Daniel Vivien

A simple model for the prediction of thermal conductivity in pure and doped insulating crystals

We present a simplified approach to evaluate the thermal conductivity of insulating crystals, taking into account chemical and structural properties. We complete the thermal conductivity evaluation in the case of point defects encountered in luminescent materials. Thermal conductivity coefficients have been determined by photothermal experiments and are in good agreement with the predicted values in Yb:Y3Al5O12 and Yb:Gd3Ga5O12 solid-state lasers.

Spectroscopic and crystal-field analysis of new Yb-doped laser materials

Crystal-field effects are very important as far as laser performances of Yb-doped materials are concerned. In order to simplify the interpretation of low-temperature spectra, two tools derived from a careful examination of crystal-field interaction are presented. Both approaches are successfully applied in the case of new Yb-doped materials, namely Ca3Y2(BO3)4 (CYB), Ca3Gd2(BO3)4 (CaGB), Sr3Y(BO3)3 (SrYBO), Ba3Lu(BO3)3 (BLuB), Y2SiO5 (YSO), Ca2Al2SiO7 (CAS) and SrY4(SiO4)3O (SYS). The 2F7/2 splitting is particularly large in these materials and favourable to a quasi-three-level laser operating scheme. Calculations performed using the point charge electrostatic model for these compounds and using a consistent set of effective atomic charges confirm the experimental results. This should permit to use this model in a predictive approach.

LNA: A new CW Nd laser tunable around 1.05 and 1.08 µm

We have investigated the CW laser properties of the lanthamide hexa-aluminate La 0.9 Nd 0.1 MgAl 11 O 19 at room temperature. When a 1 cm long crystal is pumped by an Ar+ laser (514 nm) or a Kr+ laser (752 nm), CW emission is obtained with slope efficiencies of 10 and 26 percent, respectively. A four-plate Lyot filter in the cavity forces the LNA crystal to oscillate in either of the two major bands centered al 10 820 A (tuning range 80 A) and 10 545 A (tuning range 35 A).

Theoretical and experimental investigations of a diode-pumped quasi-three-level laser: the Yb/sup 3+/-doped Ca/sub 4/GdO(BO/sub 3/)/sub 3/ (Yb:GdCOB) laser

We present a theoretical and experimental analysis of a diode-pumped Yb/sup 3+/-doped Ca/sub 4/GdO(BO/sub 3/)/sub 3/ (Yb:GdCOB) laser. A new model for a diode-pumped quasi-three-level laser is described. The effects of absorption saturation, temperature profile, and the beam quality M/sup 2/ factor of the pump diode have been taken into account, for the first time to our knowledge. We have obtained a good agreement between experimental measurements and theoretical calculations with two different pump wavelengths, 902 and 976 nm. Our model has given good predictions of the laser performances for different crystal temperatures and different M/sup 2/ factors of the pump beam. As much as 440 mW of output power (at 1082 nm) have been achieved for 640 mW of absorbed pump power at 976 nm, corresponding to one of the highest slope efficiencies (81%) ever obtained with Yb-doped lasers.

Apatite-structure crystal, Yb 3+ :SrY 4 (SiO 4 ) 3 O, for the development of diode-pumped femtosecond lasers

We report what is believed to be the first diode-pumped Yb(3+): SrY(4)(SiO(4)) (3)O laser. We investigate both the cw and the mode-locked regimes. In the cw regime an average output power of 1.05 W is demonstrated for 3.6 W of incident pump power. In the femtosecond regime a pulse duration of 94 fs for an average power of 110 mW is obtained at a central wavelength of 1068 nm. This is, to our knowledge, the first femtosecond mode-locked oscillator made with an Yb-doped apatitelike crystal. These properties are related to the unique spectroscopic properties of Yb(3+) ions in this atypical apatite-family crystal.

