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In this work we present a comparison of the quantum efficiencies (QE) of the 4I13/2→4I15/2transition of Er3+in ZBLAN (58 ZrF4-18 BaF2-5.5 LaF3-3 AlF3-15 NaF), BIGaZYT (30 BaF2-18 InF3-20 ZnF2-10 YF3-10 ThF4-12 GaF3), and silica (67.7 SiO2-16.2 SiO-11.1 CaO-5 Al2O3) glasses obtained by photoacoustic spectroscopy. In the case of ZBLAN and BIGaZYT glasses, the method takes into account the 4I11/2→4F7/2excited state absorption.

One of the most usual way to determine quantum efficiency of Rare Earth ions in solids is to compute the radiative lifetime (τo) of the considered level through Judd's theory and to measure the emission decay time (τ) ; the ratio τ/τobeing the desired quantum efficiency. By such way the quantum efficiency of Er3+at 1.55 µm in silica fibers is estimated to be 100% [1]. However a number of pitfalls can be encountered [2] in such determination, namely radiative energy transfer and "non-natural" excitation linked with crystal field splittings. The purpose of this paper is to show that, taking such effects into account, Er3+efficiency at 1.55 µm is found to be much less in silica fiber than usually accepted. We show that quantum efficiency of Er3+in silica fibers should range between 30 to 45% instead of the 100% usually believed.

Under reducing conditions, single crystal of LiYF4doped with 762 ppm of U3+were obtained. Absorption and fluorescence spectra of this system are presented as well as the energy level scheme of the lasing transition. Oscillator strength and laser cross section between Stark levels of the [MATH] transition are calculated.

ABSTRACTIn the present study, we describe a new simian immunodeficiency virus (SIV), designated SIVgsn, naturally infecting greater spot-nosed monkeys (Cercopithecus nictitans) in Cameroon. Together with SIVsyk, SIVgsn represents the second virus isolated from a monkey belonging to the Cercopithecus mitisgroup of the Cercopithecusgenus. Full-length genome sequence analysis of two SIVgsn strains, SIVgsn-99CM71 and SIVgsn-99CM166, revealed that despite the close phylogenetic relationship of their hosts, SIVgsn was highly divergent from SIVsyk. First of all, they differ in their genomic organization. SIVgsn codes for a vpuhomologue, so far a unique feature of the members of the SIVcpz/human immunodeficiency virus type 1 (HIV-1) lineage, and detailed phylogenetic analyses of various regions of the viral genome indicated that SIVgsn might be a mosaic of sequences with different evolutionary histories. SIVgsn was related to SIVsyk in Gag and part of Pol and related to SIVcpz in Env, and the middle part of the genome did not cluster significantly with any of the known SIV lineages. When comparing the two SIVgsn Env sequences with that of SIVcpz, a remarkable conservation was seen in the V3 loop, indicating a possible common origin for the envelopes of these two viruses. The habitats of the two subspecies of chimpanzees infected by SIVcpz overlap the geographic ranges of greater spot-nosed monkeys and other monkey species, allowing cross-species transmission and recombination between coinfecting viruses. The complex genomic structure of SIVgsn, the presence of a vpugene, and its relatedness to SIVcpz in the envelope suggest a link between SIVgsn and SIVcpz and provide new insights about the origin of SIVcpz in chimpanzees.

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The upconverted fluorescence dots that appear under photon-avalanche excitation in an erbium-doped fluorozirconate fiber are quantitatively analyzed with respect to the pump-mode structure into the fiber and the energy spatial diffusion between Er^3+ ions. By computing the mode-propagation constants, we show that the main luminescent structures periods correspond to beating between low-order pump modes. The observed luminescent dots result from these interferences combined with the excitation intensity threshold of avalanche upconversion. We also investigated the influence of energy-transfer spatial diffusion among Er ions by comparing the pump-mode transverse profiles deduced from avalanche and Addition de Photons par Transfert d’Energie (APTE) emissions. At high pump power the modes widths that correspond to APTE experiments are larger than those obtained from avalanche upconversion by 0.8 μm. We attribute this effect to the resonant energy transfer (^4I_11/2, ^4I_15/2)?(^4I_15/2, ^4I_11/2), which is negligible in photon avalanches because of strong excited-state absorption. From the experimental broadening we estimate that the one-dimensional diffusion length associated with this transfer is 1.13 μm.

The effect of the thermal treatments at a temperatures much below the glass crystallization on the microstructure of ZrF4-based fluoride glass doped by Er3+ions was studied. The glass characteristic temperatures and the apparent activation energy for crystallization were estimated on the basis of differential thermal experiments. The crystallization of ErxLa1−xZr3F15phase was detected in nearly transparent samples. Room temperature, Raman scattering spectra in the acoustic and optical phonon regime as well as fluorescence of Er3+ions in the infrared and visible region indicated a continuous structural modification of the glassy matrix during heat treatment and crystallization initiation.

Multiphonon relaxation rate saturation is observed for rare-earth-doped glasses for high excited-state densities. This behavior is analyzed in a statistical approach; a theoretical model for the microscopic process is proposed. A phonon bottleneck effect on radiationless relaxation related to an accepting-modes saturation is suggested. At high excited-state density, ions in a phonon diffusion volume simultaneously fill the accepting modes. The critical distance below which excited ions share a common phonon bath is related to the phonon diffusion length in the host and deduced from our model. The results are in good agreement with the phonon mean free path independently deduced from sound velocity, thermal conductivity, and heat capacity.

