Energy transfer up-conversion in Tm-doped silica fiber

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm³⁺-doped silica fibers is presented. Up-conversion luminescence was observed from the ³H₄ level of Tm³⁺ under 1586 nm pumping into the ³F₄ level. The quadratic dependence of the up-conversion luminescence at 800 nm on the 1800 nm luminescence from the ³F₄ level confirms that the ³H₄ level is populated by a two photon process. Two possible processes are proposed as mechanisms responsible for the up-conversion: excited state absorption and energy transfer up-conversion. The decay characteristics of the luminescence from the ³H₄ level were studied under direct and indirect pumping at 786 and 1586 nm, respectively. By comparing the decay waveforms to the solution of a simple set of rate equations, the energy transfer up-conversion process (³F₄, ³F₄ → ³H₄, ³H₆) was established at Tm₂O₃ concentrations greater than 200 ppm.     

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO₂ layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb³⁺-Yb³⁺ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state ²F_5/2 to the ²F_7/2 ground state in the Yb³⁺ ions. The Er implanted SiO₂ exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the ²H_11/2 or ⁴S_3/2 energy levels and from the ²H_9/2 or ⁴F_5/2 energy levels to the ⁴I_15/2 ground state, respectively. We have found that the energy transfer from Yb³⁺ to Er³⁺ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er³⁺-Yb³⁺ system.     

Growth and characterization of Nd,Yb – yttrium oxide nanopowders obtained by sol‐gel method

Nanopowders of Y₂O₃ pure, doped and codoped by Nd³⁺, Yb³⁺ were obtained by sol‐gel method. Solution with ethylene glycol was choosed as the proper solution where crystallites of powder with Nd and Yb dopants had the same size. Finally the one‐phased compounds of Y₂O₃ doped 0.5 at% Nd and 1, 2 or 4 at% Yb were obtained. Grain growth and their morphology were investigated in various temperature and time of heating. The changes of crystallite sizes and lattice constants in relation to the heating time and temperature for the composition Y₂O₃ doped 0.5 at% Nd and 2 at% Yb are presented. Y₂O₃ containing 0,5 at% of Nd exhibits intense luminescence bands centered at 920 nm, 1100 nm and 1360 nm whereas a single band at about 1020 nm appears in samples co‐doped with neodymium and ytterbium. Luminescence spectra recorded did not depend on the sample preparation procedure and size of grains. OH impurity affects critically the relaxation dynamics of luminescent ion in nanopowders. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of gold nanoparticles on the up-conversion fluorescence in Sm

Fluorescence spectra of gold colloid containing samarium ions have been recorded and analyzed at room temperature. For gold colloids, two fluorescence emission peaks are observed at 443 nm and 745 nm, respectively when the corresponding excitation wavelength is fixed at 595 nm. When amount of Sm³⁺ are added to gold colloids, the up-conversion emission peak at 443 nm increased greatly. Furthermore, increasing the gold colloids addition leads to the up-conversion luminescent intensity increases in the early stage and then decreased. We believe the enhanced up-conversion are due to the energy transferred from the fluorescence from gold nanoparticles to Sm³⁺, and make the excited state absorption (ESA) in Sm³⁺ takes place.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Luminescence of borogermanate glasses activated by Er and Yb ions

Glasses of the 25Ln₂O₃-25B₂O₃-50GeO₂ composition (mol%) where Ln = (1 − x − y) La, xEr, yYb, with an addition of Al₂O₃ have been obtained and their luminescent characteristics examined. Probabilities of spontaneous emission, peak sections of the induced radiation and quantum yields of luminescence corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions have been defined. Quantum yield of Yb³⁺ luminescence for glasses with low Yb₂O₃ concentration reaches values closed to 100%. The luminescence spectrum of Er³⁺ ions exhibits a broad peak at about 1530 nm with effective width more than 80 nm when excited by irradiation at λ = 977 nm. Spontaneous emission probability and peak stimulated radiation section for Er³⁺ luminescence band ⁴I_13/2 → ⁴I_15/2 were determined to be equal to 175 s⁻¹ and 4.9 × 10⁻²¹ cm² respectively. Effective quenching of both rare-earth activators by oscillations with ν ≈ 2630 and 2270 cm⁻¹ was found. These oscillators, most likely, represent OH-groups connected by a hydrogen bond with non-bridging oxygen atoms in the borogermanate matrix.     

