For decades, 2 μm lasers based on Tm³⁺-Ho³⁺ co-doped host media have attracted much attention in light detection and ranging (LIDAR), free-space optical communication, atmospheric sensors, and the laser microsurgery.

Tm³⁺-Ho³⁺ co-doped crystals have been proved to be the best gain-media operating at 2 μm due to the cross-relaxation process and efficient energy transfer.

In previous study, a variety of Tm3+ and Ho3+ co-doped hosts have been carried out. Rare-earth cubic sesquioxide crystals are among the most outstanding with high transmittance and low maximum phonon energy.

Considering the doping on thermal conductivity, Lu₂O₃ is the most famous one for its similar mass of the doping ions. For the above reasons, Tm³⁺ and Ho³⁺co-doped LuLu₂O₃ crystal (Tm,Ho:Lu₂O₃) can be a promising material for 2 μm lasers. However, there remain challenges in the extremely high melting point and great difficulties of crystal preparation.

Recently, a cooperative team from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, together with Guangxi University, has designed a new preparation process of growing Tm,Ho:Lu₂O₃ crystal. The structural and optical properties were investigated systematically. The result was published in Optical materials.

For the first time, researchers obtained the high-quality 5 at.% Tm, 0.5 at.% Ho:Lu₂O₃ crystal through optical floating zone method with the size of  mm3 free from any bubble or crack.

In the experiment, the Lu₂O₃ host exhibited perfect doping ability because the segregation coefficients of Tm³⁺-Ho³⁺ ions were 0.70 and 0.94,respectively, which was carried out by inductively coupled plasma atomic emission spectrometry analysis.

In addition, up-conversion experiments were carried out in order to get detailed information on the excited states dynamics. The energy transfer process between Tm³⁺:³F₄ and Ho³⁺: ⁵I₇ energy levels was reversible to a quasi-detailed balance, so the lifetime of Tm³⁺:³F₄ manifold also affected the lifetime of Ho³⁺: ⁵I₇ energy level in the equilibrium state, which could explain the fact that the fluorescence lifetime of the 2 μm emission dropped in Tm³⁺-Ho³⁺ co-doped crystals.

The paper introduced a successful preparation process for extremely high melting-point crystals and developed a new gain medium for 2 μm lasers.

This work was supported by 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.

The as-grown Tm,Ho:Lu₂O₃ crystal. (Image by SIOM)