Population dynamics of the F4 and H4 levels in highly-doped Tm:ZB(L)AN glasses

Population dynamics of the ³F₄ and ³H₄ levels in Tm³⁺ doped ZB(L)AN glasses was studied for Tm³⁺ concentrations from 0.5 to 12 mol%. Fluorescence waveforms from these levels were measured at 1.8 μm (³F₄) and 800 nm (³H₄) with both direct and indirect pumping. Decay from the ³F₄ level was found to be exponential with non-radiative decay rates proportional to the square of the Tm concentration. This indicated a process of energy migration by diffusion within the excited Tm³⁺ ions followed by quenching at sites to which the ions could migrate. The decay of the directly pumped ³H₄ level exhibited both exponential and non-exponential behavior depending on the concentration. For the lowest concentration (0.5 mol%) the decay was exponential, but at concentrations of 1, 2, 4 and 6 mol% the decay waveforms were distinctly non-exponential. The non-exponential waveforms could be fitted by the Yokota-Tanimoto model for diffusion of excited donors and dipole-dipole interactions with acceptors. This model produced values for C_DD and C_DA, the donor-donor and donor-acceptor energy transfer parameters, respectively. At the higher concentrations (8, 10, 12 mol%) the waveforms were exponential with decay rates from which the cross-relaxation parameter for the process ³H₄, ³H₆ → ³F₄, ³F₄ was obtained. When the ³F₄ level is pumped at 1660 nm, the decay of the ³H₄ level confirmed the influence of the up-conversion energy transfer process ³F₄, ³F₄ → ³H₄, ³H₆.     

1.48 μm emission properties and the cross-relaxation mechanism in chalcohalide glass doped with Tm

1.48 μm emission properties and the cross-relaxation mechanism of Tm³⁺ ions in 0.7(Ge_0.25As_0.10S_0.65) + 0.15GaS_3/2 + 0.15CsBr glass were investigated. Both the relative intensity ratio of the 1.48 μm emission to 1.82 μm and the measured lifetime of the ³H₄ level decreased with increasing Tm³⁺ concentration. When temperature decreased from room temperature to 20 K, lifetimes of the ³H₄ level increased from 670 to 970 μs. At the same time, the critical distance for cross-relaxation decreased from 1.11 to 0.93 nm. These results indicate that cross-relaxation (³H₄, ³H₆ → ³F₄, ³F₄) became less effective as temperature decreased. Analysis of the temperature dependence of cross-relaxation rates showed that cross-relaxation in Tm³⁺ is a phonon-assisted energy transfer process. The major phonon contributing to the process is from the Ga-Br vibration in [GaS_3/2Br]⁻ units.     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.     

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.     

Luminescence characteristics of Yb, La codoped yttrium oxide nanopowders

Luminescence characteristics of Yb³⁺, La³⁺ codoped yttrium oxide nanopowders were investigated. The grain size and the crystallinity of (Yb_0.05Y_0.90La_0.05)₂O₃ nanopowders increase with the increase of calcination temperature. The average grain size of the nanopowders calcined at 1100 °C is 66 nm and its cooperative up-conversion luminescence centered at 498 nm was detected due to nanometer size effect and perfect crystallinity. However, the cooperative up-conversion luminescence of (Yb_0.05Y_0.90La_0.05)₂O₃ transparent ceramics was not detected.     

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.     

Float zone growth of Dy:GdVO4 single crystals for potential use in solid-state yellow lasers

Single crystals of dysprosium-doped gadolinium orthovanadate (Dy:GdVO₄) were successfully grown by the floating zone method and their fluorescence properties were investigated. The as-grown crystals did not contain any macroscopic defects such as cracks and inclusions for any Dy-concentration of up to 4 at%. Every crystal showed optical homogeneity under observation with a polarizing microscope; that is, no low-angle grain boundaries and growth striations were detected. In the visible region, two distinct fluorescence bands were observed around 480 and 575 nm, corresponding to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions, respectively. The excitation spectrum for the emission of 573 nm indicates the possibility to use a commercially available laser diode of 450 nm as a pumping source for solid-state yellow laser.     

