Infrared (IR) nonlinear optical (NLO) material is one of the core device of all-solid-state lasers, which show wide applications in long distance laser communication, environmental monitoring and photonic technologies, etc. Currently, the commercially available IR NLO materials are mainly composed of chalcopyrite-like (CL) compounds, like AgGaS₂ (AGS), AgGaSe₂ (AGSe), and ZnGeP₂ (ZGP), which are built by tetrahedral structure units. Nevertheless, the small band gaps (E_g) induced low LIDT and two-photon absorption (TPA) in these materials have limited their further applications in current laser techniques. Hence, break through the property limitations of CL compounds, developing high performance IR NLO materials based on new structural motifs or new strategies becomes an urgent need.

Due to the unique electronic configuration, Hg element is conducive to produce highly polarized Hg²⁺ ions, resulting significant NLO effects. Meanwhile, since the 6s and 6p orbital energies in Hg are close to each other, the Hg atom displays abundant coordination modes with Se atoms, like 2-coordinated linear [HgSe₂], 3-coordinated triangular [HgSe₃], and 4-coordinated tetrahedral [HgSe₄] motifs. Hence, Hg-containing chalcogenide has been regarded as one of the most promising systems for the development of high performance IR NLO materials. Over the past decades, many Hg-based IR NLO materials have been developed, and most of them are composed of [HgSe₄] tetrahedral units. Similar to (BS₃), both linear [HgSe₂] and triangular [HgSe₃] are regarded as emergent NLO-active units, and have received considerable attention recently. Nevertheless, successful cases for the development of novel IR NLO materials based on the unique linear and planar NLO units are quite rare due to the experimental challenge.

Recently, a research group led by Prof. Shilie Pan and Prof. Junjie Li at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, designed and fabricated a new Hg-based selenide Hg₇P₂Se₁₂ (HPSe) by breaking through the Pauling's fifth rule (or parsimony rule). In this compound, three types of NLO-active units, including linear [HgSe₂], triangular [HgSe₃], and tetrahedral (MSe₄) (M= Hg/P) groups, simultaneously exist. It shows a wide transparency range (~0.84–22.8 μm), large SHG response (~1 × AGS), high laser-induced damage threshold (~2 × AGS), and a large birefringence (~0.102@2090 nm). Moreover, the compound melts congruently along with good crystal growth habits, and single crystals with a maximum size of ~7 × 5 × 2 mm³ are obtained. Theoretical analyses confirm that the large NLO response originates from the synergistic effects of Hg–Se units, (PSe₄) and nonbonding electrons. The results indicate that HPSe is a promising mid-and far-IR NLO material, and inspire a path to finding new classes of IR NLO materials that are different from traditional chalcopyrite materials by grouping the linear, planar, and tetrahedral units.

The paper was published in Advanced Functional Materials with the title: “Three-in-One”: A New Hg-Based Selenide Hg₇P₂Se₁₂ Exhibiting Wide Infrared Transparency Range and Strong Nonlinear Optical Effect. 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. 

Links: https://doi.org/10.1002/adfm.202314933

Figure 1. a) XRD pattern of well-polished HPSe single crystal (Inserts showing the optical images of grown HPSe crystals); b) Mid to far-IR transmittance spectrum of HPSe crystal, and AGS and ZGP used as the references (Inserts showing the optical images of utilized AGS, ZGP, and HPSe single crystals); c) Particle size versus SHG intensity curves for HPSe and AGS at 2.09 μmradiation; d) UV？vis？NIR transmittance spectrum of HPSe crystal; e) Statistical analysis showing the IR transmission ranges of typical selenide IR NLO crystals (Blue: the high optical transparent regions; grey: the transparent regions with evident optical absorptions).