Fluoride transparent ceramics for solid-state lasers: A review

The unique characteristics of fluoride ceramics, including phonon energy, refractive index, and nonlinear refractive index, render them highly suitable as laser matrix materials. Fluoride-based laser ceramics also offer thermal conductivity comparable to fluoride laser crystals and possess advantages in fracture toughness and thermal shock resistance. Simultaneously, they facilitate high-concentration uniform doping and the fabrication of large-sized and intricately shaped samples, rendering them highly promising as laser gain media. Importantly, thanks to the widely recognized facts above and the efforts put in by research groups from all over the world, in recent years the innovations made in manufacturing methods and the availability of increasingly advanced equipment have contributed to the exponential development of technologies for obtaining fluoride transparent ceramics with increasingly better performance. At the same time, numerous reports in recent years confirm that expectations for these materials continue to exceed achievements, and the work on their further optimization still does not lose its pace.

Recently, a team of material scientists led by Jiang Li from Shanghai Institute of Ceramics, Chinese Academy of Sciences, China presented a comprehensive review of the progress of fluoride laser ceramics, summarising the preparation of CaF₂, SrF₂ and BaF₂ based transparent ceramics, and focusing on the current status of rare earth ion doped fluoride laser ceramics for application in different wavelength bands. The current laser output level of fluoride laser ceramics is summarised and the current problems and challenges are pointed out, and future research directions are proposed. This review is of great significance for the development and performance optimisation of fluoride transparent ceramics, which will help to further enhance the applicability of fluoride transparent ceramics.

The team published their work available online in Journal of Advanced Ceramics on October 14, 2024.

“The characteristics of fluoride materials in terms of phonon energy, refractive index, nonlinear refractive coefficient, and negative thermo-optic coefficient make them popular as laser substrate materials. Meanwhile, fluoride-based laser ceramics have a thermal conductivity comparable to that of fluoride laser crystals and are more advantageous in terms of mechanical properties such as fracture toughness and micro-hardness, and it is also easy to achieve high-concentration homogeneous doping of rare-earth ions and the preparation of large-size and complex samples. Therefore, it is a very potential laser gain medium.” said Jiang Li, professor affiliation at Shanghai Institute of Ceramics, Chinese Academy of Sciences (China), a senior expert whose research interests focus on the transparent and photofunctional ceramic materials.

“It is worth noting that the inspiration and guidance provided by oxide transparent ceramics to the development of fluoride ceramics should not be overlooked. Researchers have applied the inspiration gained from the development of oxide transparent ceramics, such as fabrication techniques, sintering aid additions, microstructure modulation and composite material development, to greatly improve the performance and applicability of fluoride transparent ceramics.” said Jiang Li.

Fluoride transparent ceramics have now successfully achieved laser output in the visible and infrared spectral regions. However, the output power and slope efficiency of fluoride transparent ceramics are relatively low compared with that of fluoride single crystals, and there are no reports of their use in high-power and ultrashort-pulse solid-state lasers. In addition, the optical quality of fluoride transparent ceramics still needs to be further optimised, and the sharp decrease in transmittance at short wavelengths can lead to severe optical losses, hindering the achievement of higher power laser output. Li also put forward that future research should still focus on optimising the optical quality of fluoride transparent ceramics, reducing the preparation cost, improving the production efficiency and ensuring the reproducibility of the manufacturing process. At the same time, research on eliminating defects in fluoride transparent ceramics to improve material properties is insufficient and needs to be further investigated. In addition, the application scope of fluoride laser ceramics should be further broadened, not only to continue to study new matrix materials such as BaF₂, but also to explore in depth more rare earth ion-doped fluoride transparent ceramics.

Other contributors include Xiang Li, Qiang Liu from the School of Materials Science and Engineering at Jiangsu University in Zhengjiang, China; Chen Hu from the Shanghai Institute of Ceramics at Chinese Academy of Sciences in Shanghai, China; Dariusz Hreniak from the Institute of Low Temperature and Structure Research at Polish Academy of Sciences in Wroclaw, Poland.

This work was supported by the National Key R&D Program of China (Grant No. 2021YFE0104800), the International Partnership Program of Chinese Academy of Sciences (Grant No. 121631KYSB20200039), National Center for Research and Development (Contract No.WPC2/1/SCAPOL/2021), and the Chinese Academy of Sciences President’s International Fellowship Initiative (Grant No. 2024VEA0014).

About Author

Jiang Li holds an M.S. and Ph.D. from Shanghai Institute of Ceramics, Chinese Academy of Sciences. He is a researcher and doctoral supervisor at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, and the chief scientist of the National Key Research and Development Program (NKRDP). He is currently the associate editor of Journal of Advanced Ceramics and Journal of Inorganic Materials, associate editor of Journal of the American Ceramic Society and International Journal of Applied Ceramic Technology, editorial board member of Magnetochemistry, Advanced Ceramics, Journal of Synthetic Crystals, Journal of Functional Materials and Devices and other journals. Currently, he is mainly engaged in basic and applied research on optical functional transparent ceramics (including: laser ceramics, scintillation ceramics, magneto-optical ceramics, phosphor ceramics for illumination/display, long afterglow luminescent ceramics, etc.).

For four consecutive years, he has been selected as one of the top 2% of the world's top scientists in the ‘Career Science Impact Ranking’ published by Stanford University, USA. He has been invited to give more than 60 invited lectures (more than 20 times as session chair) at important academic conferences at home and abroad. As a project leader, he has undertaken more than 20 national and provincial projects. So far, he has published 446 papers in important academic journals both at home and abroad (including 275 papers as the first author and corresponding author); co-authored two monographs titled "A Brief Discussion on High Temperature Structural Ceramics" and " Advanced and Optical Functional Transparent Ceramics " published by Science Press; co-translated the monograph "Luminescent Materials" by the Higher Education Press; participated in the writing of some chapters in two English monographs; participated in the writing of Chapter 19 "Transparent Ceramics" in the textbook "Inorganic Nonmetallic Materials Technology "; applied for over 50 Chinese invention patents (including 25 authorized).

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