Achieving broadband ultraviolet to mid-infrared transparency in germanate-based nonlinear optical crystals Cs₃REGe₃O₉ (RE = Y, Gd)

Nonlinear optical (NLO) materials currently attract significant interest and hold paramount importance in the optoelectronic field. Nevertheless, the fabrication of NLO crystals capable of transmitting ultraviolet to mid-infrared wavelengths remains a formidable challenge. Herein, we synthesized two novel NLO crystals, namely Cs₃REGe₃O₉ (RE = Y, Gd), by employing a structural gene strategy that harmoniously merges flexible [YO₆] and [GdO₆] octahedra with rigid [GeO₄] motifs. Remarkably, Cs₃REGe₃O₉ (RE = Y, Gd) crystals exhibit substantial band gaps of 4.92 and 5.3 eV, respectively, while achieving the shortest reported ultraviolet cutoff edges at 210 and 215 nm among the boron-free germanate-based NLO materials. Importantly, these compounds also feature a remarkably broad infrared transparency range exceeding 6.8 μm. Furthermore, both compounds demonstrate a moderate second-harmonic-generation (SHG) response, comparable to that observed in KDP. Additionally, first-principles calculations unveil that the synergistic effect of the [REO₆] octahedra and [GeO₄] tetrahedra plays a pivotal role in governing the optical properties of Cs₃REGe₃O₉ (RE = Y, Gd). These explored findings offer a novel avenue for leveraging the highly desired NLO materials in the region spanning from ultraviolet to mid-infrared wavelengths.