# Progress in Transparent Nano-Ceramics and Their Potential Applications
## 3 Magneto-Optical Transparent Nano-Ceramics
### Tgg  
In 2003, [PERSON] [41] reported TGG transparent nano-ceramics for the first time. The results demonstrated that the emergence of high-quality nano-ceramics and the improved Faraday device made it possible to apply TGG transparent nano-ceramics in higher-power lasers. In 2007, [PERSON] et al. [42] reported the Faraday effect of TGG ceramics for the first time. They tested the Verdet constant of TGG transparent nano-ceramics samples at 1053 nm as a function of temperature and found that the Verdet constant was 36.4 rad/(T-m) at room temperature, which was 87 times greater than that at the temperature of liquid nitrogen (77 K). This showed that under the same magnetic field, the length of the Faraday material could be shortened to 1/87, which provided advantages for femtosecond short-pulse lasers. In addition, the test data also confirmed that the Verdet constant of TGG transparent nano-ceramics was similar to that of TGG single crystals. In 2011, [PERSON] et al. [39] systematically studied the optical characteristics and Faraday effect of TGG ceramics at room temperature. As shown in Figure 5a, the grain size distribution of the prepared TGG transparent nano-ceramics was relatively uniform, and the grain size was between 300 nm and 3 \(\upmu\)m, which was smaller than that of the YAG transparent nano-ceramics. At the same time, they tested the optical transmittance of TGG ceramic samples in the visible and near-infrared bands and compared them with TGG single crystals. It could be drawn from Figure 5b that in the 600-1400 nm band, the transmittances of TGG ceramics and single crystals were almost equal, with both higher than 80%[PERSON]  
In 2019, [PERSON] et al. [43] prepared the magneto-optical properties of TGG transparent nano-ceramics. The microstructures of the TGG powders showed that the average particle size was about 80 nm, and the powder agglomerated through small connections to the particle necks, which facilitated the densification of the ceramic body. The best microstructure of the nano-ceramics with average grain sizes of 5.32 \(\upmu\)m, fabricated by co-precipitated method and vacuum sintering methods, could be obtained while the sample was sintered at 1550 \({}^{\circ}\)C (in Figure 6)[PERSON] Additionally, they found that the best optical transmittance of TGG ceramics was close to 80% in the region of 400-1500 nm, and that the Verdet constant of the fabricated TGG ceramics decreased linearly with an increase in temperature. The results demonstrated that TGG transparent nano-ceramics could meet the requirements of magneto-optical devices working the visible-near-infrared region. In 2021, [PERSON] et al. [44]investigated the fabrication and evaluated the performance of novel TGG transparent nano-ceramics, which were doped by rare earth (RE) of Pr, Tm, and Dy. The microstructure of the powder particles was several hundred nanometers (300-700 nm) in length and several nanometers in width. After two-step sintering, no second phase was detected in the microstructure, although residual pores in the ceramic could be noticed. Therefore, the prepared ceramics all had good optical quality, and the online transmittance at 1070 nm was greater than 80%. The Verdet constant of RE:TGG transparent nano-ceramic samples (\(-143\) rad/T-m at 632.8 nm) was optimized by rare earth doping, which was about 5% higher than that of TGG transparent nano-ceramics. With the increase in power of the CW laser and pump laser, the thermal effect of optical elements in a laser system becomes more and more serious [45; 46; 47]. Thermally induced birefringence reduces the isolation ratio of the Faraday isol  
Figure 5: Microstructure of TGG transparent nano-ceramic and its transmittance spectra [39]; (**a**) SEM image showing the microstructure of a typical TGG transparent nano-ceramics sample; (**b**) Comparison of transmission spectra between TGG nano-ceramics samples and TGG single crystal. Reprinted with permission from Ref. [39]. Copyright 2011, copyright IEEE Xplore.