New Preparation Process for Growing Tm,Ho:Lu2O3 Crystal

For many decades, 2-μm lasers based on Tm³⁺-Ho³⁺ co-doped host media have garnered a great deal of attention in free-space optical communication, light detection and ranging (LIDAR), laser microsurgery, atmospheric sensors, and free-space optical communication.

It has been demonstrated that Tm³⁺-Ho³⁺ co-doped crystals are the best gain-media working at 2 μm. This claim was attributed to the efficient energy transfer and cross-relaxation process.

A wide range of Ho³⁺ and Tm³⁺ co-doped hosts has been developed in a previous study. Rare-earth cubic sesquioxide crystals are considered as one of the most remarkable products with low maximum phonon energy and high transmittance.

Given the doping on thermal conductivity, Lu₂O₃ is the most popular one for its comparable mass of the doping ions. Owing to the reasons stated above, Ho³⁺ and Tm³⁺ co-doped LuLu₂O₃ crystal (Tm,Ho:Lu₂O₃) can be a potential material for 2-μm lasers. Yet, a very high melting point as well as considerable difficulties of crystal preparation tends to pose challenges.

In the recent past, a collaborative group from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, along with Guangxi University, has developed a novel preparation process to grow Tm,Ho:Lu₂O₃ crystals. The optical and structural characteristics were systematically examined. The study outcomes have been published in Optical Materials.

Now, using the optical floating zone technique, scientists have achieved the high-quality 5 at.% Tm, 0.5 at.% Ho:Lu₂O₃ crystal for the first time. Measuring ∅4x36 mm³, the crystal does not contain any crack or bubble.

During the experiment, perfect doping ability was exhibited by the Lu₂O₃ host since the segregation coefficients of Tm³⁺-Ho³⁺ ions were respectively 0.70 and 0.94. This was performed by inductively coupled plasma atomic emission spectrometry analysis.

Up-conversion experiments were also performed to obtain complete data on the dynamics of the excited states. Moreover, the energy transfer process between Ho³⁺:⁵I₇ and Tm³⁺:³F₄ energy levels was reversible to a quasi-detailed balance; hence, the lifetime of Tm³⁺:³F₄ manifold also had an impact on the lifetime of Ho³⁺:⁵I₇ energy level in the equilibrium state.

This may elucidate the fact that the 2-μm emission’s fluorescence lifetime decreased in Tm³⁺-Ho³⁺ co-doped crystals. The study provided an effective preparation process for crystals with very high melting point and created a new gain medium for 2-μm lasers.

The National Natural Science Foundation of China, the National Key R&D Program of China, Strategic Priority Research Program (B), and the Major Project of Shanghai Science and Technology Research Foundation supported the study.