Epitaxial growth and optical characterization of BaSO4:Mn6+ layers for novel coherent light sources

Abstract

Because of the strong electron-phonon coupling of the emitting 3d orbital in transition-metal (TM) ions, 3d systems exhibit luminescence with typically several hundred nm of spectral bandwidth. TM-ion-doped materials are, therefore, of high interest for applications as tunable and short-pulse lasers. Whereas many TM-ion-doped systems suffer from excited-state absorption (ESA), systems with a d1 electron configuration possess only one excited 3d level and ESA into higher-lying 3d levels is impossible. However, ESA can occur owing to transitions into the conduction band and conduction-band-related energy levels. Mn6+ is a promising ion for a tunable laser system, and near-infrared emission from Mn6+ was observed in several host lattices. In BaSO4, the room-temperature stimulated-emission cross section is larger than the ESA cross section in the spectral range 920-1600 nm [bru97II]. Therefore, as a laser material BaSO4:Mn6+ can offer a broad tuning range. The fabrication of Mn6+-doped BaSO4 layers requires a particular growth method at low temperatures, since the Mn6+ ions tend to reduce to Mn4+ at T ≥ 600°C. Moreover, this reduction is also time-dependent. The Mn6+-doped BaSO4 layers have been grown at low temperatures using liquid-phase epitaxy (LPE). The eutectic CsCl-KCl-NaCl served as solvent for the LPE of BaSO4:Mn6+ with a low eutectic temperature of 480°C in order to keep the temperatures well below 600°C to prevent chemical reduction of Mn6+. The initial Mn6+ concentration was up to 1 mol% with respect to S6+. In contrast, the Mn6+ concentration in the layers was maximum 20% of the initial Mn6+ concentration. High quality layers with thickness of up to 580 μm, lack of large-size inclusions, and low defect concentration were achieved [ehr02]. The etch-pit density for the (011) face was 103 cm-2. The Mn6+-doped BaSO4 layers were investigated spectroscopically by absorption and emission measurements at room temperature and the incorporation of manganese solely in its hexavalent oxidation state into the layers was confirmed [ehr02]. The Mn6+ absorption bands are 2E —> 2T2 at 700-900 nm and a ligand-to-metal charge-transfer band at 500-650 nm. Excitation into these bands leads to broadband Mn6+ emission between 850 and 1600 nm. The fluorescence lifetime was measured to be 0.56μs [ehr02]. A relatively small reabsorption from the ground state is visible in the region of the fluorescence emission. With the measured spectroscopic data, analytical and numerical simulations in order to investigate the future laser potential of BaSO4:Mn6+ was performed. Since reabsorption at the laser wavelength from the ground state especially in the wings of the pump absorption may play a significant role in such a ground-state laser system, the rate equations for the excited-state population density and the photon density in the cavity were solved. The pump and laser wavelengths, the dopant concentration, and the length of the active medium were optimized with respect to minimize the laser threshold. Pumping around 800 nm and providing high reflectivity of the cavity mirrors at 1150 nm cannot enable laser emission, because the ratio of 3:1 between pump absorption cross-section and laser reabsorption cross-section is too small to allow for sufficient pump absorption and simultaneously avoid significant reabsorption of the oscillating laser light. In contrast, when pumping at 532 nm and providing high reflectivity at 1500 nm, this ratio becomes 143:1. In conclusion, laser oscillation of BaSO4:Mn6+ at 1500 nm is more likely. The damage threshold was evaluated for the pump wavelengths of 532 and 1064 nm, 1.6 × 106 W cm-2 and 5.2 × 108 W cm-2, respectively. The damage threshold and the laser threshold are similar for pumping at 532 nm and 5% Losses in the crystal. References [bru97II] T.C. Brunold, H.U. Gudel, S. Kuck, G. Huber, J. Opt. Soc. Am. B 14 (1997) 2373. [ehr02] D. Ehrentraut, M. Pollnau, S. Kuck, Appl. Phys. B 75 (2002) 59.