Alkali metal/mercury dual-metal sulfides as infrared nonlinear optical materials with high laser-induced damage thresholds

Two new mercury-based sulfides, Rb₂HgGe₃S₈ and Cs₂HgGe₃S₈, were successfully synthesized by the high-temperature solid-state reaction, and they are isostructural to crystallize in the non-centrosymmetric orthorhombic space group <i>P</i>2₁2₁2₁. The results of the experiments indicate that Rb₂HgGe₃S₈ has a large band gap of 3.26 eV, a second harmonic generation (SHG) response of 0.17 × AgGaS₂ (AGS) at the particle size of 90-125 μm, and non-phase-matchable (NPM) properties. Additionally, Cs₂HgGe₃S₈ exhibits a SHG response of 0.13 × AGS (particle size of 90-125 μm) as well as type-I phase-matchable (PM) performance at 2090 nm. Moreover, Rb₂HgGe₃S₈ and Cs₂HgGe₃S₈ exhibit high laser-induced damage thresholds of 3.1 and 2.7 × AGS, respectively. Theoretical calculations reveal that Rb₂HgGe₃S₈ and Cs₂HgGe₃S₈ are two direct semiconductors with the calculated band gap of 2.32 and 2.15 eV and the synergistic action of [HgS₄] and [GeS₄] units realizes their SHG response.