinduced optical changes in Sb-based glasses

Changes occurring in absorption coefficients when glasses in the SbPO₄–WO₃ binary system were irradiated by light, at the edge of the absorption band, were measured in real time. These glasses present good thermal and optical properties and photoinduced changes in the absorption coefficients are reversible by heat treatment around 150 °C. Subsequent recording/erasing cycles could be made without sample degradation. The sensitivity of the induced optical changes was studied for different wavelengths, light powers and energy of light dose exposures, and for different compositions of the glasses. The changes in the absorption coefficients of the glass samples were accompanied by a color change from yellow to blue, and were also characterized by visible spectroscopy. The color changes occurred through the entire volume of the glass (∼2 mm thickness) for the Ar-ion laser lines at the edge of the absorption band.     

Electronic transport properties of mixed transition metal ions doped borophosphate glasses

A series of borophosphate glasses in the composition (B₂O₃)_0.10–(P₂O₅)_0.40–(CuO)_0.50?x–(MoO₃)_x; 0.05 ? x ? 0.50 have been investigated for room temperature density and dc conductivity over the temperature range from 350 to 650 K. The density decreased with increase in MoO₃ over the composition range studied except a slight increase around 0.35 mole fraction. The observed initial decrease in conductivity with the addition of MoO₃ has been attributed to the hindrance offered by the Mo⁺ ions to the electronic motions. The observed peak-like behavior in conductivity in the composition range 0.20 – 0.50 mol% of MoO₃ is ascribed to the mixed transition metal ion effect (MTE). Mott’s small polaron hopping model has been used to analyze the high temperature conductivity data and the activation energy for conduction has been determined. The low temperature conductivity has been analyzed in view of Mott’s and Greaves variable range hopping models. It is for the first time that conduction mechanisms have been explored and MTE detected in mixed transition metal ions doped borophosphate glasses.     

Spectroscopic studies of tungsten ions in PbO–PbF2–SiO2 glasses

40PbO–(10 ? x)PbF₂–50 SiO₂:xWO₃ (where x = 1 to 7 mol%) glasses are prepared in the glass forming region. Spectroscopic studies (UV–Vis absorption, ESR, IR) are carried out for these glasses. Interesting changes are observed in the spectroscopic parameters of these glasses when the concentration of WO₃ is changing in the glass matrix. Two absorption bands are observed around at 830 and 620 nm. ESR signal are measured at room temperature for these glasses, the strength of the signal is increased and hyperfine splitting is resolved with increasing the concentration of WO₃ in the glass matrix. IR transmission gives valuable information about the nature of bonds in the glass matrix. The physical parameters along with spectroscopic parameters are measured.     

Synthesis and characterization of M2O–MgO–WO3–P2O5 (M = K, Rb, Cs) glass system

Transparent glasses of the composition M₂O–MgO–WO₃–P₂O₅ (M = K, Rb, Cs), corresponding to the crystalline phases of M₂MgWO₂(PO₄)₂, have been prepared and studied by Raman and IR spectroscopy as well as DTA. Moreover, the thermally stimulated depolarization and dc conductivity have been measured. The glass transition temperature is 797, 795 and 773 K for the K-, Rb- and Cs-containing glass, respectively. Raman and IR studies have shown that these glasses have very similar structure. The main building blocks are pyrophosphate groups, WO₆ octahedra and magnesium–oxygen polyhedra. The dc conduction in these glasses is controlled by hopping of small polarons. The potassium containing glass was shown to be very stable whereas the rubidium and cesium glasses have significantly higher tendency for crystallization and phase separation. It seems, therefore, that the potassium containing glass is a suitable material for the preparation of samples containing non-linear and ferroelectric nanocrystals of the K₂MgWO₂(PO₄)₂ phosphate.     

Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses

The glasses representing (Bi₂O₃)_x(WO₃)_y(TeO₂)_100?x?y and (PbO)_x(WO₃)_y(TeO₂)_100?x?y systems were prepared. The dilatometric glass-transition temperatures of examined glass samples were found in the region 383–434 °C, the coefficient of thermal expansion varied from 12 to 16 ppm/°C and the density ranged from 6.30₂ to 6.80₈ g/cm³. From the optical transmission measurements of thin glassy bulk samples prepared by a glass blowing, the optical gap values were found in the narrow region 3.21–3.36 eV. For the temperature interval 300–480 K, the values of the temperature coefficient of the optical band gap varied from 3.7 × 10^?4 to 5.24 × 10^?4 eV/K. It is suggested that Raman feature observed at around 350 cm^?1 can be assigned to an overlap of Raman bands attributed to WO₆ corner shared octahedra and to the following three atomic linkages: Bi–O–Te, Pb–O–Te and W–O–Te.     

