Philippe Veber

Flux growth at 1230 °C of cubic Tb2O3 single crystals and characterization of their optical and magnetic properties

In this work, we present the first crystal growth of cubic Tb2O3 single crystals by a controlled atmosphere flux method which uses a heavy metal free solvent working at less than half the melting temperature of this sesquioxide. Cubic millimeter-sized crystals extracted from as-grown boules are phase (powder XRD) and chemically (GDMS) pure and exhibit a Verdet constant in the visible and near-infrared spectral ranges, which is at least three times higher than that of a commercial Tb3Ga5O12 (TGG) crystal. The 1.36 mm thick crystals display a transmission coefficient higher than 77% over the 525 nm–1.38 μm spectral range. The absorption spectrum, magnetic susceptibility and specific heat measurements on the single crystals confirm the absence of detectable Tb4+ cations and other impurities.

Flux growth of Yb3+-doped RE2O3 (RE = Y,Lu) single crystals at half their melting point temperature

We discuss a new, cheap, simple, reliable and safe growth process of Yb3+-doped Y2O3 and Lu2O3 single crystals by a flux method, using an original, nontoxic solvent in a growth setup operative in air and at half the melting temperature of cubic RE2O3. Several mm3-sized Yb3+-doped Y2O3 and Lu2O3 single crystals were grown, the orientation, purity and Yb3+-concentration of which were characterized by means of X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy and electron probe microanalysis (EPMA). A preliminary characterization of Yb3+-doped cubic RE2O3 (RE = Y,Lu) single crystals basic spectroscopic and laser parameters is presented.

Laser demonstration with highly doped Yb:Gd 2 O 3 and Yb:Y 2 O 3 crystals grown by an original flux method

We present, to the best of our knowledge, the first laser demonstration of an Yb-doped Gd(2)O(3) cubic crystal. This crystal was obtained by the flux method using an original borate-based solvent, which was particularly well suited to the growth of rare earth sesquioxide crystals at half the working temperature of classical growth techniques. This flux method is a very interesting alternative for the production of laser sesquioxide crystals, not only because it provides access to new matrices of the cubic polymorph, but also because it permits high Yb(3+)-doping levels for these crystals. The first laser results of two highly Yb(3+)-doped sesquioxides, namely Gd(2)O(3) and Y(2)O(3), grown by this flux method are presented here, including the Ti:sapphire and diode pumping configurations. Laser efficiencies and emission spectra for these two crystals were studied and compared.

Orientation-dependent electromechanical properties of Mn-doped (Li,Na,K)(Nb,Ta)O3 single crystals

Orientation and temperature dependence of dielectric and electromechanical properties of Mn-doped (Li,Na,K)(Nb,Ta)O3 single crystals were investigated. Samples exhibited very low dielectric losses, with tanδ between 0.03 and 0.05 over a broad temperature range between room temperature and 480u2009°C. Influences of the crystallographic structure and external electric field on polarization and strain parameters are discussed. The orientation-dependent electrical properties were ascribed to the anisotropic polarization rotation in the crystals. Higher maximum polarization, coercive field, and negative strain were achieved when the electric field was oriented along one of the spontaneous polarization directions. The highest maximum unipolar strain of 0.42% (at 3u2009kV/mm) and the normalized strain d33* of 1391u2009pm/V were obtained in the [001]PC-oriented sample at 100u2009°C, which was much higher than the values obtained for the [110]PC-oriented sample. Further insight of the phase transition behavior is given by compari...

Influence of Ta5+ content on the crystallographic structure and electrical properties of [001]PC-oriented (Li,Na,K)(Nb,Ta)O3 single crystals

A series of centimeter-sized lead-free piezoelectric Li+- and Ta5+-modified (Na,K)NbO3 single crystals with an ABO3 perovskite structure was successfully grown by the top-seeded solution growth method. Effective segregation of the elements was considered in order to further develop the growth of Li+- and Ta5+-modified (Na,K)NbO3 single crystals. The crystallographic structure and electrical behaviour along the [001]PC orientation were studied. X-ray diffraction, Raman spectroscopy and dielectric studies reveal that the increase in Ta5+ content reduces the orthorhombic–tetragonal phase transition. Bipolar and unipolar strain curves were investigated at 2 kV mm−1. The highest bipolar and unipolar strains associated with a large-signal piezoelectric constant of 368 pm V−1 were achieved for Li0.02Na0.626K0.354Nb0.808Ta0.192O3. The asymmetric bipolar strain curves were observed and related to the existence of internal bias fields induced by defect dipoles, which were created during the crystal growth process.

