Layer growth of high-quality BaSO4:Mn6+ using liquid phase epitaxy

Single-crystalline host materials doped with transition-metal ions are of high interest for applications as tunable lasers. Mn6+ ions exhibit broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown in sufficient quality. The active material has to be free of inclusions or defects larger than λ/10, with λ, the wavelength of the porpagating beam. The interface between active layer and substrate must be optically flat to receive low-loss guiding properties. Finally, in the case of homo-epitaxy of BaSO4, the doped layer has to be arranged on the substrate (001) direction, because … .\ud The growth temperature of BaSO4:Mn6+ is limited by the decomposition of BaSO4 at 1590°C, its phase transition above 1010°C, and especially the chemical reduction of the manganese dopant from Mn6+ to Mn5+ above 620°C. Therefore, the growth of BaSO4:Mn6+ from a solution at lower temperatures is the most suitable method. Liquid phase growth is close to the thermodynamic equilibrium and has enabled us to grow high-quality layers.\ud First, we prepared undoped BaSO4 crystals of 10 x 5 x 1 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as solvent. Subsequently, growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained crystalline layers free of inclusions, grown in the Frank-Van der Merwe mode (layer-by-layer growth). Finally, layers of BaSO4:Mn6+ were fabricated with thicknesses up to 150 μm, at growth rates of 3 μm/h and temperatures of 500–580°C. The thickness was controllable with a precision of 0.1 μm. The Mn6+ concentration in the doped layer was up to 1 mol.% with respect to S6+.\ud In collaboration with the University of Hamburg, absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its sextavalent oxidation state. Room-temperature luminescence in the wavelength range 850-1600 nm was observed

Homoepitaxial growth of high-quality BaSO4:Mn6+ using low-temperature liquid phase epitaxy

Single-crystalline host materials doped with 3d1 transition-metal ions are of high interest for applications as tunable lasers. The Mn6+ ion exhibits broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown with sufficient optical quality. The active material has to be free of defects and inclusions larger than 1/20 of the optical wavelength. In addition, the surface of the active layer and its interface to the substrate must be optically flat to receive low-loss guiding properties.\ud The structure of the barite-phase BaSO4 contains tetrahedra which are replaced partly by the dopant complexes. The BaSO4:Mn6+ growth temperature is limited by the phase transition above 1010°C and especially the noticeable reduction of Mn6+ to Mn5+/Mn4+ above 600°C. Therefore, the growth of BaSO4:Mn6+ from a solution at lower temperatures is the most suitable method. Liquid-phase growth is close to the thermodynamic equilibrium and has enabled us to grow high-quality layers.\ud First, we grew undoped BaSO4 substrate crystals of 10 x 5 x 4 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as the solvent. Subsequently, the growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained flat layers free of inclusions with step heights of 1.4 nm, equal to 2 unit cells, and step distances of about 200 nm. Finally, layers of BaSO4:Mn6+ were grown on c-oriented faces with thicknesses up to 150 μm, at growth rates of 3 μm/h and growth temperatures of 500–550°C. The Mn6+ concentration in the doped layer was up to 1 mol% with respect to S6+.\ud Absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its hexavalent oxidation state. Room-temperature broadband luminescence in the wavelength range 850-1600 nm was observed

Epitaxial growth and spectroscopic investigation of hexavalent manganese in barium sulfate

We investigate the influence of active-ion distributions on energy-transfer upconversion (ETU) in a static upconversion regime in which energy migration is inactive and the active local environment is important for the dynamics of ETU. Neodymium is used as the probe ion. In oxide and fluoride host materials, Nd3+ possesses only one metastable excited state (4F3/2) from which strong ETU occurs. The levels excited by ETU emit weak visible luminescence, possess short (multiphonon-quenched) lifetimes and, therefore, react almost instantaneously on the dynamics of the metastable level. The chosen host material lanthanum scandium borate [1] possesses large distances between the active-ion sites and ETU occurs in the static regime [2,3]. After excitation of the metastable level, we measure concentration-dependent (10, 25, 50, 100 at.%) infra-red (direct) and visible (upconversion) luminescence decay. The upconversion luminescence decays neither quadratically with respect to the direct luminescence as would be expected from a usual rate-equation model -- even if two classes of isolated (non-ETU) and clustered (ETU) ions are assumed [4] -- nor exponentially as would be expected from a dimer model [5]. The decay curves are described in a multimer model that takes into account the real structure of the host material and assumes centers with different numbers of active nearest neighbors. With decreasing excitation at longer decay times, its solution converges to the solution of a quasi-dimer model.\ud [1] J.P. Meyn et al., IEEE J. Quantum Electron. 30, 913 (1994)\ud [2] D.A. Zubenko et al., Phys. Rev. B 55, 8881 (1997)\ud [3] V. Ostroumov et al., J. Opt. Soc. Am. B 15, 1052 (1998)\ud [4] M. Pollnau, J. Alloys Compd. 341, 51 (2002)\ud [5] D.R. Gamelin, H.U. Güdel, in Topics in Current Chemistry, Vol. 214 (Springer-Verlag, Berlin Heidelberg, 2001

