On the potential of BaSO4:Mn for broadly tunable laser emission in the near infrared spectral region

Because of the strong electron-phonon coupling of the emitting 3d orbitals 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 d 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). Mn is a promising ion for a tunable laser system, and near-infrared emission from Mn was observed in several host lattices. In BaSO4, the room-temperature stimulated-emission cross section is larger than the excited-state-absorption cross section in the spectral range 920-1600 nm [1], i.e., as a laser material BaSO4:Mn can offer a broad tuning range. The fabrication of Mn-doped BaSO4 layers requires a particular growth method at low temperatures, since the Mn ions tend to reduce to Mn at T ≥ 600°C. We have grown Mn-doped BaSO4 layers at low temperatures using liquidphase epitaxy (LPE). We used a CsCl-KCl-NaCl solvent for the LPE of BaSO4:Mn with a low solidification temperature of 480°C in order to keep the temperatures well below 600°C to prevent chemical reduction of Mn to Mn. The nominal Mn concentration was up to 1 mol% with respect to S. High quality layers with thickness of up to 580 μm, lack of largesize inclusions, and low defect concentration were achieved [2]. The Mn-doped BaSO4 layers were investigated spectroscopically by absorption and emission measurements at room temperature (see Fig. 1) and the incorporation of manganese solely in its hexavalent oxidation state into our layers was confirmed [2]. The Mn absorption bands are E → T2 at 700-900 nm and a ligand-to-metal charge-transfer band at 500-650 nm. Excitation into these bands leads to broadband Mn emission between 850 and 1600 nm. The fluorescence lifetime was measured to be 0.6 μs [2]. A relatively small reabsorption from the ground state is visible in the region of the fluorescence emission.