G. Özen

Thermal and optical properties of Tm3+ doped tellurite glasses

Ultraviolet, visible (UV/VIS) and differential thermal analysis (DTA) measurements were carried out in order to investigate the optical and thermal properties of various 0.5 mol.% Tm2O3 containing (1 - x)TeO2 + xLiCl glasses in molar ratio. The samples were prepared by fusing the mixture of their respective reagent grade powders in a platinum cricuble at 750 degrees C for 30 min. DTA curves taken in the 23-600 degrees C temperature range with a heating rate of 10 degrees C/min reveal a change in the value of the glass transition temperature, Tg, while melting was not observed for the glasses containing LiCl content less than 50 mol.%. These glasses were found to be moisture-resistant. However, the glasses with LiCl content higher than 50 mol.%, in which a melting peak was observed at Tc = 401 degrees C, were moisture-sensitive. Absorption measurements in the UV/VIS region of the glasses without Tm2O3 content show that the Urbach cutoff occurs at about 320 nm and, is relatively independent of the LiCl content. Six absorption bands were observed in the Tm2O3 doped glasses corresponding to the absorption of the 1G4, 3F2, 3F3 and 3F4, 3H5 and 3H4 levels from the 3H6 ground level of Tm3+ ions. The spectra also show that the integrated absorption cross-section of each band depends on the glass composition. Judd-Ofelt theory was used to determine the Judd-Ofelt parameters as well as the radiative transition probabilities for the metastable levels of Tm3+ ions in (0.3)LiCl + (0.7) TeO2: 0.01 Tm2O3 glass which is moisture-resistant.

RED TO BLUE UP-CONVERSION EMISSION OF TM3+ IONS IN YB3+-DOPED GLASS-CERAMIC

A detailed study of the spectroscopic properties of PbF2+GeO2+WO3 glass ceramics doped with Tm3+ and codoped with Tm3+ and Yb3+ ions upon 680 nm dye laser light excitation has been made. The absorption, up‐conversion emission, excitation, and time‐resolved spectra were measured as a function of TmF3 and YbF3 concentrations at room temperature. An enhancement of blue emission centered at 478 nm in the codoped sample was observed. The optium concentrations of TmF3 and YbF3 for this blue emission are about 0.2 and 15 mol %, respectively. It is suggested that the results are due to energy transfer between Yb3+ and Tm3+ ions. Measured oscillator strengths and radiative rates for several transitions are compared with calculated values using the Judd–Ofelt theory with reasonable agreement between theory and experiments for sample doped with Tm3+ ions.

Effect of cross relaxation on the 1470 and 1800 nm emissions in Tm3+:TeO2–CdCl2 glass

We have used spectroscopic methods to investigate the effect of cross relaxation on the 1470 and 1800 nm emissions in Tm3+:TeO2–CdCl2 glass for Tm3+ ion concentrations of 0.05, 0.1, 0.25, 0.5 and 1.0 mol%. The emission spectra obtained under 800 nm excitation reveal the existence of energy transfer via cross relaxation among the Tm3+ ions. As a result, as the thulium concentration is increased, the strength of the 1470 nm emission is found to decrease in relation to that of the 1800 nm emission. The analysis of the time evolution of the 3F4 emission due to the transition further shows that the electronic mechanism responsible for the ion–ion interaction can be identified as an electric dipole–dipole energy transfer process and occurs in the diffusion-limited relaxation regime. The average critical distance parameter which provides a measure for the strength of cross relaxation was further determined to be 17.9 A.

Thermal properties and optical transition probabilities of Tm3 + doped TeO2-WO3 glass.

Glasses with the composition of (1 - x)TeO2 + (x)WO3, where x = 0.15, 0.25 and 0.3 were prepared and, their thermal and absorption measurements were carried out. Differential thermal analysis (DTA) curves taken in the 23-600 degrees C temperature range with a heating rate of 10 degrees C/min reveal a change in the value of the glass transition temperature, Tg, while crystallization was not observed for the glasses containing a WO3 content of more than 15 mol%. All the glasses were found to be moisture-resistant. The absorption bands corresponding to the absorption of the 1G4, 3F2, 3F3 and 3F4, 3H5 and 3H4 levels from the 3H6 ground level of the Tm3+ ion were observed in the optical absorption spectra. Integrated absorption cross-sections of each band except that of 3H5 level was found to vary with the glass composition. Judd-Ofelt analysis was carried out for the samples doped with 1.0 mol% Tm2O3. The omega2 parameter shows the strongest dependence on the host composition and it increases with the increasing WO3 amount. The value of omega4 increases rather slowly while the value of omega6 is practically independent of the composition. The strong dependence of the parameter omega2 indicates that this parameter is related to the structural change and the symmetry of the local environment of the Tm3+ ions in this glass.

Downconversion and upconversion dynamics in Pr-doped Y3Al5O12 crystals

We have investigated the spectroscopic properties of praseodymium ion in a Y3Al5O12 sample. We obtained the luminescence spectra by exciting selectively the sample at 460 and 599nm, resulting in the excitation of the P03 and D21 levels, respectively. With excitation in the P03 level we obtained emission from this level and from the D21 level. We used time resolved spectra and decay pattern inspection to assign spectral lines to specific radiative transitions. With excitation in the D21 level we observed luminescence emitted by the same level and upconverted emission from the P03 level and the (4f5d) band. The experimental data indicate that the mechanism responsible for the upconversion processes is excited state absorption. We found the upconverted emission from P03 and the (4f5d) band to be proportional to the square of the excitation energy and to the cube of the excitation energy, respectively.

