Mixed Sesquioxides for Solid-State Lasers (MiSS-S)

• Applicant: Dr. Christian Kränkel
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Synthesis and Properties of Functional Materials ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523111523

The cubic sesquioxides Lu2O3, Sc2O3 and Y2O3 are excellent host materials for efficient high-power solid-state lasers. However, their high melting points around 2450 °C place very high demands on suitable crystal growth methods. Rhenium is the only crucible material suitable for these high melting points. However, rhenium crucibles are expensive to produce and very sensitive to oxygen in the growth atmosphere. Growth of sesquioxides by the Czochralski method is moreover complicated by a strong self-absorption of the thermal radiation emitted at the high melting temperatures. Since crucible-free growth methods were not successful, yet, single-crystalline sesquioxides are currently not available. The project MiSS-S thus aims to explore the Czochralski growth of the mixed sesquioxide crystal (Y_(x)Sc_(1-x))2O3 with x ≈ 0.5 or in short YScO3, and to investigate its suitability as a laser host material for using Tm3+ as an example. In the solution series between Y2O3 and Sc2O3, the melting temperatures at compositions close to YScO3 decrease to values below 2200 °C. In our recent experiments we demonstrated for the first time that such low temperatures enable the growth by the Czochralski method from conventional iridium crucibles. However, the crystals grown so far suffer from considerable stress and corresponding depolarization losses. The fundamental thermodynamic analyses carried out within this project with a particular focus on the role of the doping ion will revel the origin this stress. In combination with a systematic variation of the growth parameters, in particular the seed crystal orientation, the growth and cooling rates, the isolation setup as well as the growth atmosphere, this should enable to reduce the stress and grow large YScO3 crystals of high optical quality. Our focus will be on Tm3+-doped YScO3, as this doping ion should benefit in particular from the unique properties of mixed sesquioxides. Due to their strong crystal field, we expect unusually long free-running laser wavelengths exceeding 2.1 µm in the laser experiments to be performed within the project. Such values are only known from sesquioxides for the laser transition under investigation. Moreover, the inhomogeneous broadening of the absorption and emission bands is advantageous for pumping with high-power laser diodes or generating ultrashort pulses. Corresponding experiments on mode-locking of Tm:YScO3 will be performed by our partners. The project work will include thermodynamic analyses, crystal growth experiments and laser experiments as well as extensive work on structural, chemical and optical characterization of the crystals in order to provide feedback into crystal growth and to determine the required dopant concentrations and crystal dimensions for the planned laser experiments. The results of the project will also be highly relevant for the doping of YScO3 with other rare earth ions.

• DFG Programme: Research Grants
• International Connection: Japan
• Cooperation Partners: Professorin Dr. Clara Saraceno; Dr. Masaki Tokurakawa