Adolfo Speghini

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 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.

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.

1.3 μm emitting SrF2:Nd3+ nanoparticles for high contrast in vivo imaging in the second biological window

Novel approaches for high contrast, deep tissue, in vivo fluorescence biomedical imaging are based on infrared-emitting nanoparticles working in the so-called second biological window (1,000–1,400 nm). This allows for the acquisition of high resolution, deep tissue images due to the partial transparency of tissues in this particular spectral range. In addition, the optical excitation with low energy (infrared) photons also leads to a drastic reduction in the contribution of autofluorescence to the in vivo image. Nevertheless, as is demonstrated here, working solely in this biological window does not ensure a complete removal of autofluorescence as the specimen’s diet shows a remarkable infrared fluorescence that extends up to 1,100 nm. In this work, we show how the 1,340 nm emission band of Nd3+ ions embedded in SrF2 nanoparticles can be used to produce autofluorescence free, high contrast in vivo fluorescence images. It is also demonstrated that the complete removal of the food-related infrared autofluorescence is imperative for the development of reliable biodistribution studies.

Optical spectroscopy of lanthanide ions in ZnO–TeO2 glasses

Zinc tellurite glasses of compositions 19ZnO-80TeO2-1Ln2O3 with Ln = Eu, Er, Nd and Tm were prepared by melt quenching. The absorption spectra were measured and from the experimental oscillator strengths of the f-->f transitions the Judd-Ofelt parameters ohm(lambda) were obtained. The values of the ohm(lambda) parameters are in the range usually observed for oxide glasses. For Nd3+ and Er3+, luminescence spectra in the near infrared were measured and the stimulated emission cross sections sigma(p) were evaluated for some laser transitions. The high values of sigma(p), especially for Nd3+, make them possible candidates for optical applications. Fluorescence line narrowing (FLN) spectra of the Eu3+ doped glass were measured at 20 K, and the energies of the Stark components of the 7F1 and 7F2 states were obtained. A crystal field analysis was carried out assuming a C2v site symmetry. The behaviour of the crystal field ratios B22/B20 and B44/B40 agrees reasonably well with the values calculated using the geometric model proposed by Brecher and Riseberg. The crystal field strength at the Eu3+ sites appears to be very low compared to other oxide glasses.

A spectroscopic investigation of trivalent lanthanide doped Y2O3 nanocrystals

Nanocrystalline yttrium oxide doped with the lanthanide ions Sm3+, Dy3+ or Tm3+ with a particle diameter of approximately 10?nm was prepared by propellant synthesis. The Y2O3:Sm3+ nanocrystals showed a visually dominant red emission from the transition and a strong concentration dependence of both the emission spectra and decay was observed. The decay time of the 4G5/2 excited state in the 1?mol% nanocrystalline Y2O3:Sm3+ sample was determined to be longer than in the identically doped bulk (microcrystalline) sample. We have shown that the lifetime of this state in the nanocrystalline material is strongly dependent on the index of refraction of the medium surrounding the nanoparticles and a 0.75 filling factor was obtained. Yellow emission was observed from the Y2O3:Dy3+ nanocrystals from the transition. The decay curves of the 4F9/2 state deviated from exponentiality and were fitted with the Inokuti?Hirayama model in order to probe the energy transfer processes. The model revealed that a dipole?dipole mechanism was responsible for the interactions and a critical transfer distance (R0) of 6.05???was obtained. Tm3+ doped Y2O3 nanocrystals showed predominantly blue emission from the and transitions. Red-to-blue up-conversion was detected following excitation with 656?nm into the 3F2 state which occurred via a two-photon ESA/ETU process.