**5. Oxide crystal**

Normally, oxide-based materials have a high laser damage threshold and many of them have limited IR transparency within the range of 6 μm. Here, we would like to explain the new oxide-based NLO materials, which are very important for the development of IR transparent regions beyond 6 μm. Several new NLO oxide crystals were identified, which have high IR cutoff wavelengths that are up to 6 μm. For instance β-BaTeMo2O9 [53], MnTeMoO6 [54], Cs2TeW3O12 [55], V2Te2O9 [56], Li3VO4 [57] , M2LiVO4 (M = Rb, Cs) [58], Li2K4TiOGe4O12 [59], and Rb4Li2TiOGe4O12 [60]. Moreover, for the oxide-based NLO material, IR transparent range is a difficult factor to attain. Pb17O8Cl18 was new IR-NLO material discovered by Pan and Poeppelmeier et al. [61] in 2015. Pb17O8Cl18 single crystal was synthesized by modified spontaneous crystallization in an open model. Tao's et al. [62] reported a new LiNbO3-type NLO crystal. Li2ZrTeO6 crystal (size: 16 × 15 × 12 mm3 ) is grown by top-seeded solution growth (TSSG) method. To maintain the structural qualities of LiNbO3, Zr4+and Te6+ were substituted for Nb5+ to form Li2ZrTeO6 and it crystallized in the trigonal crystal system with space group *R*3 (**Figure 5**). LiNbO3 belongs to the space group, *R*3*c,* which has a close structural feature with Li2ZrTeO6 [63]. La3SnGa5O14 is a new

*Recent Advances in Infrared Nonlinear Optical Crystal DOI: http://dx.doi.org/10.5772/intechopen.108173*

**Figure 5.** *Crystal structures of Li2ZrTeO6.*

IR-NLO crystal, it belongs to the langasite family and has a space group *P*321 [64]. Polycrystalline La3SnGa5O14 was produced using a solid-state reaction and a single crystal were grown by Czochralski method. The single crystal of Pb17O8Cl18 has a bandgap of 3.44 eV and it has high IR transparency (13.9 μm). La3SnGa5O14 has a wide energy gap value of 4.60 eV and transparency of 10 μm.

The SHG responses of Pb17O8Cl18 showed a response at 2090 nm and 1064 nm, which is phase-matchable, and it is two times stronger than that of AgGaS2 and four times higher than that of KDP. Li2ZrTeO6 showed a huge powder SHG behavior at 1064 nm, which is 2.5 times higher than that of KDP [63]. For Li2ZrTeO6 and LiNbO3, there is a variation in the SHG responses, which is closely correlated, and it is due to the different sizes of octahedral distortions in their crystal structure. The SHG efficiency of La3SnGa5O14 is 0.4 times that of AgGaS2 and when compared to AgGaS2 (13 μm) it has wide IR transparency. Pb17O8Cl18 has an LDT of 12.8 MW/cm2 (**Table 7**). These characteristics reveal that Pb17O8Cl18 is one of the good mid-IR-NLO crystals for the next generation. Li2ZrTeO6 has an outstanding optical performance and a very high LDT greater than 1.3 GW/cm2


**Table 7.** *Space group, band gap, transparency range, and SHG value of oxide crystal.* and it was more than 22 times that of LiNbO3. It has a higher IR transparent range that is up to 7.4 μm. La3SnGa5O14,which has the highest LDT of 846 MW/cm2 , is an alternative NLO crystal in the mid-IR region. It is transparent beyond 10 μm. The langasite family provided valuable information to design a new NLO crystal in the IR area.
