Ehrentraut, D.

Pollnau, Markus

English

High-quality layers of BaSO4:Mn6+ were grown by liquid-phase epitaxy. Spectroscopic investigation and numerical calculation using the measured spectroscopic parameters show that lasing at the long-wavelength side should be possible under 532-nm excitation

University of Twente Research Information

On the potential of BaSO4:Mn6+ for broadly tunable laser emission in the near infrared spectral region  D. Ehrentraut and M. Pollnau Applied Photonics Laboratory, Institute of Biomedical Imaging, Optics and Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland  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 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 excited-state-absorption cross section in the spectral range 920-1600 nm [1], i.e., 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 Mn5+ at T ≥ 600°C. We have grown Mn6+-doped BaSO4 layers at low temperatures using liquid-phase epitaxy (LPE). We used a CsCl-KCl-NaCl solvent for the LPE of BaSO4:Mn6+ with a low solidification temperature of 480°C in order to keep the temperatures well below 600°C to prevent chemical reduction of Mn6+ to Mn5+. The nominal Mn6+ concentration was up to 1 mol% with respect to S6+. High quality layers with thickness of up to 580 µm, lack of large-size inclusions, and low defect concentration were achieved [2].  The Mn6+-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 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 0.6 µs [2]. A relatively small reabsorption from the ground state is visible in the region of the fluorescence emission. 400 600 800 1000 1200 14000102030 Absorption Emission Abs. Coeff. α [cm-1]Wavelength [nm] Photon Flux [s-1] Fig. 1: Absorption and emission spectra of BaSO4:Mn6+.  With the measured spectroscopic data, we performed a numerical simulation in order to investigate the future laser potential of BaSO4:Mn6+. Since reabsorption 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 eqations for the excited-state population density and the photon density in the cavity were solved with longitudinal and radial resolution. The pump and laser wavelengths, dopant concentration, and length of the active medium were optimized with respect to a possible minimization of the laser threshold.   We find that pumping around 800 nm and providing high reflectivity of the cavity mirrors at 1150 nm cannot enable laser emission, because the ratio of ~5:1 between pump absorption cross-section and laser reabsorption cross-section is too small to allow for sufficient pump abssorption 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 ~140:1. We conclude that laser oscillation of BaSO4:Mn6+ at the longer wavelength side of the emission should be possible.  [1] T.C. Brunold, H.U. Güdel, S. Kück, G. Huber, J. Opt. Soc. Am. B 14, 2373 (1997). [2] D. Ehrentraut, M. Pollnau, S. Kück, Appl. Phys. B 75, 59 (2002). 

On the potential of BaSO<sub>4</sub>:Mn<sup>6+</sup> for broadly tunable laser emission in the near infrared spectral region

https://ris.utwente.nl/ws/files/6137548/CLEO-2003_Ehrentraut.pdf

On the potential of BaSO<sub>4</sub>:Mn<sup>6+</sup> for broadly tunable laser emission in the near infrared spectral region

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