Flux growth and liquid phase epitaxy of undoped and Mn6+-doped sulfates, tungstates, and molybdates

The Mn6+ ion is a promising activator ion for tunable and short-pulse laser materials because of its broadband luminescence in the spectral region 850-1600 nm and its simple 3d1 electronic configuration, which excludes an occurrence of undesirable exited-state absorption into higher 3d levels. However, hexavalent manganese can be stabilized only in the tetrahedral oxo-coordination and easily reduces to Mn5+ or Mn4+ at temperatures above 600°C. Recently, flux [1] and liquid-phase epitaxy (LPE) [2] growth of Mn6+-doped sulfates has been reported, while except for BaMoO4:Mn6+ [3] investigations on the mechanically more stable alkaline-earth-metal molybdates and tungstates as possible host materials for efficient Mn6+ incorporation have as yet not been reported.\ud We investigated the growth conditions of undoped and Mn6+-doped MAO4, with M = Ca, Sr, Ba and A = S, Mo, W, from the ternary NaCl-KCl-CsCl solvent at temperatures 480-600°C. The growth rates increase in the series tungstates < molybdates < sulfates and depending on the cation, in the series Ca < Sr < Ba. The dopant ion Mn6+ can be easily incorporated into BaSO4, less well into BaMoO4 and BaWO4, whereas for Ca- and Sr-containing tungstates and molybdates no significant doping was found, independent on the concentration of Mn6+ in the liquid solution. Moreover, reduction of the Mn6+ ion cannot be avoided, even at the presence of oxidizing additives such as K2CO3 or NaOH.\ud LPE was employed for growing Mn6+-doped layers of BaAO4 compounds. Growth velocities of 3-5 µm/h in the temperature interval from 490-540°C from chloridic solution, containing 0.3-1mol% of K2MnO4 with respect to the solute, delivered dark-pink BaSO4 and slightly green BaMoO4 and BaWO4 layers up to 200 µm in thickness. With respect to high Mn6+ doping levels, BaSO4 is the most suitable host material and its further investigation under different initial concentrations of manganese is currently underway.\ud \ud [1] T.C. Brunold, H.U. Güdel, Inorg. Chem. 36, 1946 (1997).\ud [2] D. Ehrentraut, M. Pollnau, Appl. Phys. B 75, 59 (2002).\ud [3] T.C. Brunold, H.U. Güdel, Chem. Phys. Lett. 249, 77 (1996)

Rente mit 67 - ein unvermeidbarer Schritt

Ende August hat die „Kommission für die Nachhaltigkeit in der Finanzierung der Sozialen Sicherungssysteme“ („Rürup-Kommission“) der Bundesregierung ihren Bericht vorgelegt. Hierin wird, ebenso wie im Bericht der „Herzog-Kommission“ „Soziale Sicherheit _ zur Reform der sozialen Sicherungssysteme“ für den CDU-Bundesvorstand von Ende September, die in der Öffentlichkeit heftig umstrittene Heraufsetzung des Renteneintrittsalters auf 67 Jahre gefordert. Ist eine „Rente mit 67“ unvermeidbar? --

Orientational and Translational Cooling in Two Dimensional Systems of Granular Needles

We present molecular dynamics results of a two-dimensional gas of inelastic needles. At high number densities such a system forms a macroscopically ordered nematic phase. A simple dissipation model is introduced which takes into account geometry and number density of interacting bodies. Freely cooling of the dissipative system leads to&nbsp; orientational clusters (bundles), local inelastic collapse and decrease of the order parameter