Local study on the MoO4 units in AgI-doped silver molybdate glasses

The short-range order around molybdenum has been investigated in AgI-doped silver molybdate glasses (AgI)_x(Ag₂MoO₄)_1?x (with x = 0.67 and 0.75) by Mo–K edge EXAFS measurements as a function of temperature. The difference from crystalline Ag₂MoO₄ is weak. A softening of the Mo–O nearest-neighbours bond has been detected, but the MoO₄ units still exhibit high rigidity. Above the T_g temperature, no meaningful evidence of local structural and dynamical modifications has been observed around molybdenum.     

Dielectric properties,conductivity, UV–visible and infrared spectroscopy of PbO P2O5 NaF glasses containing WO3

Homogeneous (50P₂O₅–(30 ? x)PbO–20NaF–xWO₃ where x = 0.0, 5, 10 and 15 mol%) glasses were synthesized using a melt-quenching method. The short range structures of the phosphate samples were examined by Fourier transform infrared spectroscopy. The infrared spectral studies have pointed out the existence of conventional PO₄, WO₄ and WO₆ structural units in the glass network, the number of WO₄ tetrahedra decreases as WO₃ concentration increases. The optical transmittance and reflectance spectrum of the glasses have been recorded in the wavelength range of 190–1100 nm. The values of the optical band gap E_op for all types of electronic transitions and refractive index have been determined and discussed. The real and imaginary parts ε₁ and ε₂ of the dielectric constant have been determined. The type of electronic transitions in the present glass system is indirectly allowed and the high values for the refractive index and dispersion are recorded due to the high polarizability of tungsten ions. The results of refractive indices as determined reveal the homogeneity of samples and were found to depend on the glass composition. The electrical properties of the glasses were investigated by ac conductivity from 0.12 to 100 kHz for temperatures ranging from room temperature to 600 K. The study of dielectric properties suggested increase in the insulating character of the glass system with increase in the content of WO₃. The ac conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from W⁴⁺ to W⁵⁺ ions.     

Optical properties of erbium-doped fluorochlorozirconate glasses

We report the optical properties of a fluorochlorozirconate (FCZ) glass with the composition 53% ZrF₄, 20% NaF, 3.5% AlF₃, 3% LaF₃, 0.5% InF₃, (20 ? x)% BaCl₂, x% BaF₂ with x varying from 0% to 2%, and doped with various amounts of trivalent erbium by the addition of ErCl₃. Annealing of the as-prepared glass in inert (N₂) or reducing (5%H₂ + 95%Ar) atmospheres at temperatures that ensure the conversion of the glass into a glass-ceramic by the nucleation of BaCl₂ nanocrystals, does not significantly change any of Er³⁺ related absorption and photoluminescence (PL) characteristics. We have carried out a Judd–Ofelt analysis of the absorption spectra and obtained Ω₂ = (1.92 ± 0.3) × 10^? 20 cm², Ω₄ = (0.88 ± 0.16) × 10^? 20 cm² and Ω₆ = (0.59 ± 0.08) × 10^? 20 cm², and also the radiative lifetimes of the ⁴I_13/2→⁴I_15/2, ⁴I_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 bands. The radiative lifetime from the Judd–Ofelt analysis for the ⁴I_13/2→⁴I_15/2 band is in good agreement with the experimentally measured PL decay time. The examination of the optical properties of powdered samples with different average particle size does not show any photon trapping effects. We have determined the spectral absorption and emission cross-sections and then estimated the possible spectral optical gain for varying degrees of relative populations of the ⁴I_13/2 and ⁴I_15/2 manifolds.     

