Bruno Viana
PSL Research University
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Featured researches published by Bruno Viana.
Advanced Materials | 2010
Oleg Lupan; Thierry Pauporté; Bruno Viana
Nanowire (NW)-based light-emitting diodes (LEDs) have drawn great interest because of their many advantages compared to thin-fi lm-based devices. Marked improved performances are expected from nanostructured active layers for light emission. Nanowires can act as direct waveguides and favor light extraction without use of lenses and refl ectors. Moreover, the use of wires avoids the presence of grain boundaries and then the emission effi ciency is boosted by the absence of nonradiative recombinations at the joint defects. The junctions between the nand p-type semiconductors can also be of very high quality with low interfacial strain and defect density as a result of a reduced contact area between the two materials in the case of epitaxial heterojunctions. Integration of nanowires is especially promising for the preparation of short-wavelength emitters such as superluminescent UV-light-emitting diodes and laser diodes. GaN is the main wide-bandgap semiconductor ( E g = 3.39 eV) used for the preparation of blue and UV LEDs. However GaN nanowire arrays are diffi cult to grow. [ 1 , 2 ] Alternative materials for the preparation of nanostructured UV LEDs mainly include ZnO. [ 3–5 ] ZnO has a similar wide bandgap of 3.37 eV and a larger exciton binding energy of 60 meV (compared to 29 meV in the case of GaN) that should favor light emission at room temperature. [ 3 , 6 ] ZnO and GaN share the same wurtzite hexagonal structure. A lot of research efforts have focused on the preparation of p-type ZnO to realize LEDs based on homojunctions made of this semiconductor. [ 7–9 ] For the moment, however, reproducible and stable p-type ZnO material with suffi cient high conductivity and carrier concentration is still at the development phase. An alternative promising approach is to grow n-type ZnO NWs on p-type GaN. [ 10–17 ] ZnO NWs can be grown by various techniques. Most of these efforts for LED applications have been focused on vapor phase growth. Recently, devices have
Journal of the American Chemical Society | 2011
Thomas Maldiney; Aurélie Lecointre; Bruno Viana; Aurélie Bessière; Michel Bessodes; Didier Gourier; Cyrille Richard; Daniel Scherman
Focusing on the use of nanophosphors for in vivo imaging and diagnosis applications, we used thermally stimulated luminescence (TSL) measurements to study the influence of trivalent lanthanide Ln(3+) (Ln = Dy, Pr, Ce, Nd) electron traps on the optical properties of Mn(2+)-doped diopside-based persistent luminescence nanoparticles. This work reveals that Pr(3+) is the most suitable Ln(3+) electron trap in the diopside lattice, providing optimal trap depth for room temperature afterglow and resulting in the most intense luminescence decay curve after X-ray irradiation. This luminescence dependency toward the electron trap is maintained through additional doping with Eu(2+), allowing UV-light excitation, critical for bioimaging applications in living animals. We finally identify a novel composition (CaMgSi(2)O(6):Eu(2+),Mn(2+),Pr(3+)) for in vivo imaging, displaying a strong near-infrared afterglow centered on 685 nm, and present evidence that intravenous injection of such persistent luminescence nanoparticles in mice allows not only improved but highly sensitive detection through living tissues.
Optics Letters | 2006
Y. Zaouter; J. Didierjean; François Balembois; G. Lucas Leclin; Frédéric Druon; Patrick Georges; Johan Petit; Philippe Goldner; Bruno Viana
The experimental demonstration of a diode-pumped passively mode-locked femtosecond laser based on an Yb3+:CaGdAlO4 single crystal is reported. The oscillator is directly diode pumped by a high-brightness 5 W fiber coupled laser diode, and pulses are produced by use of a semiconductor saturable-absorber mirror. It permits the production of pulses as short as 47 fs at 1050 nm, which are to our knowledge the shortest laser pulses obtained from an oscillator based on Yb3+-doped bulk materials. The average power is 38 mW, and the repetition rate is 109 MHz.
Applied Physics Letters | 2003
Romain Gaume; Bruno Viana; Daniel Vivien; J.P. Roger; D. Fournier
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.
