# Progress in Transparent Nano-Ceramics and Their Potential Applications
## 2 Transparent Nano-Ceramics for Solid-State LASERs
### Doped YAG Transparent Nano-Ceramics  
**Nd3+-doped**. In 1995, Nd:YAG transparent ceramics for solid-state lasers with continuous wave (CW) laser emissions were first reported by [PERSON] and [PERSON] [12], and the nano powers of Y\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and Nd\({}_{2}\)O\({}_{3}\) were used as starting materials with the average particle diameters of 60, 400, and 500 nm, respectively. The optical scattering loss of Nd:YAG was about 0.9%/cm. The experiment demonstrated that the performance of solid-state lasers could be obtained with an oscillation threshold of 309 mW and a slope efficiency of 28%, respectively. In 2002, [PERSON] et al. [13] developed Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\) optical ceramic materials based on highly transparent nanocrystalline YAG. The pore volume concentration of YAG transparent ceramics was 1 ppm, and the average diameter of particles was about 10 \(\upmu\)m. The grain boundary width was only about 1 nm. The results showed that in the preliminary comparative laser experiment of Nd:YAG ceramic and single-crystal rods, the output power of 88 and 99 W were obtained, respectively. This means that it could be used in high-power, solid-state lasers. Compared with single-crystal Nd:YAG, the light-to-light efficiency of Nd:YAG transparent ceramics with nanocrystalline needs to be further improved. However, it will become a good substitute for the widely used Nd: YAG single crystal, due to its low manufacturing cost, for different types of solid-state lasers. In 2010, [PERSON] et al. [14] first obtained 1 at.% Nd:YAG nano powder with an average particle size of 100 nm by using a reverse-strike precipitation method. Then, the Nd:YAG transparent nano-ceramics were prepared by the hot isostatic pressing (HIP) method. They found that the optical properties were significantly different with different sintering and HIP parameters. The infrared transmittance of the fabricated sample was 80%, and its emission spectrum was the same as a 1 at.% Nd:YAG single crystal. In 2011, [PERSON] et al. [15] sintered Nd:YAG transparent ceramics at 1600 \({}^{\circ}\)C with B\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) double-sintering additives. They adopted the solid-state reaction method to prepare the Nd:YAG transparent ceramics and \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) (>99.99%, 100-300 nm), Y\({}_{2}\)O\({}_{3}\) (>99.999%, 50 nm), and used Nd\({}_{2}\)O\({}_{3}\) (>99.99%, 200 nm) nano powders as the starting materials. Additionally, the B\({}^{3+}\): Si\({}^{4+}\) atomic ratio ranged from 0.5 to 2 while keeping the total doping level at 1.35 mol%. The results demonstrated that the relative density of the samples exceeded 99.9% and the transmittance in the visible band was as high as 84%. They also found that densification could be completed at about 100 \({}^{\circ}\)C lower than the normal sintering temperature since B\({}^{3+}\) greatly improved the driving force of densification. In 2014, [PERSON] et al. [16] also utilized the solid-state reaction method to fabricate a Nd:YAG transparent ceramic, and investigated its phase formation and densification mechanism in the sintering process. As depicted in Figure 3, the particle size of Al\({}_{2}\)O\({}_{3}\), Y\({}_{2}\)O\({}_{3}\) starting powders, as well as 2.88 Y\({}_{2}\)O\({}_{3}\)-0.12 Nd\({}_{2}\)O\({}_{3}\)-5 Al\({}_{2}\)O\({}_{3}\) powder mixture and Y\({}_{2}\)O\({}_{3}\) powders after planetary ball milling for 15 h, ranged from 80 to 800 nm. The results showed that using Y\({}_{2}\)O\({}_{3}\) nano powder, under bimodal particle size distribution (D\({}_{50}\approx\) 160 nm and 400 nm), could make the shrinkage effect higher than the expansion effect in the formation of the YAG phase during sintering. Additionally, the transmittance of the prepared 4 at.% Nd:YAG sample (1 mm thick) at 650 nm was 80%, which was close to that of Nd:YAG single crystal. In addition, [PERSON] et al.