Near infrared and blue cooperative emissions in Yb-doped GeO2-PbO glasses

In previous years there has been great interest in new materials for photonic devices operating at infrared (IR) and visible (VIS) regions. We report here near infrared and blue cooperative luminescence properties for Yb³⁺-doped GeO₂-PbO glasses. Luminescence and lifetime measurements in the VIS and near-IR regions were performed to investigate the spectroscopic characteristics of the glasses. Intense emissions around 507 and 1010 nm were observed using 980 and 808 nm excitation, respectively. The VIS lifetimes (∼0.4 ms) are about half of their respective near infrared ones (∼0.8 ms), as expected for materials in which the VIS emission is caused by the cooperative effect. Regarding the IR emission, the glasses exhibited a high absorption cross-section (1.2 × 10⁻²⁰ cm²) at 978 nm and an emission cross-section of 0.6 × 10⁻²⁰ cm², at 1010 nm, with a minimum pump intensity of 2.8 kW/cm². These results suggest this glass composition as a potential material to be used in devices operating in the VIS and IR spectral range, such as 3-D displays and infrared lasers.     

Thermal stability, spectra and laser properties of Yb: lead-zinc-telluride oxide glasses

A series of new glasses of 70TeO₂-(20 − x) ZnO-xPbO − 5La₂O₃-2.5K₂O-2.5Na₂O (mol%) doped with Yb³⁺ is presented. Thermal stability, spectra and laser properties of Yb³⁺ ions have been measured. It found that 70TeO₂-15PbO-5ZnO-5La₂O₃-2.5K₂O-2.5Na₂O composition glass had fine stability ((T_x−T_g)>190 °C), high-stimulated emission cross-section of 1.25 pm² for the ²F_5/2 → ²F_7/2 transition and existed measured fluorescence lifetime of 0.94 ms and the broad fluorescence effective linewidth of 72 nm. Evaluated from the good potential laser parameters, this system glass is excellent for short pulse generation in diode pumped lasers, short pulse generation tunable lasers, high-peak power and high-average power lasers.     

Investigation of infrared emission and lifetime in Tm-doped 75TeO2:20ZnO:5Na2O (mol%) glasses: Effect of Ho and Yb co-doping

In this paper we describe fabrication and characterization of rare-earth-doped active tellurite glasses to be used as active laser media for fiber lasers emitting in the 2 μm region. The base composition is (mol%): 75TeO₂-20ZnO-5Na₂O with different concentrations of Tm³⁺, Yb³⁺ and Ho³⁺ as dopants or co-dopants. Optical properties of doped glasses were studied and pumping at 800 nm and at 980 nm were tested in order to compare the efficiency of two pumping mechanisms. Optical characterization carried out on glasses containing only Tm³⁺ ions indicated the optimum concentration of Tm₂O₃ in terms of emission efficiency as 1 wt%. The addition of 5 wt% of Yb₂O₃ to Tm³⁺-doped glasses led to the best results in terms of intensity of fluorescence emission and of lifetime values. Yb and Ho co-doped Tm-tellurite glass was measured in emission.     

Evolution of Yb environment and luminescence properties under ionizing irradiation in aluminoborosilicate glasses

Yb-doped aluminoborosilicate glasses were irradiated with 2.3 MeV electron and gamma rays at different doses ranging between 10⁴ and 2.6 × 10⁹ Gy and the local structure around the Yb³⁺ ions has been studied using a combination of NMR, EPR and photoluminescence spectroscopic techniques. The spectroscopic results indicate the presence of two distinct Yb³⁺ sites in these glasses and their relative fractions depend on both the Yb concentration and the irradiation dose. These two sites can be attributed to Yb³⁺ ions with and without Yb next-nearest neighbors. The evolution of the shape of the ⁷F_5/2 → ⁷F_7/2 infrared emission band under irradiation is explained by a preferential reduction of the Yb³⁺ ions with Yb next-nearest neighbors at high integrated doses (> 10⁸ Gy). This interpretation is supported by a strong decrease of the visible cooperative luminescence under irradiation due to the reduction into the Yb pairs. In other terms, it shows an interesting effect of ionizing irradiation on the Yb clusters. A low intensity emission band appears in irradiated samples at 390 nm that can be attributed to Yb²⁺. Finally, the decrease of the Yb³⁺ fluorescence lifetime observed under irradiation appears to be primarily due to electronic interaction of these ions with Non-Bridging Oxygen Hole Centre defects created by the ionizing radiation.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

Thermal, chemical, optical properties and structure of Er-doped and Er/Yb-codoped P2O5-Al2O3-ZnO glasses

