Raffaella Rolli

Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics

We show that oxy-fluoride glass ceramics, with typical composition, 32(SiO2):9(AlO1.5):31.5(CdF2):18.5(PbF2):5.5(ZnF2): 3.5(ErF3) mol % have potential applications in telecommunications. Upon heat treatment, Er3+ nucleates the growth of the nanocrystalline β-PbF2, which acts as its host. Heat treatment at 440 °C for 5 h and at 390 °C for 3 h gave rise to ∼12 and ∼2.5 nm diameter crystals, respectively. The emission band of Er3+ in the 1.54 μm telecommunications window (4I13/2→4I15/2 transition, at the half-height width) was 75 nm in the former and 90 nm in the latter case, while 4I13/2↔4I15/2 absorption and emission bands became wavelength divergent in both cases. Also in the latter case, the spectrum was flat from 1.53 to 1.56 μm. The evolution of spectral behavior is explained by changes in average site geometry of the Er3+ dopant, related to the α→β phase transition of PbF2, which is stimulated by heat treatment.

Erbium-doped tellurite glasses with high quantum efficiency and broadband stimulated emission cross section at 1.5 μm

Optical transitions of Er 3þ ion in two tellurite glasses of molar composition 75TeO2:12ZnO:10Na2O:2PbO:1Er2O3 and 75TeO2:12ZnO:10Na2O:2GeO2:1Er2O3 were investigated. The measured absorption and emission spectra were analysed by Judd–Ofelt and McCumber theories, in order to obtain radiative transition rates and stimulated emission cross sections. It was found that these glasses have high and broadband absorption and stimulated emission cross sections at 1.5 lm. For the metastable 4 I13=2 level, by comparing the measured lifetime with the calculated radiative decay time, quantum efficiency higher than 80% was found.

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.

Er-doped silica-based waveguides prepared by different techniques: RF-sputtering, sol-gel and ion-exchange

Erbium-activated silica-based planar waveguides were prepared by three different technological routes: RF-sputtering, sol–gel and ion exchange. Various parameters of preparation were varied in order to optimize the waveguides for operation in the NIR region. Particular attention was devoted to the minimization of the losses and the increase of the luminescence efficiency of the metastable 4I13/2 state of the Er3+ ion. Waveguide properties were determined by m-line spectroscopy and loss measurements. Waveguide Raman and luminescence spectroscopy were used to obtain information about the structure of␣the prepared films and about the dynamical processes related to the luminescence of the Er3+ ions.

Phonon sidebands and vibrational properties of Eu3+ doped lead germanate glasses

Abstract The phonon sideband spectra of the 7 F 0 → 5 D 0 and 7 F 0 → 5 D 2 electronic transitions of Eu 3+ are reported and assigned on the basis of the vibrational Raman spectra for a series of lead garmanate glasses of molar compositions ( x − 1)PbO · (100 − x )GeO 2 · 1Eu 2 O 3 with x = 30, 40 and 50 mol%. The samples were prepared by melting the appropriate amount of the oxides at 1100°C for 2 h, and quenching and annealing at temperatures from 320 to 380°C for 15 h. The electron-phonon coupling strength of the GeO stretching modes coupled to the 7 F 0 → 5 D 2 transition was evaluated by determining the ratio of the intensity of the phonon sideband to the intensity of the zero phonon line. With increasing lead content, the coupling strength appears to increase (within the experimental uncertainties) from 0.010 ( x = 30 mol%) to 0.012 ( x = 50 mol%).

Erbium-Activated Silica-Titania Planar Waveguides

Abstract(100 − x)SiO2-(x)TiO2-ErO3/2 planar waveguides, with 7 ≤ x ≤ 20 have been prepared by sol-gel route using the dip-coating technique. The thickness of the films was optimized to support a single propagating mode at 1550 nm, with a confinement coefficient higher than 0.75. The process of densification of the gel and the devitrification with the growth of TiO2 nanocrystals were studied by Raman scattering. Devitrification is important only for x ≥ 15, but it was not possible to obtain full densification of the samples, even at the lowest TiO2 content, without the appearance of nanocrystals.Emission in the C telecom band was observed; the spectral width of the 4I13/2 → 4I15/2 transition of Er3+ slightly increases with the titania content. For x ≤ 12 most of Er3+ ions (about 65%) decay exponentially with a lifetime of about 8 ms.

