Marco Bettinelli

Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+, Yb3+ nanocrystals

The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2H11/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (4F7/2→4F9/2 and 4F9/2←4I11/2) was responsible for directly populating the 4F9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The 4F9/2 level was populated via a nonresonant mechanism that involved the 4F9/2←4I13/2 transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanis...

Growth, spectroscopic characterization, and laser performance of Nd:LuVO_4, a new infrared laser material that is suitable for diode pumping

The spectroscopic properties and laser operation of a new neodymium-doped vanadate crystal, Nd:LuVO4, grown by the flux technique are reported. Polarized absorption and emission spectra were recorded at low and room temperatures, excited-state absorption was measured near 1060 and 1340 nm, and laser emission at 1066 nm was obtained after pumping near 809 and 880 nm.

980 nm excited upconversion in an Er-doped ZnO–TeO2 glass

In this letter, we investigate the upconversion properties of 19ZnO–80TeO2–1Er2O3 glass after excitation into the 4I11/2 level using 980 nm radiation. At an excitation power density of 880 W/cm2, green emission [(2H11/2, 4S3/2)→4I15/2] dominates the upconversion spectrum with an efficiency of 0.16%. Temporal studies reveal that the 4I11/2 level is the intermediate state by which the two-step upconversion process occurs. Excited-state absorption and phonon-assisted energy transfer are discussed as possible mechanisms for the upconversion.

Effect of glass composition on Judd–Ofelt parameters and radiative decay rates of Er3+ in fluoride phosphate and phosphate glasses

For fluoride phosphate and phosphate glasses, Judd–Ofelt Ωt parameters, spontaneous emission probabilities, radiative lifetimes and branching ratios of several Er3+ transitions were determined from the absorption spectra by the Judd–Ofelt treatment. The compositional changes of Ω2 and Ω6 are attributed to changes in the bonding between Er3+ and surrounding ligand groups. The phosphate groups play a special role. They show electron donation from the 2p orbitals of the oxygen ions, as well π-electron donation from the resonating P–O π-bond. Larger electron donation from the ligand anions leads to larger Ω2, but smaller Ω6. By contrast, larger π-electron donation from the phosphate groups increases Ω6. The changes of Ω2 with increasing phosphate content and with variation of modifier ions are ascribed to changes in the content and polarizability of the oxygen ions, leading to changes in the electron donation from the ligands at the rare earth (RE) sites. In the case of Pb2+, changes in the degree of asymmetry at the RE sites could also be important. The smaller Ω6 parameter of phosphate glasses compared with fluoride phosphate glasses is caused by larger σ-electron donation due to larger oxygen content and by smaller π-electron donation due to smaller delocalization of the π-bonds. The changes of Ω6 with phosphate content, in the case of fluoride phosphate glasses and with variation of modifier ions, are dominated by changes in the π-electron donation.

Visible upconversion of Er3+ doped nanocrystalline and bulk Lu2O3

Abstract We report the luminescence and upconversion spectra of nanocrystalline and bulk Lu1.98Er0.02O3. After direct excitation at 488 nm or using the upconversion wavelengths (804 and 650 nm), blue, green and red emissions were observed for both samples under investigation. A temperature study of the bulk sample showed that the upconversion of the 4 F 9/2 → 4 I 15/2 and the 4 S 3/2 → 4 I 15/2 emission show maxima at 140 and 180 K, respectively, which is due to the competition of two different phonon-assisted processes. We have shown that upconversion occurs via a sequential absorption of three photons and via a phonon-assisted energy transfer (PET) process, with the latter being more efficient at higher Er3+ concentrations.

