Nadège Ollier

Clusters dissolution of Yb3+ in codoped SiO2-Al2O3-P2O5 glass fiber and its relevance to photodarkening

Using a combination of pulse electron paramagnetic resonance and photoluminescence spectroscopy, we demonstrate the major role of phosphorous rather than aluminium in the rare-earth dissolution process, an essential advance in telecommunication and solid laser fields. Our results also provide new insight into the micro-structural origin of the photodarkening process occurring in Yb doped fiber.

Evidence of AlOHC responsible for the radiation-induced darkening in Yb doped fiber

Using a combination of experimental techniques such as optical absorption, Raman scattering, continuous wave and pulse Electron Spin Resonance (ESR), we characterize a set of γ-irradiated Yb(3+) doped silica glass preforms with different contents of phosphorous and aluminum. We demonstrate that when P is introduced in excess compared to Al, nearly no radiodarkening is induced by γ-rays. On the other hand, when Al>P, a large absorption band is induced by radiation. Thermal annealing experiments reveal the correlation between the decrease of the optical absorption band and the decrease of the Al-Oxygen Hole Center (AlOHC) ESR signal, demonstrating the main role of AlOHC defects in the fiber darkening. HYSCORE (HYperfine Sublevel CORElation) pulse-ESR experiments show a high Al-P nuclear spin coupling when P>Al and no coupling when Al>P. This result suggests that both AlOHC and POHC creation is inhibited by Al-O-P linkages. Confronting our data with previous works, we show that the well-known photodarkening process, meaning losses induced by the IR pump, can also be explained in this framework.

A structural approach by MAS NMR spectroscopy of mechanisms occurring under β-irradiation in mixed alkali aluminoborosilicate glasses

Two series of mixed Na/K and Na/Li aluminoborosilicate glasses have been irradiated with electrons of 1.8 MeV at doses close to 109 Gy. Si, B and Al glass former environment changes under irradiation have been probed by MAS NMR spectroscopy. It was shown that the mixed alkali effect acts on the middle range compositions by operating with a selective blockage in irradiated glasses, depending on the alkali nature and concerning either modifier or charge compensator alkalis.

Impact of rare earth element clusters on the excited state lifetime evolution under irradiation in oxide glasses

Rare earth doped active glasses and fibers can be exposed to ionizing radiations in space and nuclear applications. In this work, we analyze the evolution of (2)F(5/2) excited state lifetime in Yb(3+) ions in irradiated aluminosilicate glasses by electrons and γ rays. It is found that the variation of lifetimes depends on the Yb(3+) clusters content of the glasses for irradiation doses in the 10(2)- 1.5∙10(9) Gy range. In particular, glasses with high clustering show a smaller decrease in lifetime with increasing radiation dose. This behavior is well correlated to the variation in paramagnetic defects concentration determined by electron paramagnetic resonance. This effect is also observed in Yb(3+) doped phosphate and Er(3+) doped aluminosilicate glasses, inferring that clustering plays an important role in irradiation induced quenching.

Radiation hardening in sol-gel derived Er3+-doped silica glasses

The aim of the present paper is to report the effect of radiation on the Er3+-doped sol-gel silica glasses. A possible application of these sol-gel glasses could be their use in harsh radiation environments. The sol-gel glasses are fabricated by densification of erbium salt-soaked nanoporous silica xerogels through polymeric sol-gel technique. The radiation-induced attenuation of Er3+-doped sol-gel silica is found to increase with erbium content. Electron paramagnetic resonance studies reveal the presence of E′δ point defects. This happens in the sol-gel aluminum-silica glass after an exposure to γ-rays (kGy) and in sol-gel silica glass after an exposure to electrons (MGy). The concentration levels of these point defects are much lower in γ-ray irradiated sol-gel silica glasses. When the samples are co-doped with Al, the exposure to γ-ray radiation causes a possible reduction of the erbium valence from Er3+ to Er2+ ions. This process occurs in association with the formation of aluminum oxygen hole centers...

