Enhanced Second-Harmonic-Generation Efficiency and Birefringence in Melillite Oxychalcogenides Sr2MGe2OS6 (M = Mn, Zn, and Cd)

Abstract

Simultaneous regulation and control of non-linear second-harmonic-generation (SHG) coefficients (deff) and linear birefringence (Δn) of non-centrosymmetric (NCS) crystals is a crucial aspect of improving the non-linear optical (NLO) performance, yet it remains a huge challenge in modern laser techniques and science. Herein, a series of melilite-type oxide-derived NCS oxychalcogenides, Sr2MGe2OS6 (M = Mn, Zn, and Cd), are successfully designed and synthesized through a facile partial isovalent chemical substitution strategy. These isostructural compounds crystallize in the tetragonal NCS space group P4̅21m (no. 113) and feature unique two-dimensional Cairo pentagonal tiling layers composed of heteroligand [GeOS3] and tetrahedral [MS4] asymmetric building units (ABUs). Compared to the parent melilite-type oxides, the derived title compounds not only successfully realize the phase matchability transformation [i.e., from non-phase-matching (NPM) to phase-matching (PM)] but also greatly improve the IR-NLO performance (especially, simultaneously boosting deff and Δn). Sr2CdGe2OS6 exhibits the best comprehensive performance among oxychalcogenides, including a wide PM cutoff edge (>525 nm), a strong deff (0.8 × AgGaS2), a giant laser-induced damage threshold (19.2 × AgGaS2), a large band gap (3.62 eV), as well as a broad transmission cutoff region (0.28–12.0 μm). Furthermore, highly distorted [GeOS3] ABU in this melilite-type structure is proved to be the profitable bifunctional NLO-active unit, which can contribute to the search and design of novel IR-NLO crystals with a large Δn and a strong deff based on the experimental results and theoretical calculations. This work demonstrates the first examples of PM melilite-type oxychalcogenides and offers new perspectives on the phase matchability transformation through the partial isovalent chemical substitution approach, which will play a constructive role in the future design of high-performance IR-NLO heteroanionic materials.