Journal of The Optical Society of America B-optical Physics | 2002
Paul-Henri Haumesser; Romain Gaume; Bruno Viana; D. Vivien
Spectroscopic and laser properties at ∼1 µm of seven new ytterbium-doped crystalline materials—silicates Y2SiO5,Ca2Al2SiO7,SrY4(SiO4)3O, borates Ca3Y2(BO3)4,Ca3Gd2(BO3)4,Sr3Y(BO3)3, and Ba3Lu(BO3)3— are investigated. Absorption and emission cross sections are strongly dependent on the crystalline host. Good agreement is obtained between emission cross sections calculated by the reciprocity method and by the Fuchtbauer–Ladenburg method. The wavelength dependence of the gain cross sections are also determined. Except for Ba3Lu(BO3)3, which is of poor crystalline quality, laser oscillations in a plano-concave cavity are obtained for all the presented hosts. From optical spectroscopy and laser results, large tunability of the emission is expected for these materials. Slope efficiencies are in the range 40%–60% with very low threshold values.
Journal of Physics: Condensed Matter | 2003
Ludivine Pidol; A. Kahn-Harari; Bruno Viana; Bernard Ferrand; Pieter Dorenbos; J.T.M. de Haas; C.W.E. van Eijk; E. Virey
Cerium doped lutetium pyrosilicate Lu2Si2O7:Ce3+ (LPS), a new inorganic scintillator, displays particularly promising performance. This material can be readily pulled from the melt. A high light output (average value: 26 300 ph MeV−1), a relatively good energy resolution (9%) and a fast decay time (38 ns) without afterglow make this new scintillator very attractive, in particular for medical imaging. Optical characterizations and scintillation properties of LPS:Ce large single crystals are presented, including timing properties and study of the scintillation yields as a function of incident energy.
Optics Letters | 2005
Johan Petit; Philippe Goldner; Bruno Viana
It is demonstrated that 2% Yb: CaGdAlO4 (called CAlGO) presents favorable thermomechanical properties with a high measured thermal conductivity (Kc = 6.3 and Kc = 6.9 W m(-1) K(-1). A laser oscillation in this material at 1016 nm is demonstrated for the first time to our knowledge while pumping at 979 nm. This implies a very small quantum defect (3.5%). A simple new figure of merit that takes into account thermomechanical properties and quantum defects is proposed here to compare the resistance of materials under high-power laser pumping. Consequently, Yb:CAlGO is similar to garnets and sesquioxides in regard to laser power resistance.
Journal of Physics: Condensed Matter | 2001
Paul-Henri Haumesser; Romain Gaume; Bruno Viana; E. Antic-Fidancev; Daniel Vivien
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.
Journal of Physics: Condensed Matter | 2003
Pieter Dorenbos; L Pierron; L.E Dinca; C.W.E. van Eijk; A. Kahn-Harari; Bruno Viana
The properties of Ce3+ in CaBPO5, LiCaPO4 and Li2CaSiO4 have been studied in the 100–400 nm wavelength region with time resolved spectroscopy at temperatures between 10 and 300 K. Emphasis is on the relationship between the energies of the five 5d levels of Ce3+ and the crystalline environment. Good agreement between predicted and observed average energy of the 5d configuration is demonstrated. In addition values for the bandgap of the host materials, Stokes shifts and luminescence decay time data are presented.
Optics Letters | 2007
Justine Boudeile; Frédéric Druon; Marc Hanna; Patrick Georges; Yoann Zaouter; Eric Cormier; Johan Petit; Philippe Goldner; Bruno Viana
We demonstrate the generation of 68 fs secant hyperbolic pulses at a 105 MHz repetition rate with an average power of 520 mW from a diode-pumped Yb(3+):CaGdAlO(4) mode-locked laser. A semiconductor saturable absorber allows passive mode locking, and a 15 W diode laser is used to pump directly the crystal. To our knowledge this represents the highest average power ever obtained for a sub-100 fs diode-pumped Yb-bulk laser.