Optical spectroscopy of nanocrystalline cubic Y2O3:Er3+ obtained by combustion synthesis

We report the emission and upconversion spectra and the lifetimes of Er3+ doped Y2O3 nanocrystalline and bulk samples. We found that when the nanocrystal and bulk samples of Y1.80Er0.20O3 were excited at 815 nm, the overall emission intensity was stronger for the bulk sample. However, the relative intensity of the (2H11/2, 4S3/2)→4I15/2/4F9/2→4I15/2 transitions is 2:3 and 3:2 for the nanocrystal and bulk sample, respectively. The decay times obtained for the nanocrystalline samples are in general significantly faster than those observed for the bulk sample. We attribute this to the adsorption of CO2 on the surface of the nanocrystalline samples. The effect of Er3+ concentration on the decay time of the nanocrystalline samples is also discussed.

Luminescence Dynamics in Tb3+-Doped CaWO4 and CaMoO4 Crystals

Single crystals of CaWO(4) and CaMoO(4) doped with Tb(3+) have been grown by the flux growth method. Their luminescence properties have been investigated in the 10-600 K temperature range under different experimental conditions. In spite of very similar spectra at low temperature upon excitation at 365 nm, the crystals show a very different behavior as the temperature is raised or the excitation wavelength is changed. These differences have been accounted for on the basis of models that take into consideration the position of the energy levels of the rare earth relative to the bandgap of the host material.

Optical spectra of yttrium phosphate and yttrium vanadate single crystals activated with Dy3

Abstract The optical spectra of YPO 4 :Dy 3+ and YVO 4 :Dy 3+ single crystals have been measured at 10 and 298 K. The luminescence spectra are dominated by a strong emission in the 575 nm region. Other bands have been observed in the 480, 665, 750 and 840 nm regions. They have been assigned to transitions from 4 F 9/2 to lower lying states on the basis of the energy level scheme deduced from the 10 K absorption spectra. The Stark structure of the manifolds observed in the low temperature spectra has been reliably reproduced by a standard crystal field (CF) calculation. The room temperature absorption spectra of YVO 4 :Dy 3+ have been analyzed using the Judd–Ofelt parameterization scheme, and the intensity parameters have been determined. Using these data the radiative lifetime for the 4 F 9/2 level has been calculated. The comparison with the effective decay times deduced from time-resolved measurements allows evaluation of the efficiency of the non-radiative processes.

The electronic structure of zircon-type orthovanadates: Effects of high-pressure and cation substitution

The electronic structure of four ternary-metal oxides containing isolated vanadate ions is studied. Zircon-type YVO4, YbVO4, LuVO4, and NdVO4 are investigated by high-pressure optical-absorption measurements up to 20 GPa. First-principles calculations based on density-functional theory were also performed to analyze the electronic band structure as a function of pressure. The electronic structure near the Fermi level originates largely from molecular orbitals of the vanadate ion, but cation substitution influences these electronic states. The studied orthovanadates, with the exception of NdVO4, undergo a zircon-scheelite structural phase transition that causes a collapse of the band-gap energy. The pressure coefficient dEg/dP shows positive values for the zircon phase and negative values for the scheelite phase. NdVO4 undergoes a zircon-monazite-scheelite structural sequence with two associated band-gap collapses.

Spectroscopic investigation of zinc-borate glasses doped with trivalent europium ions

Abstract 4ZnO · 3B 2 O 3 glasses undoped and doped with 0.5–2.5% Eu 3+ were studied using absorption, emission and excitation spectroscopies, fluorescence line narrowing, Raman scattering and lifetime measurements. The vibrational spectra of the undoped and doped glasses are very similar and indicate the presence of borate groups already observed in binary metaborate glasses. The Judd-Ofelt parameters and the asymmetry ratio derived from the f-f transitions of the Eu 3+ ion give information on the average local structure around the impurity ions. Crystal-field parameters obtained from site selective emission spectra are close to those found for various silicate glasses, and their major ratios are close to predictions of the Brecher and Riseberg model. A very strong phonon sideband is coupled to the 5 D 0 ← 7 F 0 transition; a value of 0.11 is obtained for the electron-phonon coupling strength. Comparison with Raman spectra indicates that the sideband is due to an Eu 3+ O stretching/BO 3 3− mode. No energy transfer is found in the most diluted samples.