This study was explored in series of the optical, thermal, and structure properties based on 60P₂O₅-10Al₂O₃-30ZnO (PAZ) glasses system that doped with varied rare-earth (RE) elements Yb₂O₃/Er₂O₃. The glass transition temperature, softening temperature and chemical durability were increased with RE-doping concentrations increasing, whereas thermal expansion coefficient was decreased. In the optical properties, the absorption and emission intensities also increase with RE-doping concentrations increasing, When Er₂O₃ and Yb₂O₃ concentrations are over than 3 mol% in the Er³⁺-doped PAZ system and Yb³⁺-doped concentration is over than 3 mol% for Er³⁺/Yb³⁺-codoped PAZ system, the emission intensity significantly decreases presumably due to concentration quenching, formation of the ions clustering, and OH⁻ groups in the glasses network. It is suggested that the maximum emission cross-section (σ_e) is 7.64 × 10^− 21 cm² at 1535 nm is observed for 3 mol% Er³⁺-doped PAZ glasses. Moreover, the maximum σ_e × full-width-at-half-maximum is 327.8 for 5 mol% Er³⁺-doped PAZ glasses.     

Broadband infrared luminescence of Ni-doped silicate glass-ceramics containing lithium aluminate spinel nanocrystals

Transparent Ni²⁺-doped SiO₂-Al₂O₃-Ga₂O₃-Li₂O (LGAS) glass-ceramics embedding lithium aluminate spinel nanocrystals was prepared. After heat treatment, LiAl₅O₈ crystallite was precipitated in the glasses, and its size was about 3 nm. It was confirmed from the absorption spectra that the ligand environment of Ni²⁺ ions changed from the trigonal bi-pyramid fivefold sites in the as-made glass to the octahedral sites in the glass-ceramics. Upon excitation at 980 nm, broadband infrared luminescence centered at around 1250 nm with full width at half maximum (FWHM) more than 250 nm was observed originating from the ³T₂(³F) → ³A₂(³F) transition of Ni²⁺ in octahedral sites. The broadband near-infrared (NIR) emission from Ni²⁺-doped glass-ceramics can be as host materials for broadband optical amplifier.     

Er photoluminescence enhancement in Ag-doped sol-gel planar waveguides

This paper reports on the study of the effects of silver (Ag^o) nanoparticles on the optical and spectroscopic properties of Er³⁺-doped silica-based gels and glasses, including active bulk materials and planar waveguides for integrated optics. Two different procedures for silver and erbium ion incorporation into the glassy matrices have been investigated: the direct incorporation of a metal salt (AgNO₃ and/or Er(NO₃)₃) into the sol-gel solution, as well as a modified sol-gel process, based on pore-doping of a precursor gel with AgNO₃ and/or Er(NO₃)₃ solutions. The study of the parameters determining the average size and size distribution of the nanoparticles, together with their influence on the sol-gel material densification and Er³⁺ photoluminescence at 1.5 μm, has been performed by means of transmission electron microscopy, plus ultra-violet/visible and photoluminescence spectroscopies. The Ag^o colloidal nanoparticles, obtained by thermal precipitation, were approximately spherical, homogeneously distributed and they exhibited an average size between ∼2 and 15 nm, depending on the silver content and heat treatment performed. They are shown to be responsible for a remarkable enhancement of the Er³⁺ photoluminescence intensity, which is mainly due to the increase of the local electric field around the Er³⁺ ions, due to the surface plasmon resonance of the Ag^o nanoparticles.     

Microstructure and dielectric properties of La2O3 films prepared by ion beam assistant electron-beam evaporation

Ultrathin La₂O₃ gate dielectric films were prepared on Si substrate by ion assistant electron-beam evaporation. The growth processing, interfacial structure and electrical properties were investigated by various techniques. From XRD results, we found that the La₂O₃ films maintained the amorphous state up to a high annealing temperature of 900 °C for 5 min. From XPS results, we also discovered that the La atoms of the La₂O₃ films did not react with silicon substrate to form any La-compound at the interfacial layer. However, a SiO₂ interfacial layer was formed by the diffusion of O atoms of the La₂O₃ films to the silicon substrate. From the atomic force microscopy image, we disclosed that the surface of the amorphous La₂O₃ film was very flat. Moreover, the La₂O₃ film showed a dielectric constant of 15.5 at 1 MHz, and the leakage current density of the La₂O₃ film was 7.56 × 10⁻⁶ A/cm² at a gate bias voltage of 1 V.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

The spectrum and laser properties of ytterbium doped phosphate glass at low temperature