Enhanced Tm3+ blue emission in Tm, Yb, co‐doped fluorophosphate glasses due to back energy transfer processes

Up‐conversion emission spectra of fluorophosphate glasses doped with Tm3+ and co‐doped with Tm3+ and Yb3+ ions were obtained under two different excitation wavelengths (657 and 683 nm). Both samples show a stronger emission centered at 450 nm than the ones centered at 478 and 363 nm when 3F2 level of Tm3+ is excited at 657 nm. If 3F3 level of Tm3+ is excited with the 683 nm light the 478 nm emission of the double doped sample becomes much stronger (about 200 times) than the same emission of the single doped sample. We propose an up‐conversion mechanism to explain the spectra which is in agreement with quadratic dependence of the emission intensities on the excitation power under both excitations.

Biocomposite films based on alginate and organically modified clay

Sodium alginate/sodium montmorillonite hybrid films were prepared by casting from the suspension of sodium alginate and different clay samples. Clay samples had been modified with a cationic surfactant, a cationic polymer, and a small polar molecule, respectively. Benzethonium chloride, polyethyleneimine and urea were used as clay modifiers. The composite films begin to disintegrate at a higher temperature and with less weight loss than the pure alginate films. This suggests an enhancement of the film thermal stability due to the modification of the alginate with clay samples.

Spectroscopic analysis of Tm3+:LuAG

We studied the spectroscopic properties of two thulium-doped Lu3Al5O12 (Tm:LuAG) samples with Tm3+ concentrations of 0.5 and 5 at.%. Judd–Ofelt theory was used to analyse the absorption spectra and to determine the radiative transition rates. Fluorescence measurements were further performed to determine the luminescence quantum efficiencies. The average radiative lifetimes of the 3H4 and 3F4 levels were calculated to be 1041 ± 143 µs and 17.7 ± 3.4 ms, respectively. We observed a sharp increase in the strength of cross relaxation for the 5% Tm:LuAG sample evidenced by the much shorter fluorescence lifetime of 42.3 µs for the 3H4 level, in comparison with 851 µs for the 0.5% Tm:LuAG sample. This was further supported by the relative emission measurements at 1470 and 1800 nm. The measured fluorescence lifetime of the 3F4 level showed a smaller decrease from 11.2 ms (0.5% doping) to 7.1 ms (5% doping). By using the Judd–Ofelt theory, the stimulated emission cross section was further calculated to be 1.2 ± 0.2 × 10−21 cm2 at 2023 nm, corresponding to the free running wavelength of Tm:LuAG lasers. Finally, the critical distance parameter R0 for cross relaxation was determined to be 10.2 ± 0.8 A from the fluorescence decay data.

Stimulated and spontaneous emission probabilities of Tm3+ in TeO2-CdCl2 glass: the role of the local structure

Abstract We present the results of a study which uses experimental and theoretical methods to investigate the characteristics of the 1450- and 1800-nm luminescence bands of Tm 3+ -doped TeO 2 –CdCl 2 glasses as a function of the CdCl 2 fraction. These glasses are potentially important in the design of fiber-optic amplifiers and fiber lasers. In the experiments, the glass samples were prepared by fusing a mixture of their respective reagent grade powders in a platinum crucible at 750 °C for 30 min. The spectroscopic properties were then investigated by measuring the absorption, fluorescence intensities, and the fluorescence lifetimes at room temperature by using diode and Ti:sapphire pump lasers operating around 800 nm. In the theoretical calculations, Judd–Ofelt analysis was used to assess the effect of the glass composition on the radiative transition probabilities, quantum luminescence efficiencies, and the stimulated emission cross-sections. The results indicate that the luminescence quantum efficiency, while remaining approximately constant around 10% for the 1450-nm band, increases with increasing CdCl 2 concentration for the 1800-nm emission. Furthermore, the stimulated emission cross-sections for the 1450- and 1800-nm transitions were found to be largest for the sample with 30 mol% CdCl 2 .

Microstructural characterization and crystallization kinetics of (1−x)TeO2–xLiCl (x=0.6–0.4 mol) glasses

Abstract On the basis of DTA analyses, three (1− x )TeO 2 – x LiCl ( x =0.3, 0.35 and 0.4 mol) glasses doped with 0.005 mol of Tm 2 O 3 were crystallized in the vicinity of 400°C. X-ray investigations for samples heated to 425°C (above the peak crystallization temperature) followed by quenching in air, revealed the presence of the paratellurite (TeO 2 ) as the only crystallizing phase in all glass compositions. SEM investigations revealed that the paratellurite crystals formed in these glasses as a result of surface crystallization were trigonal in shape, between 40 and 50 μm in length, 7 and 15 μm in width and 5 and 9 μm in depth. DTA analyses were carried out on the 0.7TeO 2 –0.3LiCl glass at different heating rates and an activation energy value of 238 kJ/mol for surface crystallization was determined graphically from a Kissinger-type plot using the analysis of Matusita and Sakka (Matusita, K. and Sakka, S., Kinetic study on crystallization of glass by differential thermal analysis — criterion on application of Kissinger plot. J. Non-cryst. Solids. 38&39 , 741–746.).