Room-temperature CW laser operation at /spl sim/1.55 /spl mu/m (eye-safe range) of Yb:Er and Yb:Er:Ce:Ca/sub 2/Al/sub 2/SiO/sub 7/ crystals

Room-temperature CW laser operation at 1.55 /spl mu/m of Yb:Er:Ca/sub 2/Al/sub 2/SiO/sub 7/ (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, Ce/sup 3+/, decreases the Er/sup 3+4/I/sub 11/2/ excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of /spl sim/5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented.

Nucleation efficiency of erbium and ytterbium fluorides in transparent oxyfluoride glass-ceramics

Oxyfluoride glasses (GeO 2 –PbO–PbF 2 ) doped with erbium and/or ytterbium fluorides were prepared. Highly transparent glass-ceramics, containing β–PbF 2 nanocrystallites, were successfully obtained by controlled glass devitrification and were studied as they could lead to promising optical applications. To characterize the samples, differential thermal analysis, x-ray diffraction, and transmission electron microscopy were performed, revealing a variation of the crystallites size, the crystallites number and β–PbF 2 crystallization temperature according to the doping ion. Indeed, the analyses indicated differences between erbium and ytterbium fluorides in promoting the crystallization of the fluoride phase. Although both fluorides act as seeds for β–PbF 2 heterogeneous nucleation, erbium fluoride has higher nucleation efficiency than ytterbium fluoride and runs the nucleation process in co-doped samples. Energy dispersive x-ray microanalysis insured high rare-earth segregation into the crystallites, proving the formation of a solid solution Pb 1− x−y Er x Yb y F 2 + x + y , also confirmed by the unit cell parameter study.

Long-lifetime fluorescence and crystal field calculation in Cr4+-doped Li2MSiO4, M = Mg, Zn

Abstract The diffuse reflectance spectra of Cr:Li2MSiO4, M=Mg, Zn, indicate that in these compounds, chromium ions occur in the 4+, 5+, and 6+ oxidation states simultaneously. Under selective excitation in a Cr4+ absorption band, a very long fluorescence decay time is observed for both compounds: ∼110 μs at room temperature and ∼300 μs at 30 K. These are by far the longest fluorescence lifetimes reported for Cr4+ activated materials. In Cr4+:Li2MSiO4, the 1E excited state level lies below the lowest component of the 3T2 level and the fluorescence decay time is dominated by the long-lifetime 1E level for which the transition to the ground state is spin-forbidden. The reverse situation occurs for the other Cr4+ doped compounds and their fluorescence lifetimes, governed by the short-lifetime 3T2 state, are only a few microseconds. A crystal field calculation, performed for Cr4+:Li2MSiO4, confirms the above interpretation and supports the localization of Cr4+ at the silicon site in this compound.

Quasi-three-level 946 nm CW laser emission of Nd:YAG under direct pumping at 885 nm into the emitting level

Abstract The possibility to enhance the laser emission characteristics and to reduce the heat generation for the 946 nm Nd:YAG laser by direct pumping into the emitting level is discussed. Improved laser characteristics are demonstrated with a 1 at.% Nd:YAG component under 885 nm pumping into the thermally activated transitions Z2→R1 and Z3→R2 of the 4 I 9/2 → 4 F 3/2 absorption.

Optical and ESR investigations of lanthanum aluminates LaMg1-xMnxO19 single crystals with magnetoplumbite-like structure

Abstract New lanthanum aluminates LaMAl11O19 (M2+ = Ni, Co, Mn, Mg1−xMnx, 0 ≤ x ≤ 1), with magnetoplumbite-like structure have been obtained as single crystals. This paper is particularly devoted to the Mn2+ and Mg 2+ Mn 2+ mixed compounds, which exhibit promising luminescent properties. Several characteristics of the crystals are given. The absorption spectra of the materials, as grown, are assigned to Mn2+ ions in tetrahedral sites. After annealing in air new absorptions attributed to octahedral Mn3+ ions, appear. The ESR spectra of Mn2+ in all these crystals exhibit axial symmetry. For x ≤ 0.25 they arise from isolated Mn2+ ions in slightly distorted tetrahedral sites and reveal a strong disorder effect. For x ≥0.5 the spectra consist of a single line, attributed to clusters of magnetically interacting Mn2+ ions.