Yannick Ledemi

Structural Investigations of Glass Ceramics in the Ga2S3―GeS2―CsCl System

Transparent glass ceramics have been prepared in the Ga(2)S(3)-GeS(2)-CsCl pseudoternary system using appropriate heat treatment time and temperature. In situ X-ray diffraction at the heat treatment temperature and (133)Cs and (71)Ga solid-state nuclear magnetic resonance have been performed in function of annealing time to understand the crystallization process. Both techniques have evidenced the nucleating agent role played by gallium with the formation of Ga(2)S(3) nanocrystals. On the other hand, cesium is incorporated very much later into the crystallites during the ceramization. Moreover, the addition of CsCl, which is readily integrated into the glassy network, permits us to shift the optical band gap toward shorter wavelength. Thus, new glass ceramics transmitting in the whole visible range up to 11.5 mum have been successfully synthesized from the (Ga(2)S(3))(35)-(GeS(2))(25)-CsCl(40) base glass composition.

Frequency upconversion in a Pr3+ doped chalcogenide glass containing silver nanoparticles

We report on orange-to-blue frequency upconversion (UC) in Pr3+ doped chalcogenide glass (Ga10Ge25S65) doped with Ag2S and heat treated under different conditions to nucleate silver nanoparticles (NPs). The experiments were performed using 7ns pulses from a dye laser that operates at ∼590nm, in resonance with the H43→D21 transition of Pr3+ ions. The enhancement observed in the UC emission at ∼494nm, ascribed as P03→H43 transition of the Pr3+ ion, is attributed to the large local field acting on the emitting ions due to the presence of the metallic NPs.

White light and multicolor emission tuning in triply doped Yb3+/Tm3+/Er3+ novel fluoro-phosphate transparent glass-ceramics

New Yb3+, Er3+ and Tm3+ doped fluoro-phosphate glasses belonging to the system NaPO3–YF3–BaF2–CaF2 and containing up to 10 wt% of rare-earth ion fluorides were prepared and characterized by differential scanning calorimetry, absorption spectroscopy and up-conversion emission spectroscopy under excitation with a 975 nm laser diode. Transparent and homogeneous glass-ceramics have been reproducibly obtained with a view to manage the red, green and blue emission bands and generate white light. X-ray diffraction as well as electron microscopy techniques have confirmed the formation of fluorite-type cubic nanocrystals at the beginning of the crystallization process while complex nanocrystalline phases are formed after a longer heat-treatment. The prepared glass-ceramics exhibit high optical transparency even after 170 h of thermal treatment. An improvement of up-conversion emission intensity – from 10 to 160 times larger – was measured in the glass-ceramics when compared to the parent glass, suggesting an important incorporation of the rare-earth ions into the crystalline phase(s). The involved mechanisms and lifetime were described in detail as a function of heat-treatment time. Finally, a large range of designable color rendering (from orange to turquoise through white) can be observed in these materials by controlling the laser excitation power and the crystallization rate.

Ytterbium-doped glass-ceramics for optical refrigeration

We report for the first time the characterization of glass-ceramics for optical refrigeration. Ytterbium-doped nanocrystallites were grown in an oxyfluoride glass matrix of composition 2YbF(3):30SiO(2)-15Al(2)O(3)-25CdF(2)-22PbF(2)-4YF(3), forming bulk glass-ceramics at three different crystalisation levels. The samples are compared with a corresponding uncrystalised (glass) sample, as well as a Yb:YAG sample which has presented optical cooling. The measured X-ray diffraction spectra, and thermal capacities of the samples are reported. We also report for the first time the use of Yb:YAG as a reference for absolute photometric quantum efficiency measurement, and use the same setup to characterize the glass and glass-ceramic samples. The cooling figure-of-merit was measured by optical calorimetry using a fiber Bragg grating and found to depend on the level of crystallization of the sample, and that samples with nanocrystallites result in higher quantum efficiency and lower background absorption than the pure-glass sample. In addition to laser-induced cooling, the glass-ceramics have the potential to serve as a reference for quantum efficiency measurements.

Development of ytterbium-doped oxyfluoride glasses for laser cooling applications.

Oxyfluoride glasses doped with 2, 5, 8, 12, 16 and 20 mol% of ytterbium (Yb3+) ions have been prepared by the conventional melt-quenching technique. Their optical, thermal and thermo-mechanical properties were characterized. Luminescence intensity at 1020 nm under laser excitation at 920 nm decreases with increasing Yb3+ concentration, suggesting a decrease in the photoluminescence quantum yield (PLQY). The PLQY of the samples was measured with an integrating sphere using an absolute method. The highest PLQY was found to be 0.99(11) for the 2 mol% Yb3+: glass and decreases with increasing Yb3+ concentration. The mean fluorescence wavelength and background absorption of the samples were also evaluated. Upconversion luminescence under 975 nm laser excitation was observed and attributed to the presence of Tm3+ and Er3+ ions which exist as impurity traces with YbF3 starting powder. Decay curves for the Yb3+: 2F5/2 → 2F7/2 transition exhibit single exponential behavior for all the samples, although lifetime decrease was observed for the excited level of Yb3+ with increasing Yb3+ concentration. Also observed are an increase in the PLQY and a slight decrease in lifetime with increasing the pump power. Finally, the potential of these oxyfluoride glasses with high PLQY and low background absorption for laser cooling applications is discussed.