Effect on the fluorescence branching ratio of different synergistic ligands in neodymium complex doped PMMA

A series of neodymium complexes Nd(TTA)₃L_x (where TTA = α-thenoyltrifluoroacetonato, L_x (x = 1–5) = H₂O, triophenylphosphine oxide (Tppo), 2,2-bipyridine (Bipy), 1,10-phenanthroline (Phen) and 2- (N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (Dpbt) were synthesized and incorporated in poly(methyl methacrylate). Their absorption spectra were measured and analyzed using Judd–Ofelt theory. Near-infrared luminescent spectra were studied and the radiative properties have been stimulated. Laser parameters such as effective bandwidths (Δλ_eff), stimulated emission cross-sections (σ_e), and gain bandwidth (σ_g) had also been calculated and compared with other systems. The effect on the fluorescence branching ratio (β) in the ⁴F_3/2→⁴I_11/2 transition of different synergistic ligands had been investigated and the relation between β and Judd–Ofelt parameter Ω₂, Ω₄, Ω₆ was discussed. In conclusion, among the five neodymium complexes, Nd(TTA)₃Dpbt has the largest Ω₂ parameter (33.72 × 10^? 20 cm²), stimulated emission cross-sections, which is found promising to be a candidate for laser materials in further application.     

Quantum Size Effect and very localized random laser in ZnO@mesoporous silica nanocomposite following a two-photon absorption process

ZnO@mesoporous silica nanocomposite was prepared by the impregnation of a CMI-1 material in a Zn(NO₃)₂ solution followed by calcination under O₂. Intensive characterization was carried out by N₂ adsorption–desorption measurements, scanning and transmission electron microscopy. The optical properties of the ZnO@mesoporous silica nanocomposite were studied by photoluminescence spectroscopy. Quantum Size Effect was firstly demonstrated by subjecting the sample to a 254 nm excitation light, and was further confirmed by using a 680 nm excitation laser beam, which implies a two-photon absorption process. By focusing the 680 nm laser beam on different places in the sample, a very localized random laser effect, also induced by a two-photon absorption process, was detected.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Preparation,crystallization behavior and magnetic properties of nanoparticles magnetic glass–ceramics in the systems Fe2O3 CoO MnO2, Fe2O3 NiO MoO3 and Fe2O3 CoO V2O5

Ferrimagnetic glass–ceramics were prepared in the systems Fe₂O₃ CoO MnO₂ (S1), Fe₂O₃ NiO MoO₃ (S2) and Fe₂O₃ CoO V₂O₅ (S3). Small amount of H₃BO₄ was added to make the melting process easier. The samples were characterized using DTA, XRD, TEM and EDX. Sequence of crystallization was studied by applying heat-treatment at 800 and 1000 °C for 4 h. CoFe₂O₄ with crystallite sizes of ≈ 14–20 nm was successfully prepared beside FeCoOBO₃ and Co₃BO₅ in S1. NiMoO₄, (FeNi₂)O₂(BO₃) and NiO with crystallite size ≈ 56–79 nm were crystallized in S2. CoFe₂O₄, FeCoOBO₃ and Co₃BO₅ with crystallite size ≈ 6–8 nm were crystallized in S3. Magnetic hysteresis cycles were analyzed with a maximum applied field of 20 kOe at room temperature. From the obtained hysteresis loops Ms records higher values for S1 and S3 and lower value for S2, while coercivity reach maximum for S2. The variable, magnetic, data range gives a wide range for different applications.     

FTIR and UV–VIS optical absorption spectra of gamma-irradiated MoO3-doped lead borate glasses

Structure and optical properties of MoO₃-doped lead borate glasses which contain high PbO content (60, 70 and 80%) have been studied using Fourier transform infrared (FTIR) and ultraviolet–visible (UV–VIS) spectroscopic tools. FTIR spectra reveal absorption bands which are characteristic for various structural units of borate network, mainly BO₃ triangles and BO₄ tetrahedra, in addition to the PbO_n (where n = 3 and/or 4) structural units. UV–VIS optical absorption spectra reveal broad intense charge transfer UV bands due to Pb^2 + ions in the range 320–385 nm. Within this range, molybdenum ions, preferably Mo^3 + and Mo^5 +, can interfere at about 360–385 nm. Additionally, molybdenum ions give a weak visible band at about 850–860 nm. The optical absorption spectra of the studied glasses show marked resistance to successive gamma irradiation up to 5 Mrad. This shielding behavior can be related to the present high content of the high atomic mass Pb^2 + ions. Changes in the atomic structure before and after gamma irradiation are described and explained.