Infrared nonlinear optical crystals (NLOCs), as key devices for laser frequency conversion, have important applications in all-solid-state lasers. Current commercial IR nonlinear optical crystals mainly include chalcopyrite-type compounds composed of tetrahedral groups, such as AgGaS2 (AGS), AgGaSe2 and ZnGeP2 (ZGP). However, due to their respective intrinsic performance defects, these materials can no longer fully satisfy the demands of the current infrared laser technology development. Therefore, there is an urgent need for the design of new infrared nonlinear optical materials based on new motifs or new strategies to break through the limitations of the existing material properties and obtain new high performance infrared nonlinear optical materials with novel structures.
The Crystal Materials Research Center of Xinjiang Institute of Physical and Chemical Technology, Chinese Academy of Sciences has been devoted to the research of new optoelectronic functional crystals. Previous studies have shown that mercury has a unique electronic configuration, which is conducive to the formation of highly polarized Hg2+ ions, resulting in a significant nonlinear optical response. Meanwhile, Hg has abundant coordination forms and can form linear [HgSe2], planar [HgSe3] and triangular-conical [HgQ4] (Q = S, Se) nonlinearly active radicals in the process of bonding with sulfur elements. Due to the limitation of Bowling's fifth rule (saving rule), most of the pre-synthesized Hg-based sulfur compounds contain only a single nonlinear reactive motif, and the compounds composed of [HgQ4] tetrahedra are predominant, which limits the chemical and structural diversity of the Hg-based compounds. On the basis of previous research, the research team led by researcher Pan Shilie and researcher Li Junjie at the Crystal Materials Research Center of Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences, proposed a "three-in-one" design strategy in the system of Hg-based sulfur compounds, i.e., three kinds of non-linearly active but different polarizability anisotropies of ［HgQn] (n = 2, 3, 4) groups in an attempt to break the limitation of the "saving rule" in a single compound, and synthesized the first mercury-based infrared nonlinear optical material Hg7P2Se12 (HPSe), which contains [HgSe2], [HgSe3], and [HgSe4] at the same time. The compound exhibits a large second-order octave response (∼1 × AGS) under a two-micrometer light source, and the crystal has a wide infrared cutoff (∼22.8 μm) and a high laser damage threshold (∼2 × AGS), suggesting that breaking the "rule of conservation" and increasing the diversity of groups is an effective strategy for design. This suggests that breaking the "saving rule" and increasing the diversity of groups is an effective strategy for designing new infrared nonlinear optical materials with novel structure and excellent performance. This result will inspire researchers to explore more new infrared nonlinear optical materials with excellent comprehensive performance.
The research results were published in full text in Advanced Functional Materials by Wiley, with Xinjiang Institute of Physics and Chemical Technology (XICT) as the sole completion unit, Shiliu Pan and Junjie Li as the corresponding authors, and postdoctoral fellow Yu Chu and doctoral student Hongshan Wang as the co-first authors. The research work was funded by the Talent Program of Chinese Academy of Sciences, National Natural Science Foundation of China and Natural Science Foundation of Xinjiang Autonomous Region.
Figure 1 Structure and optical properties of HPSe crystals. (a) XRD patterns of HPSe single crystals (the inserted figure is an optical photo of HPSe); (b)?IR transmission patterns of HPSe, AGS, and ZGP single crystals (the inserted figure is an optical photo of HPSe, AGS, and ZGP); (c) powder doubling effect of HPSe samples; (d) UV transmission spectra of HPSe single crystals; and (e) typical selenide IR nonlinear Statistical analysis of the optical transmission range of optical materials. where blue color represents the high transmittance region.