Evidence of transient species occurring in the reduction process of trivalent lanthanides under 2.5 MeV electron irradiation by in situ cathodoluminescence and time-resolved photoluminescence

In situ cathodoluminescence and time-resolved photoluminescence measurements were carried out on Sm-doped glasses during a 2.5 MeV electron irradiation. These experiments allow characterizing more precisely the mechanisms operating under irradiation and more specifically the reduction processes of Sm3+ to Sm2+ ions. Sm2+ emission lines appear in the first steps of the irradiation, however, the D50→F70–2 emission lines of Sm2+ were not observed on the cathodoluminescence spectrum. Moreover, two sites of Sm2+ have been evidenced in the glasses; the formation of each species is clearly different. Relaxation processes of Sm3+ and Sm2+ leads to consider the existence of different transient states of Sm3+ and Sm2+ species which are annealed after irradiation. We propose some synthetic schemes of the reduction mechanisms produced during the irradiation.

New insights on P-related paramagnetic point defects in irradiated phosphate glasses: Impact of glass network type and irradiation dose

P-related paramagnetic point defects were studied in irradiated Yb-doped phosphate glasses by electron paramagnetic resonance spectroscopy (X and Q-bands). A strong impact of the glass network type on the defect nature is shown. In all glasses, r-POHC defects formation is in strong correlation with Q2 tetrahedra amount supporting the structure of r-POHC. Ultra-phosphate glasses contain the larger defect type: Peroxy radicals, P1, P2, and P4 defects whose formation is linked to Q3 tetrahedra presence. In meta-phosphate and poly-phosphate glasses, peroxy radicals appear with r-POHC thermal recovery. In meta-phosphate glasses, a combination of P1 and P3 defects was evidenced for the first time, whereas in poly-phosphate glasses, only P3 defects were identified. Dose effect as well as defect recovery were analyzed.

Suppression mechanism of radiation-induced darkening by Ce doping in Al/Yb/Ce-doped silica glasses: Evidence from optical spectroscopy, EPR and XPS analyses

Yb3+/Al3+ co-doped silica glasses with different Ce2O3 contents were prepared using the sol–gel method combined with high-temperature sintering. Changes in refractive index, absorption, emission and fluorescence lifetime of these glasses caused by X-ray irradiation were recorded and analyzed systematically. It is found that co-doping with certain amount of Ce could greatly improve the radiation resistance without evident negative effects on the basic optical properties of the Yb3+ ions in the near-infrared region. The nature of the radiation-induced color centres and the mechanism by which Ce prevented the formation of these centres were studied using optical absorption, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) methods. Direct evidence confirmed that trapped electron centres (Yb2+/Si-E′/Al-E′) and trapped hole centres (Al-OHCs) were effectively inhibited by Ce doping, which was correlated to the coexistence of the redox couple Ce3+/Ce4+ in the glasses. These result...

Interplay between photo- and radiation-induced darkening in ytterbium-doped fibers

This Letter demonstrates a remarkable interplay between photo- and radiation-induced darkening of ytterbium-doped alumino-silica optical fibers operated in amplifying conditions and harsh environments (as, e.g., in space-based applications). Influences of the pump power, ionizing dose, and dose rate on this interaction are characterized. The pump is capable of accelerating or slowing down the radiation-induced darkening build-up depending on the ionizing dose. The steady-state photo-radio-darkening level is independent of the dose and at least equal to the equilibrium level of pure photo-darkening. This lower limit is notably reached at low dose rates, including those encountered in space. We, therefore, argue that photo-resistant ytterbium-doped fibers will resist against a space mission, whatever the dose.

Direct Evidence for Trivalent Titanium in Artificially Irradiated (electrons) Oxide Glasses

Ti-doped borosilicate and disilicate glasses were irradiated with high-energetic electrons in order to study Ti(IV) reduction processes. Reduction efficiency was actually confirmed by an important Ti(III) EPR signal around g=1.93 in irradiated samples. The Ti(III) amount increases with the irradiation dose. Moreover, the Ti(Ill) environment depends on the glass composition. Hence, the reduction process efficiency is strongly correlated to the Ti(IV) environment in the pristine glass. The Ti environment was analyzed by Ti K-edge XANES in order to understand the reduction mechanisms. Pre-edge analysis of non-irradiated samples indicates the presence of four, five and six coordinated tetravalent Ti sites in borosilicate glasses, whereas in disilicate the Ti(IV) coordination is a mixture between five-fold and six-fold. In irradiated samples, the Ti pre-edge shows a slight low-energy shift associated to an intensity decrease in disilicate glasses. Correlation between EPR and XANES experiments allows for interpretation of the pre-edge variation in terms of Ti(III) formation. Profiles (gin resolution) were also performed to measure the proportion of Ti(HI) on different points across the irradiated DS. No significant difference in XANE S spectra was observed suggesting that the production of Ti(III) is homogeneous in the volume.