Optical spectroscopy and upconversion luminescence in Nd3+ doped Ga10Ge25S65 glass

Optical properties of a neodymium (Nd3+) doped glass having composition based on the (Ga2S3)–(GeS2) system are reported. Transition probabilities, radiative lifetimes, and branching ratios related to Nd3+ levels were determined. Frequency upconversion (UC) luminescence due to nonresonant excitation at 1064 nm was observed at ≈535, ≈600, and ≈670 nm. The dependence of the UC intensity on the laser intensity and on the Nd3+ concentration as well as the dynamics of the luminescence process were studied. The results indicate that two-photon absorption by isolated Nd3+ ions and energy transfer among pairs of Nd3+ ions contribute to the UC luminescence.

Control of the radiative properties via photon-plasmon interaction in Er3+ -Tm3+ -codoped tellurite glasses in the near infrared region.

The novelty of this paper is that it reports on the tuning of the spectral properties of Er3+ -Tm3+ ions in tellurite glasses in the near-infrared region through the incorporation of silver or gold nanoparticles. These noble metal nanoparticles can improve the emission intensity and expand the bandwidth of the luminescence spectrum centered at 1535 nm, covering practically all the optical telecommunication bands (S, C + L and U), and extended up to 2010 nm wavelength under excitation by a 976 nm laser diode. Both effects are obtained by the combined emission of Er3+ and Tm3+ ions due to efficient energy transfer processes promoted by the presence of silver or gold nanoparticles for the (Er3+)4I(11/2)→(Tm3+)3H5, (Er3+)4I(13/2)→(Tm3+)3H4 and (Er3+)4I(13/2)→(Tm3+)3F4 transitions. The interactions between the electronic transitions of Er3+ and Tm3+ ions that provide a tunable emission are associated with the dynamic coupling mechanism described by the variations generated by the Hamiltonian H DC in either the oscillator strength or the local crystal field, i.e. the line shape changes in the near-infrared emission band. The Hamiltonian is expressed as eigenmodes associated with the density of the conduction electron generated by the different nanoparticles through its collective free oscillations at each resonance frequency of the nanoparticle and their geometric dependence. A complete description of photon-plasmon interactions of noble metal nanoparticles with the Er3+ and Tm3+ ions is provided.

Germanate-tellurite composite fibers with a high-contrast step-index design for nonlinear applications

The fabrication process and characterization of composite step-index fibers with a large refractive index difference (Δn = 0.336 at 1.54 μm) between the tellurite glass-made core and the germanate-tellurite glass-made cladding are presented. In order to fabricate these composite fibers, the composition of the cladding glass was selected because of its thermal and optical properties corresponding to those of the core glass. This work demonstrates that even if these two glasses have relatively different chemical compositions, their association results in a good quality fiber. This fiber design combines strong confinement of the optical modes inside its core and good environmental stability for nonlinear applications in the mid-infrared.

Similar behaviors of sulfide and selenide-based chalcogenide glasses to form glass ceramics

In this paper, the strong influence of alkali halide in chalcogenide glasses is reminded, leading for the first time to highly transparent glasses from the visible range up to 11μm. The behavior of crystallization has been demonstrated to be similar in sulfide and selenide glasses containing gallium as well. The structural evolution of several glass compositions from the Ge-Ga-S or Ge-Ga-Se systems leading to reproducible glass ceramics has been studied by XRD, NMR and thermal analysis. Whatever the composition, gallium plays a fundamental role as nanosized domains appear by phase separation between Ge rich regions and Ga rich regions. The determination of the appropriate crystallization time and temperature has permitted to obtain new passive and active glass-ceramics with a broadened transmittance region thanks to the incorporation of various alkali halides. In the first case, the controllable generation of nanocrystals leads to an increase of the main thermo-mechanical properties. In the second case, the incorporation of rare-earth ions inside the glass ceramics has exacerbated their photoluminescence properties. The possibility to combine the ceramization process with the shaping has also been demonstrated.

Upconversion luminescence in Er3+ doped Ga10Ge25S65 glass and glass-ceramic excited in the near-infrared

were used as excitation sources. The green ( 2 H11/2 ! 4 I15/2; 4 S3/2 ! 4 I15/2) and red ( 4 F9/2 ! 4 I15/2) photoluminescence (PL) signals due to the Er 3þ ions were characterized. The PL decay times were influenced by energy transfer among Er 3þ ions, by cross-relaxation processes and by energy transfer from the Er 3þ ions to the host material. The PL from the Er 3þ ions hosted in the crystalline phase was distinguished only when the glass-ceramic was excited by the 1480nm pulsed laser. The excitation pathways responsible for the green and red PL bands are discussed to explain the differences between the spectra observed under continuous-wave and pulsed excitation. V C 2013