Alkali metal partial substitution-induced improved second-harmonic generation and enhanced laser-induced damage threshold for Ag-based sulfides

Abstract

Nonlinear optical (NLO) materials in the middle infrared (IR) region play an important role in IR laser output. However, their applications are restricted by the imbalance between the large second-harmonic generation (SHG) responses and high laser-induced damage thresholds (LIDTs) for the most successful commercial ones. Aiming at this issue, two new pentanary chalcogenides, LiAgIn₂GeS₆ (1) and NaAgIn₂GeS₆ (2), are derived from the parent Ag₂In₂GeS₆ (0) via Li or Na partial substitution of the Ag element. Their structures can be described as the InS₄ and GeS₄ tetrahedra built [(In₂GeS₆)²⁻]_n chains, which are further linked by S(1) and S(2) atoms to form three-dimensional polyanionic frameworks, providing cavities occupied by the cations. 0–2 exhibit phase-matchable SHG effects of 0.5/0.8/1.2 × AgGaS₂ and high LIDTs of 2.0/4.3/2.7 × AgGaS₂, respectively. Both SHG efficiencies and LIDTs enhanced concurrently after the introduction of the alkali metal Li or Na into the structures of 0, realizing a perfect balance between these two most important NLO properties. The DFT calculation results suggest that the Li or Na partial substitution of Ag is beneficial for enlarging the band gaps and LIDTs. The dipole moment analysis reveals the reasons for the improvement in NLO responses. The chemical partial cation substitution strategy in this study indicates an effective route in the search for new promising NLO materials.