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 (E_g) 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 balanced band gap and 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 and fabricated a series of new chalcogenides in A^IB^II₃C^III₃Q^VI₈ family with unprecedented windmill-like [Mg₃M^III₃Q₂₄] (M^III = Al, Ga; Q = S, Se) units constructed by alternated [MgQ₆] octahedra and [M^IIIQ₄] tetrahedra, by coupling AEM octahedra with large HOMO-LUMO gap and NLO-active tetrahedra units. The compounds show stable structural framework and adjustable optical properties. Among them, NaMg₃Ga₃Se₈ shows a balanced large SHG response (~ 1 ×AGS), and wide band gap in selenide (2.77 eV), suitable birefringence (0.079@546 nm), and high LIDT (~ 2.3 ×AGS), which could be a potential candidate for IR NLO materials. The results enrich the chemical diversity of chalcogenides, and open an avenue for the design and fabrication of new IR NLO materials based on the octahedral and tetrahedral coupled strategy.

The paper was published in Journal of the American Chemical Society (J. Am. Chem. Soc., 2022, 144, 21916-21925）.

Link：https://pubs.acs.org/doi/10.1021/jacs.2c08318

Figure 1 (a) The coordination environments of Ag, Mg and Ga atoms, and the structures of [GaS₄]_∞, [Ga₂S₇]_∞, [MgS₆]_∞ and [Mg₂S₉]_∞ chains; (b) The isolate Ga-S chains viewed along c direction; (c, e) The formed [Mg₃Ga₃S₂₄] units composed by [MgQ₆] octahedra and [M^IIIQ₄] tetrahedra; (d) The 3D structure of AgMg₃Ga₃S₈; (f) The [Ga₆S₁₈] units in AGS; (g)The 3D structure of AGS

Figure 2 (a) SHG intensity versus particle sizes with AGS as the references at 2.09 μm radiation; (b) Experimental band gap; (c) The experimental and calculated birefringence for NaMg₃Ga₃Se₈ crystal; (d) The promising selenide IR NLO materials with E_g > 2.3 eV and SHG ≥ 1 × AGS. AGS and AGSe used as the references