The infrared wavelength of 1?5 μm contains important communication and atmospheric transmission window, which has great application value. At present, the luminescent materials used in this band mainly include rare-earth ion-doped crystal and glass.Because of the large absorption and emission cross section and low phonon energy, crystal material is widely used in the field of solid state lasers. However, the long preparation cycle and limited sample size restrict its further development. Glass material possesses excellent machinability and good fluorescence characteristics, being widely used in communication, laser and other fields. Oxide glass is mainly used in near-infrared (NIR) range being limited by its large phonon energy. Rare-earth ion-doped fluoride glass can be used in visible and mid-infrared (MIR) range, but limited by its poor chemical and mechanical properties. Rare-earth ion-doped glass ceramics shows excellent optical properties from crystal materials, and good physical and chemical properties from glass materials,making it a promising infrared luminescent material.
One of the most important properties of glass ceramic is the transmittance, as the crystalline phase in glass would cause light scattering inevitably. Based on the scattering theory, one could find that the influence factors of transmittance of glass ceramics include the size and distribution of the crystalline phase and the refractive index difference between the crystalline phase and the glass phase. To realize transparent glass ceramics, the size of the crystalline should be less than 30 nm, and the refractive index difference should be less than 0.3.
The status and characteristics of rare-earth ion-doped infrared glass ceramics, including oxyfluoride silicate, tellurate and germanate glass ceramics, were summarized and analyzed. The application and research progress were overviewed. Oxyfluoride silicate glass ceramics is the most widely investigated material. Phase separation would generally happen, leading to the formation of fluorine-rich nanophase before the crystallization process, and would affect the crystalline phase properties, such as crystalline size.The improvement of fluorescence characteristic is summarized, including the aspects of fluorescence intensity, lifetime and bandwidth. Anti-glass phase can be precipitated in tellurate glass ceramics, which is difficult to realize in other glass or preparation alone, giving the material system unique properties. The influence of crystalline phase in tellurate glass ceramics is carefully discussed. Infrared germanate glass ceramics is reported in few glass composition systems, and their properties are introduced. With the greatly improved fluorescence properties, glass ceramic could be used in the fields of infrared fiber laser, infrared whispering gallery modes (WGM) resonator microcavity laser, temperature sensing and luminous anti-counterfeiting and so on, the application cases reported so far are summarized and introduced.
Summary and prospects Glass-ceramics shows excellent optical properties from crystals, and good physical and chemical properties from glass materials, facilitating its important application in infrared wavelength range. Based on the scattering theory, the factors affecting the transmittance of glass ceramics are introduced, and the conditions for realizing transparent glass ceramics are summarized. The most widely reported glass ceramics systems, including oxyfluoride silicate, tellurate and germanate glass ceramics, are introduced. The crystallization mechanism is analyzed, but remains unclear, for example, how to understand the precipitation of anti-glass phase in tellurate glass ceramics. A variety of low phonon energy fluoride or oxide crystalline phases can be precipitated in germanate glass ceramics. However, the infrared germanate glass ceramics is limited to a few systems, and there is plenty of room for exploration of infrared germanate glass ceramics luminous materials. Finally, the application of infrared glass ceramics is introduced, especially in the field of laser. Under the condition of balancing the scattering loss caused by the crystalline phases, glass ceramics shows better laser performance than the precursor glass, which indicates that glass ceramics luminous materials have great application prospects in the field of infrared laser. However, there is few reports on other applications of rare-earth ion-doped glass ceramics in the infrared range at present. It’s expected that the development of relative applications based on unique properties of glass ceramics will promote rare-earth ion-doped glass ceramics to become the next generation infrared luminous materials.