Rare-earth-activated fluoride and tellurite glasses: optical and spectroscopic properties

In this work we report on spectroscopic properties of Er3+-doped aluminum fluorophosphate glasses of molar composition 29.8AlF3:3.5 MgF2: 19.8 CaF2: 10.9 SrF2: 12.8 BaF2: 8.4 YF3:9.8 ZrF4:4 NaPO3:1 ErF3 and 30 AlF3:3.5 MgF2:20 CaF2: 11 SrF2: 13 BaF2: 8.4 YF3: 10 ZrF4: 4 NaPO3: 0.1 ErF3, and Er3+-doped tellurite glasses of molar composition 75 TeO2: 12 ZnO: 10 Na2O: 2 PbO: 1 Er2O3 and 75 TeO2: 12 ZnO: 10 Na2O: 2 GeO2: 1 Er2O3. Absorption and Stokes luminescence upon visible excitation were measured. Emission in the third telecom window was observed upon excitation at 514.5 nm and the lifetime of the 4I13/2 level was measured. IR-to-visible up conversion emission under continuous-wave laser excitation at 976 nm was observed. The up conversion results in a strong green emission and a weaker red emission, whose intensity shows a quadratic dependence on the excitation power, indicating that two photons are involved in the process. Lifetime measurements were performed in order to study the dynamical behavior of the up conversion emission and to clarify the contributions of excited state absorption and energy transfer to the up conversion process.

Erbium-activated silica–titania planar waveguides on silica-on-silicon substrates prepared by rf sputtering

Abstract Erbium-activated silica–titania planar waveguides were prepared by radio-frequency (rf) sputtering technique. Silica-on-silicon substrates obtained by plasma-enhanced chemical vapor deposition (PECVD) and rf sputtering (RFS) were employed. The refractive indices, the thickness and the propagation losses of the waveguides were measured. The refractive index and the roughness of the silica substrates produced by RFS appear to be dependent on the thickness. Thermal annealing, which is a necessary condition to obtain light propagation, induces a decrease of the refractive index in the silica substrates. The waveguide deposited on PECVD substrate exhibits several propagating modes with an attenuation coefficient 1.7 dB/cm compared with 12.2 dB/cm measured for the waveguide deposited on silica substrate produced by RFS technique. Emission of the 4 I 13/2 → 4 I 15/2 transition with a 53 nm bandwidth was observed.

Er3+/Yb3+ Co-Activated Silica-Alumina Monolithic Xerogels

Monolithic silica xerogels doped with different concentrations of Er3+, Yb3+ and Al3+ were prepared by sol-gel route. Densification was achieved by thermal treatment in air at 950°C for 120 h with a heating rate of 0.1°C/min. We studied the luminescence properties of the 4I13/2 → 4I15/2 emission band of Er3+ as a function of the Al/Er/Yb concentration and we paid particular attention to the alumina effects. Raman spectroscopy and Vis-NIR absorption were used to monitor the degree of densification of the glasses and the residual OH content.

Yellow-to-blue frequency upconversion in Pr3+-doped aluminium fluoride glasses

Abstract Rare-earth-doped fluoride glasses are attractive materials for photonics, since they combine a transmission region from ultraviolet to infrared and low vibrational cut-off energy. We report on the spectroscopic properties of aluminium fluorophosphate glasses doped with 0.1, 0.6, 1, 3 and 10 mol% Pr 3+ . Yellow-to-blue upconversion was observed upon pulsed and continuous-wave (CW) excitation at 575 nm. The temporal evolution of the upconverted fluorescence is characterised by rise and decay times depending on Pr 3+ concentration. The upconversion intensity has a quadratic dependence on incident pump power, indicating a two-step process. The upconversion emission is described in the framework of a two-ion energy transfer mechanism. Emission in the second telecom window ( 1.3 μm ) was measured upon excitation at 976 nm.