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

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)

Liquid phase epitaxy and optical investigation of KYb(WO4)2 thin layers

In recent years, Yb3+ has attracted much attention as an activating ion because of its small quantum defect for laser emission from 2F5/2 to 2F7/2 at ~1.03 µm [1], which provides high efficiency and reduced heat generation. Of high practical interest is the thin-disk laser concept [2], which possesses a tremendous advantage over rod lasers because of its axial-cooling approach and consequent weak thermal lensing and good beam quality.\ud A promising material for Yb3+ thin-disk lasers is KYb(WO4)2 (KYbW) [3]. It can be grown from high-temperature solutions [4]. Nevertheless, the growth of high-quality, single-crystalline layers with thickness in the range of the absorption length of ~13 µm at 981 nm has as yet not been reported. A suitable substrate material is KY(WO4)2 (KYW), but the relatively large differences in the thermal expansion coefficients between KYW and KYbW along the [100], [001], and especially [010] directions [5] favor low temperatures for the hetero-epitaxial growth.\ud For the first time, we demonstrate liquid phase epitaxy (LPE) of KYbW layers. The layers were grown at start temperatures as low as 520°C, which is favorable in order to decrease the thermal stresses due to the differences in the thermal expansion coefficients of substrate and layer. Moreover, the choice of [010]-oriented substrates bypasses the large difference in the thermal expansion coefficient along the [010] direction. KY1-xYbx(WO4)2 layers with varying x = 0.03-1.00 were grown by LPE. The chloride solvent consisted of the eutectic composition [6] 24.4 mol.% KCl, 30.4 mol.% NaCl, and 42.2 mol.% CsCl. The growth temperature spanned the range from 580 to 500°C and the cooling rate was 0.67-1.00 Kh-1. Crack-free, transparent KYbW layers were grown on (010) substrates.\ud Spectroscopic investigations have shown that the lifetime of ~250 µs measured in our LPE-grown KYbW layers is dominated by radiative decay and is very similar to that measured in top-seeded-solution-grown bulk samples [4]. Fast energy migration among the Yb3+ ions and energy transfer to small amounts of Tm3+ and Er3+ ions present in the YbCl3 reagent lead to visible upconversion luminescence in the layers under 981-nm excitation.\ud \ud [1] T.Y. Fan, IEEE J. Quantum Electron. 29, 1457 (1993).\ud [2] A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, H. Opower, Appl. Phys. B 58, 365 (1994).\ud [3] P. Klopp, U. Griebner, V. Petrov, X. Mateos, M.A. Bursukova, M.C. Pujol, R. Solé, J. Gavaldà, M. Aguiló, F. Güell, J. Massons, T. Kirilov, F. Díaz, Appl. Phys. B 74, 185 (2002).\ud [4] M.C. Pujol, M.A. Bursukova, F. Güell, X. Mateos, R. Solé, J. Gavaldà, M. Aguiló, J. Massons, F. Díaz, P. Klopp, U. Griebner, V. Petrov, Phys. Rev. B 65, 165121 (2002).\ud [5] M.C. Pujol, X. Mateos, R. Solé, J. Massons, J. Gavaldà, F. Díaz, M. Aguiló, Mater. Sci. Forum 378-381, 710 (2001).\ud [6] D. Ehrentraut, M. Pollnau, S. Kück, Appl. Phys. B 75, 59 (2002)

Liquid phase epitaxy and spectroscopic investigation of optically active KYb(WO4)2 thin layers

In recent years, Yb3+ has attracted much attention as an activating ion because of its small quantum defect for laser emission from 2F5/2 to 2F7/2 at ~1.03 µm, which provides high efficiency and reduced heat generation. A promising material for Yb3+ lasers is KYb(WO4)2 (KYbW) [1]. It can be grown from high-temperature solutions [2]. A suitable substrate material for the growth of single-crystalline layers with thicknesses in the range of the absorption length of ~13 µm at 981 nm is KY(WO4)2 (KYW).\ud We demonstrate the liquid phase epitaxy (LPE) of KYbW layers at start temperatures as low as 520°C from the chloride solvent KCl-NaCl-CsCl. This temperature is favorable in order to decrease the thermal stresses due to the differences in the thermal expansion coefficients of substrate and layer. Moreover, the choice of [010]-oriented KYW substrates bypasses the large difference in the thermal expansion coefficient along the [010] direction. Our spectroscopic investigations show that the fluorescence lifetime of ~250 µs measured in our LPE-grown KYbW layers is dominated by radiative decay and is very similar to that measured in top-seeded-solution-grown bulk samples [2]. Fast energy migration among the Yb3+ ions and energy transfer to small amounts of Tm3+ and Er3+ ions present in the YbCl3 reagent lead to visible upconversion luminescence in the layers under 981-nm excitation.\ud \ud [1] P. Klopp, U. Griebner, V. Petrov, X. Mateos, M.A. Bursukova, M.C. Pujol, R. Solé, J. Gavaldà, M. Aguiló, F. Güell, J. Massons, T. Kirilov, F. Díaz, Appl. Phys. B 2002, 74, 185\ud [2] M.C. Pujol, M.A. Bursukova, F. Güell, X. Mateos, R. Solé, J. Gavaldà, M. Aguiló, J. Massons, F. Díaz, P. Klopp, U. Griebner, V. Petrov, Phys. Rev. B 2002, 65, 16512

Low-temperature flux growth of sulfates, molybdates, and tungstates of Ca, Sr, and Ba and investigation of doping with Mn 6 +

The growth of undoped and Mn6+-doped molybdates and tungstates of alkali-earth metals and BaSO4 has been investigated. Single crystals were grown by the flux method within the temperature range of 600-475°C, using the ternary NaCl-KCl-CsCl solvent. Sizes of undoped crystals increase within the series tungstates<molybdates<sulfate and, depending on the cation, within the series Ca2+≈Sr2+<Ba2+. The Mn6+ ion tends to be reduced to Mn5+/Mn4+ with time in the chloride solution, but can be partly stabilized by the addition of alkali-metal carbonates or hydroxides. The incorporation of Mn6+ is governed by the coordination of the MnO4 2- tetrahedron in the crystal. No significant doping was found for Ca and Sr compounds and only small amounts of Mn6+ were incorporated into BaMoO4 and BaWO4. Crystals with orthorhombic space group Pnma such as BaSO4 exhibit significantly higher doping levels. The Mn6+ distribution in each crystal varies due to manganese reduction with growth time. Temperature-, time-, and concentration-dependent spectroscopy of BaSO4:Mn6+ was performe