1540 nm fiber laser excited upconversion luminescence in erbium-doped lead-fluoride nanocrystals

Upconversion luminescence in erbium-doped PbGeO₃-PbF₂-CdF₂-based vitroceramic under 1540 nm infrared excitation is investigated. Luminescence signals around 410, 525, 550, 660 and 850 nm were generated and attributed to the ²H_9/2, ²H_11/2, ⁴S_3/2 and ⁴F_9/2 transitions to the ⁴I_15/2 ground-state, and ⁴S_3/2-⁴I_13/2, respectively. The erbium ions excited-state emitting levels were populated through a combination of stepwise ground-state absorption, phonon-assisted excited-state absorption and cross-relaxation processes. The results also disclosed that all emission signals obtained with vitroceramic samples presented intensities three times higher when compared to the precursor glass samples. In addition, the red emission signal at 660 nm for 1540 nm pumping exhibited an expressively high intensity when compared to the green signal.     

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.     

Dynamics of excited state relaxation and frequency upconversion in Tm3+ and Tm3+/Tb3+ doped ZBLAN glass

The effects of activator concentration on the relaxation of the ¹I₆, ¹D₂, ¹G₄ and ³H₄ levels of Tm³⁺ were investigated by the analysis of the fluorescence decay curves in Tm³⁺ and Tm³⁺/Tb³⁺ doped ZBLAN glasses. UV and blue upconversion luminescence bands around 362 and 450 nm were observed by 655 nm laser excitation in all the samples. The upconversion mechanism was attributed to excited state absorption (ESA) by analyzing the decay profiles and the intensity dependence.     

Improvement of the Tm:H4 level lifetime in silica optical fibers by lowering the local phonon energy

The role of some glass network modifiers on the quantum efficiency of the near-infrared fluorescence from the ³H₄ level of Tm³⁺ ion in silica-based doped fibers is studied. Modifications of the core composition affect the spectroscopic properties of Tm³⁺ ion. Adding 17.4 mol% of AlO_3/2 to the core glass caused an increase of the ³H₄ level lifetime up to 50 μs, 3.6 times higher than in pure silica glass. The quantum efficiency was increased from 2% to approximately 8%. On the opposite, 8 mol% of PO_5/2 in the core glass made the lifetime decrease down to 9 μs. These changes of Tm³⁺ optical properties are assigned to the change of the local phonon energy to which they are submitted by modifiers located in the vicinity of the doping sites. Some qualitative predictions of the maximum achievable quantum efficiency are possible using a simple microscopic model to calculate the non-radiative de-excitation rates.     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Growth, refined structural and spectroscopic characteristics of Tm-doped NaGd(WO4)2 single crystals

The details of Tm³⁺-doped NaGd(WO₄)₂ single-crystal growth are discussed, the results of precise investigations of its structural and spectroscopic characteristics, as well as the analysis of cross-relaxation process of Tm³⁺ ions (³H₄→³F₄, ³H₆→³F₄) in this crystal are presented. Based on the Judd–Ofelt theory, three intensity parameters, spontaneous emission probabilities, fluorescence branching ratios and fluorescence quantum efficiency from ³H₄ and ³F₄ levels were refined.     

Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses

In this paper, optical properties of 75TeO₂-20ZnO-5Na₂O host glass doped with concentration of Tm³⁺ up to 10 %mol were studied in order to assess the most suitable rare earth content for short cavity fiber lasers. Raman spectroscopy revealed a change in the glass structure while increasing Tm³⁺ content, similar to the well known addition of alkali ions in a glass. Influence of the fabrication process on the OH⁻ content was determined by FTIR measurements. Refractive index of Tm³⁺ doped tellurite glasses was measured at five different wavelengths ranging from 533 nm to 1533 nm. Lifetime and emission spectra measurements of the Tm³⁺ doped tellurite glasses are reported.     