Fluorescence decay time of resonantly excited RE multiplets were recorded as a function of excitation power density in different glass hosts. The multiphonon nonradiative relaxation rates are reduced at high excited state concentration for large energy gaps. The results are analysed in a statistical approach and the observed phonon bottleneck effect related to an accepting mode saturation. At high excited state density, several ions lying in the phonon diffusion volume simultaneously fill the common accepting mode set of the host. The critical distance below which excited ions share a common phonon bath is related to the phonon diffusion length lcin the host. Theoretical fit of experimental data allows the deduction of lc. The results are compared with the phonon mean free path in the host independently derived from sound velocity measurements and estimation of thermal conductivity and heat capacity of the investigated glasses.

In this paper, we present our work about the near I.R. emission broadband (3T2 → 3A2) of Ni2+-doped BaLiF3 around 1.4 μm. R.T. emission spectra, absorption spectra, laser cross-section and lifetime doping ion concentration dependence are reported. High values of fluorescence lifetime and emission cross-section are evidenced.

Experimental demonstration of multiphonon assisted anti-Stokes and Stokes excitation of RE3+ions in different hosts, that is in a weak coupling ion-lattice interaction case, is obtained. An exponential behaviour of excitation probabilities with respect to energy difference between excitation and electronic line is found just as for multiphonon assisted non radiative decay and energy transfer. Exponential parameters αASand αSfor anti-Stokes and Stokes excitation are measured and liked to the Miyakawa-Dexter α parameter for decay, leading to estimates for Pekar-Huang-Rhys coupling parameter S0of about 0.04 in agreement with values obtained by other authors by one-phonon spectra or non radiative decay measurements.

Determination of exponential parameters for both Stokes and anti-Stokes sidebands by excitation of a ZBLAN: Er3+ fibre and bulk samples gives values of 9.4×10-3 cm and 14.1×10-3 cm, respectively. Observed values are larger than equivalent parameters usually found for nonradiative transitions. As examples, the measured parameters are used to determine the multiphonon sideband absorption between the 4I11/2 and the 4I13/2 electronic transitions of Er3+ and also between 1G4 and 3F4 of Pr3+. Available energy ranges for sideband pumping of distributed amplifiers are given as well as the minimum multiphonon background losses intrinsic to the rare-earth plus pure glass base combination.

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Infrared laser action and tunability of the solid state vibronic system MgF2: Ni2 + is obtained by pumping with the 7525 Å radiation of a c.w. krypton laser. Limitations to laser efficiency are attributed mainly to internal losses caused by excited state absorption and energy transfer phenomena. These phenomena are analysed and interpreted using data provided by absorption and fluorescence (decay and intensity) measurements in the infrared and visible regions. The energy transfer process is discussed in terms of the Dexter and Inokuti-Hirayama theories. The dominant inter-ion interaction is then attributed to the exchange mechanism, and a critical ion concentration for energy transfer of about 0·25% Ni2+ is deduced. This shows in turn that the laser efficiency of MgF2: Ni2+, when it is pumped by a krypton laser, could be improved by using samples with lower concentrations.

Based on the understanding of the role played by the multiphonon sideband absorption as the initial step in photon avalanche up-conversion, such an effect in Er3+and in a ZBLAN glass and fibre at room temperature was obtained for the first time. The pump wavelength is far from any electronic absorptions from the ground state of the Er3+ion, but corresponds to excited state absorption. The distinct characteristics such as rise time, threshold and non-linear dependence on pumping have been demonstrated. The dynamic process of the photon avalanche has been analyzed and mechanisms are proposed.

Absorption outside the commonly considered electronic transition absorption regions in rare-earth-ion-doped ZBLAN fluoride glass have been investigated in detail and compared with those in halide crystals. Exponential decays with respect to energy have been found both in Stokes and anti-Stokes excitations, and the exponential parameters have been measured. By comparing with exponential parameters for multiphonon non-radiative decay, it is found that the phonons coupled to multiphonon side band absorption are generally smaller than the highest phonon cut off frequency which is known to be operative in non-radiative decay.

The largest recent advance in the field of solid state materials is the exponential development of the Erbium doped fibre amplifiers, the so-called EDFAs, which are already implemented in many telecommunication systems. One is already interested in the evolution of such amplifiers towards more compact devices which could be obtained through Erbium doped wave guide amplifiers. The conditions for wave guide amplifiers are first discussed showing that it will be difficult to limit the active length to less than a few tenths of a centimetre in usual glasses without having to increase the active ion concentration up to the point where self-quenching reduces prohibitively the quantum efficiency of the amplifying transition. This leads us to consider new glassy materials where the inhomogeneous linewidth of the active ions shall be reduced in order to correlatively increase the gain cross-sections. An historical review of such an approach will be presented. Then we show that composite materials of the vitroceramic type where the active rare earth ion stays only in the microcrystalline phase, provide materials with reduced inhomogeneous broadening and with crystal type optical spectra though obtained by glass technics. Evaluations of the gain cross-sections indicate increases of at least 100%. Such composite materials may so pave the way for wave guide amplifiers.

We have developed mirrorless quasi-monochromatic laser sources made of stoichiometric neodymium compounds (Nd_0.75:La_0.25P_4O_15 and NdCl_3· 6H_2O) pumped by nanosecond laser pulses. We find that short subnanosecond and narrow-bandwidth (0.15-nm) pulses are generated in both compounds when they are pumped at high intensities. This emission is spatially incoherent as shown by a speckle statistics analysis. Its origin is discussed in terms of collective effects. This shows that poor optical materials with strongly quenched emission may be useful for generating incoherent short pulses.