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 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 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 alkali and alkaline-earth diamond-like (DL) infrared NLO material Li₄MgGe₂S₇ with excellent opical properties. The introduction of rigid alkali metal and alkaline-earth metal LiS₄ and MgS₄ tetrahedra effectively broadens the band gap of DL compound to 4.12 eV (the largest one in the reported quaternary metal chalcogenides), generating a high laser damage threshold of 7 × AgGaS₂ at 1064 nm. Furthermore, Li₄MgGe₂S₇ displays a suitable SHG response (0.7 × AgGaS₂) with a type I phase-matching behavior. The results indicate that Li₄MgGe₂S₇ is a promising IR NLO material for the high-power laser application and provide an insight into the design of new DL compound with outstanding IR NLO performances. The paper was published in Angew. Chem. Int. Ed. with the title of “Li₄MgGe₂S₇: The First Alkali and Alkaline-earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap”. What’s more, based on a flexible and rigid tetrahedral motifs combined strategy, a defective DL chalcogenide Hg₃P₂S₈ (HPS) was rationally designed and fabricated. exhibits a phase-matching SHG response of ~ 3.6 × AGS (the largest one in the reported ternary sulfides), high LIDT (~ 3 × AGS), wide IR transparent region (0.43 - 16.3 μm), moderate birefringence (0.043 at 1064 nm), good physicochemical stability and crystal growth habits, which could be a promising NLO candidate for high-power, high-efficiency laser output in the mid-IR region. The paper was published in Chem. Mater. with the title of “Hg₃P₂S₈: A New Promising Infrared Nonlinear Optical Material with a Large Second-Harmonic Generation and a High Laser-Induced Damage Threshold”. These works were financially supported by CAS President’s Talent Program, National Natural Science Foundation of China, and Xinjiang Key Research and Development Program.

Links: https://doi.org/10.1002/anie.202107613;