Erbium doped fluoride glass-ceramics waveguides fabricated by PVD

Transparent amorphous and glass-ceramics waveguides in the system ZrF₄-LaF₃-ErF₃-AlF₃ (ZELA) have been fabricated by physical vapor deposition (PVD). The ceramming process was studied by means of X-ray diffraction and transmission electron microscopy for different deposition temperatures. With increasing deposition temperature, formation of La_xEr_1−xF₃ nanocrystals with x ∼ 0.3 was observed. The decay curves of the ⁴I_13/2 level in the glass-ceramics with 14.5 mol% Er³⁺ gave evidence of the presence of erbium both in the amorphous matrix (τ = 8.6 ms) and in the crystal phase (τ = 2.2 ms). The decrease of lifetime was due to clustering of erbium incorporated in LaF₃ crystal lattice. No significant increase of attenuation loss was detected after waveguide cerammization (1.3 dB/cm at 1304 nm).     

Crystal growth and luminescent properties of Pr-doped K(Y,Lu)3F10 single crystal for scintillator application

Pr1%:K(Y_1−xLu_x)₃F₁₀ (x=0, 0.2, 0.4) single crystals were grown by the μ-PD method. All the grown crystals were greenish and perfectly transparent without any inclusions or cracks. Radioluminescence spectra and decay kinetics of the Pr1%:K(Y,Lu)₃F₁₀ crystals were measured. Emission from the Pr³⁺ 5d–4f transition, peaking around 260 nm and of the decay time of around 22 ns were observed. The 5d–4f emission intensities of the Pr1%:K(Y,Lu)₃F₁₀ crystals were higher than that of the standard BGO scintillator.     

Composition dependent upconversion of Er-doped PbF2-TeO2 glasses

The upconversion properties of Er³⁺ ions were studied for heavy metal oxyfluoride tellurite glass hosts xPbF₂-(100−x)TeO₂ under 975 nm excitation. The intense green (529 and 545 nm) and relative weak red (657 nm) emissions corresponding to the transitions ⁴S_3/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively, were simultaneously observed at room temperature. The PbF₂ content has an important influence on upconversion luminescence emission. With increasing PbF₂ content, the intensities of green (529 nm) and red (657 nm) emissions increase slightly, while the green (545 nm) emission increases significantly. These results indicate that PbF₂ has more influence on the green (545 nm) emission than the green (529 nm) and red (657 nm) emissions. The intense green emission observed suggest that Er³⁺-doped heavy metal oxyfluoride tellurite glasses can become candidates for developing upconversion optical devices.     

Spectroscopic properties and energy transfer in Ga2O3– Bi2O3–PbO–GeO2 glasses codoped with Tm3+ and Ho3+

This paper reports on the spectroscopic properties and energy transfer in Ga₂O₃–Bi₂O₃–PbO–GeO₂ glasses doped with Tm³⁺ and/or Ho³⁺. From the optical absorption spectra of Tm³⁺, Judd–Ofelt intensity parameters, radiative transitions probabilities, fluorescence branching ratios, and radiative lifetimes have been calculated using Judd–Ofelt theory. The measured differential scanning calorimetry result shows that the glass exhibits excellent stability against devitrification with ΔT = 129 °C. The measured luminescence spectra show that the ³H₄ → ³F₄ transition of Tm³⁺ upon 808 nm laser diode excitation possess a broad full width at half-maximum of ∼126 nm. The maximum value calculated stimulated emission cross-section and the measured lifetime of ³H₄ level from the 1.47-μm transition are ∼4.73 × 10⁻²¹ cm² and ∼0.239 ms, respectively. It is noticed that codoping of Ho³⁺ could significantly enhanced the ratio of the intensity of 1.47–1.80 μm by energy transfer via Tm³⁺: ³F₄ → Ho³⁺: ⁵I₇.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.     

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.