Infrared nonlinear optical (NLO) materials, as the core devices of all-solid-state lasers, have tremendously contributed to the progress of many advanced technologies, such as long distance laser communication, environmental monitoring and photonic technologies, etc. Compared with the ultraviolet NLO materials, the development of mid- and far-IR NLO materials is limited by the requirements of a wide IR transparent window, high laser damage threshold (LDT) and strong NLO effect. To date, only a few chalcopyrite-like (CL) candidates including AgGaS2 (AGS), AgGaSe2 (AGSe),nand ZnGeP2 (ZGP) are commercially available in these regions, owing to their large second harmonic generation (SHG) responses and wide IR transparency. Nevertheless, the small band gaps (Eg) induced low LDT and two-photon absorption (TPA) in these materials have limited their further applications in high-power lasers. Hence, the exploration of new IR NLO materials with wide band gap and large SHG response is an urgent need but still challenging due to the competition between the wide band gap and large NLO coefficient in one material.

　　Recently, a research group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, has designed the first defect-CL (DCL) alkali-earth metal (AEM) selenide IR NLO material, DCL-MgGa2Se4 with excellent optical properties. The introduction of rigid and alkaline-earth metal MgSe4 tetrahedra effectively broadens the band gap of DCL IR NLO materials to 2.96 eV, generating a high laser damage threshold of 3 × AgGaS2 at 1064 nm. Furthermore, DCL-MgGa2Se4 displays a suitable SHG response (0.9 × AgGaS2) with a type I phase-matching behavior. The results indicate that DCL-MgGa2Se4 is a promising mid- and far-IR NLO material for the high-power laser application and provide an insight into the design of new DCL compound with outstanding IR NLO performances. The paper was published in Adv. Sci. with the title of  The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material”. This work was financially supported by National Youth Talent Program of China, National Natural Science Foundation of China, and Xinjiang Key Research and Development Program.

　　Article: https://doi.org/10.1002/advs.202106120

Figure 1. (a) Crystal structures and the corresponding E_hull of each phase of of MGSe (I - VI) and (b) phonon dispersion spectrum of MGSe-I.

Figure 2. (a) Asymmetric unit in DCL-MgGa₂Se₄; (b) Isolated [MgSe₄] tetrahedra in bc plane; (c) [Ga1Se₄] tetrahedra in bc plane; (d) [Ga2Se₄] tetrahedra in bc plane; (e) [Ga₂Se₇]^8- anionic framework constructured by corner-sharing [Ga1Se₄] and [Ga2Se₄]; (f) The 3D structure of DCL-MgGa₂Se₄.

Figure 3. (a) SHG intensity vs particle sizes with AGS as the references at 2.09 μm radiation, (b) experimental band gap, (c) optical transmittance spectrum (measured by using single crystal) of DCL-MgGa₂Se₄. (d) The experimentally verified PM selenide IR NLO materials (without cationic co-occupation) with E_g ≥ 2.0 eV.