Growth and spectroscopic properties of 6Li- and 10B-enriched crystals for heat-scintillation cryogenic bolometers used in the rare events searches

We present the crystal growth of centimeter-sized 6Li6Eu(10BO3)3 and Li6(Eu,Gd)(BO3)3 single crystals by combined Czochralski and Kyropoulos methods, initiated on specifically oriented seeds, with a heretofore unexplored concentration range for heat-scintillation cryogenic bolometers application. The crystal structure and lattice thermal expansion of the former, obtained for the first time by single crystal X-ray diffraction (XRD), and the spectroscopic characterizations together with a consistent set of related thermodynamical properties measurements (magnetic susceptibility, specific heat), also unknown to date, are discussed. Finally, preliminary scintillation measurements at low temperature between 300 and 1100 nm under X-ray excitation are shown.

Crystal growth and optical characterizations of Yb3+-doped LiGd6O5(BO3)3 single crystal: a new promising laser material

Single crystals of a new oxyborate, Yb-LGOB, were obtained by an original flux method and interesting spectroscopic properties of this new laser crystal are reported.

Anisotropy of the spectroscopic, optical and thermo-mechanical properties of Li 6 Eu 1-x Gd x (BO 3 ) 3 crystals optimized for heat-scintillation cryogenic bolometers

Optical, spectroscopic and thermo-mechanical properties of monoclinic Li6Eu1-xGdx(BO3)3 (x = 0,0.25,0.35) bulk single crystals, grown to be used in the design of heat-scintillation cryogenic bolometers (HSCBs), were investigated. The linear thermal expansion was determined along the a, b, c and c* directions over the temperature range 303–873 K, and its tensor principal coefficients were calculated for both x = 0.25 and x = 0. In addition, the anisotropic thermal conductivity was measured over the temperature range 20-400 K in Li6Eu0.75Gd0.25(BO3)3 (LGEB7, x = 0.25), and the principal components of its tensor at 300 K were established. Spectroscopic properties such as polarized absorption, polarized emission, Raman spectroscopy and optical refractive indices are also reported for the first time. Based on the polarized emission spectra, the line and oscillator strengths, the radiative lifetimes and fluorescent branching ratios were obtained. The intensity parameters Ωλ (λ = 2,4) were obtained and then predicted via the Judd–Ofelt theory. The crystal field parameters and the 7F1 level splitting were discussed using the simple overlap model (SOM) and the method of equivalent nearest neighbours (MENN). The 613 nm emission originates from the 5D0→7F2 transition and the associated stimulated emission peak cross section reaches its maximum value in π-polarization, ≈1.07 × 10−20 cm2 in LGEB7. The thermo-mechanical characterizations and spectroscopic analysis in LGEB7 suggest that this crystal has better optical properties than the Eu3+-doped Li6Y(BO3)3 crystals. However, its potential for solid-state laser applications is still quite speculative, unless a clever thermo-mechanical management of the crystal under laser operation is designed.

Correlation between Luminescence and EPR Spectroscopy as Evidence of Ytterbium Pair Formation in Li6Ln(BO3)3:Yb3+ (Ln=Gd, Y) Borate Single Crystals

Synthesized powders and grown single crystals of nominal compositions Li(6)Ln(BO(3))(3):Yb(3+) (Ln=Y, Gd) were investigated by means of powder and single-crystal X-ray diffraction (XRD), as well as optical near-IR spectroscopy in conjunction with electron paramagnetic resonance (EPR) spectroscopy. The appearance of two distinct zero-phonon lines suggests the existence of two kinds of Yb(3+) ions in the single crystals. The XRD results exclude the possibility of a phase transition occurring between room and low temperatures. EPR spectra of single crystals show the presence of both isolated ions and pairs of ytterbium ions substituted for Y(3+). A strong temperature dependence of the intensity of Yb-Yb pairs resonance lines coincides with temperature dependence of emission peak at 978 nm, confirming a common origin of the defect giving rise to these spectra. Calculated from EPR spectra, the distance between pairs of Yb(3+) is in good agreement with crystallographic ones: R=3.856 Å, R(cryst) =3.849 Å.

Scintillating bolometers for fast neutron spectroscopy in rare events searches

Neutrons are a relevant background in rare events physics. Detectors based on fast neutron-induced nuclear reactions are commonly used for fast neutron spectroscopy. In this subject, scintillating bolometers provide an excellent energy resolution and particle discrimination by the simultaneous measurement of the heat and emitted light. Our group has constructed several 6Li and 10B based massive scintillating bolometers (LiF, Li6Eu(BO3)3, Li6Gd(BO3)3), with energy resolutions ranging from 16 to 200 keV. First results of a 32 gr 6LiF scintillating bolometer enriched at 95% in 6Li operated at 20 mK are presented. The use of this material in a multi-target cryogenic dark matter experiment, like EURECA, would allow monitoring the incident neutron flux in the detector during the data-taking.