Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS₄. With an NLO effect as large as that in AgGaS₂, KYGeS₄ breaks through the limitation of energy band gap, i.e., the “3.0 eV wall”, in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS₄ is ascribed to the decreased covalency of Y–S bonds by transferring charge from [YS₇] to [GeS₄] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.

Inspired by the oxide SrB₄O₇with the largest known band gap among all borate crystals, the chalcogenide BaAl₄S₇with similar structure may possess attractive NLO properties.

The new nonlinear optical material SrZnSnS₄ is a two-dimensional layer structure with a large band gap (3.37 eV). The orderly arrangement of [ZnS₄]⁶⁻ and [SnS₄]⁴⁻ groups mainly contribute to the second harmonic generation effect (∼LiGaS₂).

An infrared (IR) nonlinear optical (NLO) material, Ag₃Ga₃SiSe₈, has been synthesized for the first time. It crystallizes in the tetragonal chalcopyrite AgGaSe₂ structure type and the substitution of Ga³⁺ with Si⁴⁺ generates an equal amount of vacancies at the Ag⁺ position. Ag₃Ga₃SiSe₈ is a congruent melting compound with a melting temperature of 784 °C. The second-harmonic generation (SHG) effect of Ag₃Ga₃SiSe₈ is similar to that of AgGaSe₂, as confirmed by the first-principles studies. Moreover, the diffuse reflectance spectra reveal that the energy band gap of Ag₃Ga₃SiSe is 2.30 eV, 0.44 eV larger compared with AgGaSe₂ (1.86 eV). The birefringence of Ag₃Ga₃SiSe₈ is predicted to be above 0.1 at the wavelength of 1 μm, significantly larger than that of AgGaSe₂ (Δn ~ 0.02). Our results indicate that Ag₃Ga₃SiSe₈ may possess improved optical properties compared with AgGaSe₂ and has a promising application as a good IR NLO material.

The new nonlinear optical material Sr₃MnSn₂S₈ is a noncentrosymmetric structure with a large band gap of 3.02 eV. In addition, Sr₃MnSn₂S₈ possesses a good second harmonic generation (SHG) response, approximately equivalent to that of AgGaS₂, and a high laser damage threshold (LDT) about 3 times that of AgGaS₂ (AGS).