Low-temperature absorption and fluorescence spectra of the Yb³⁺ ions were measured in phosphate glass with compositions of (60-65)P₂O₅-(4-8)B₂O₃-(5-10)Al₂O₃-(10-15)K₂O-(5-10)BaO-(0-2)La₂O₃-(0-2)Nb₂O₅-(4-8)Yb₂O₃ (mol%). Temperature dependence of lifetime of Yb³⁺:²F_5/2 level was investigated. Laser performance of sample pumped by 940 nm laser diode at low temperature were presented. At 8 K, laser oscillation of diode pumped Yb³⁺: phosphate glass yielded a slope efficiency of 4% and a maximum power of 2 mW, the peak laser wavelength is 1001 nm.     

Optical properties of and concentration quenching in Bi2O3-B2O3-Ga2O3 glasses

Er₂O₃-doped Bi₂O₃-B₂O₃-Ga₂O₃ glasses were prepared by the conventional melt-quenching method, and the Er³⁺:⁴I_13/2 → ⁴I_15/2 fluorescence properties are studied for different Er³⁺ concentrations. when the Er₂O₃ concentration increases from 0.03 to 3.0 mol%, the measured lifetime of Er³⁺:⁴I_13/2 level decrease from 2.24 to 0.9 m s, and from 0.25 to 0.20 m s for the Er³⁺:⁴I_11/2 level. The fast energy migration among Er³⁺ ions cause the reduction of lifetime of the ⁴I_13/2 level, whereas the change in the ⁴I_11/2 level is mainly due to a cooperative upconversion process (⁴I_11/2, ⁴I_11/2) → (⁴F_7/2, ⁴I_15/2). Based on the dipole-dipole interaction theory, the interaction parameter, C_Er,Er, for the migration rate of Er³⁺:⁴I_13/2 ↔ ⁴I_13/2 was calculated to be 32 × 10⁻⁴⁰ cm⁶ s⁻¹.     

Fluorescence of copper, manganese and antimony ions in phosphate glass host

The paper presents a study based on luminescence characteristics of phosphate glasses containing Cu²⁺, Mn⁴⁺ and Sb³⁺. The glass samples obtained by a wet chemical route belong to Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ oxide system. The oxide composition of the glass samples is calculated to obtain a vitreous network composed of metaphosphate chains bonded by modifier ions (Li⁺, Ba²⁺ and La³⁺) and fluorescent ions. The absorption spectra of the samples were acquired in the UV domain in order to establish the excitation wavelength for each fluorescent ion. The absorption peaks of Sb³⁺ ion are ranged at 285 nm and 250 nm, Mn²⁺ ion at 280 nm and 365 nm, Cu²⁺ ion at 295 nm and 313 nm. The luminescence peaks of Cu²⁺, Mn⁴⁺ and Sb³⁺ ions are found in the visible domain at different wavelengths, depending on the oxidation state and coordination symmetry of each fluorescent ion. The fluorescence of Sb³⁺ ion has a strong signal at 450 nm and a weak one at 465 nm, Mn²⁺ ion shows a fluorescence peak at 600 nm and the pair Cu²⁺/Cu⁺ ions reveals a fluorescence emission at 460 nm.     

A partial molar volume for La2O3 in silicate melts

The densities of several La-bearing silicate melts distributed along binary joins have been measured using the double-bob Archimedean method. The results show that the addition of La₂O₃ leads to an increase in the melt density. From these density data, the partial molar volume of La₂O₃ in silicate melts has been determined. Distinct values for partial molar volume of La₂O₃ have been obtained (i.e., 44.01 and 37.04 cm³/mol at 1273 K for the La₂O₃-Na₂O-SiO₂ and La₂O₃-CaO-MgO-Al₂O₃-SiO₂ systems, respectively) indicating a compositional dependence of the partial molar volume of La₂O₃. Nevertheless, a single value for the partial molar volume of La₂O₃ (i.e., 43.47 cm³/mol at 1273 K) suffices to describe the molar volume of melts within errors in the entire multi-component system La₂O₃-Na₂O-CaO-MgO-Al₂O₃-SiO₂ and for La₂O₃ concentrations not greater than 7 mol%. In addition, this work points out that the molar volumes behave ideally within the composition range investigated (i.e., linear variation of the molar volume along the binary joins). Distinct values for the partial molar volume of La₂O₃ obtained in this study and their differences with the molar volumes of molten La₂O₃ given in the literature raise the possibility however that this ideality is not maintained within the entire system.