# Progress in Transparent Nano-Ceramics and Their Potential Applications
## 2 Transparent Nano-Ceramics for Solid-State LASERs
### Doped YAG Transparent Nano-Ceramics
[17] studied the effect of Nd dopant and LiF additive on the microwave dielectric and optical properties of transparent YAG ceramics in the spark plasma sintering (SPS) process in 2016. The SEM images demonstrated that the size of YAG nano powder was almost between 50 and 100 nm, and the infrared transmittance of the sample was 81.8% after sintering at 1360 \({}^{\circ}\)C. In 2021, [PERSON] et al. [18] comparatively analyzed the influence of tetraethoxysilane (TEOS) additives on the sintering kinetics of Nd:YAG transparent ceramics. The vacuum sintering method was used to evaluate the densification process and sintering kinetics of Nd:YAG transparent ceramic samples. The densification rate of ceramic samples rose dramatically when the amount of TEOS was raised from 0 to 3.0 wt.%. The experiment showed that the transmittance of the 0.5 wt.% TEOS sample reached 75% in the near-infrared region. **Ho-doped**. Under direct pumping, the Ho\({}^{3+}\) ion emits a quasi-three-level emission at 2.0 \(\upmu\)m, exploited for efficient CW lasing [19; 20]. Additionally, infrared lasers have also been made with Ho:YAG transparent ceramics. In 2015, [PERSON] et al. [21] fabricated nano powders generated by laser ablation and then used two ways to make Ho:YAG transparent ceramics. The nano powders were made up of near-spherical particles with an average size of 8-14 nm and specific surface areas of 83.8 and 46.0 m\({}^{2}\)/g for the Al\({}_{2}\)O\({}_{3}\) and Ho:Y\({}_{2}\)O\({}_{3}\) particles, respectively. The results revealed that the transparent ceramics produced by their proposed method had better transmittance (82%) in the infrared band. Additionally, the slope efficiency of laser oscillations in the fabricated Ho:YAG transparent ceramic sample (1 mm thick) for pumping power was 40% (at 1.85 \(\upmu\)m). In 2018, [PERSON] et al. [22] demonstrated a Ho:Y\({}_{2}\)O\({}_{3}\) ceramic laser with high power, which fabricated Ho:Y\({}_{2}\)O\({}_{3}\) ceramics by vacuum sintering and HIP methods. The in-band pumping method produced a 2117 nm laser with an output power of 24.6 W, nearly an order of magnitude higher than other ceramics. For high-power, solid-state lasers, therefore, Ho-doped sesquioxide ceramics are ideal materials. **Er-doped**. Er-doped YAG transparent ceramics have very low levels of quantum defects, and their laser behavior is IR transitions at 1.5 and 3 \(\upmu\)m. In 2011, [PERSON] et al. [23] demonstrated a 0.5 at.% Er:YAG ceramic laser, which exhibited CW emission at 1617 nm and had a slope efficiency of 51.7%. In 2015, [PERSON] et al. [24] reported a passively Q-switched ceramic Er:YAG laser using a saturable absorber, which emitted 1617 nm. The experimental result confirmed that the laser could reach a peak power of 11.3 kW. In 2018, a laser adopted by 0.5 at.% Er:YAG transparent ceramics, with a resonantly pumped eye-safe, was developed by [PERSON] et al. The fabricated ceramics adopted a two-step approach, combining SPS+HIP methods [25]. In their study, high-purity 0.5 at.% Er\({}^{3+}\):YAG powder with a specific surface area of 7 m\({}^{2}\)/g and an average size of 271 nm was used. The experimental results confirmed that the light-light efficiency of this laser was 20%, and the maximum slope efficiency was 31%. Figure 3: SEM images of Al\({}_{2}\)O\({}_{3}\) (**a**), Y\({}_{2}\)O\({}_{3}\) (**b**) starting nano powders, as well as 2.88 Y\({}_{2}\)O\({}_{3}\)–0.12 Nd\({}_{2}\)O\({}_{3}\)–5 Al\({}_{2}\)O\({}_{3}\) powder mixture (**c**) and Y\({}_{2}\)O\({}_{3}\) powders after planetary ball milling for 15 h (**d**) [16]. Reprinted with permission from Ref. [16]. Copyright 2014, copyright ELSEVIER. **Tm-doped**. The Tm\({}^{3+}\) concentration should be at least 6% to guarantee efficient down-conversion energy transfer [26]. Experiments showed that the transparent ceramics doped with Tm\({}^{3+}\) have good light transmittance [27]. [PERSON] et al.