Balanced infrared nonlinear optical performance achieved by modulating the covalency and ionicity distributions in the electron localization function map

Abstract

The nonlinear optical (NLO) efficiency (d_ij) and laser-induced damage threshold (LIDT) of a material are mainly determined by their covalency and ionicity, respectively, the incompatibility between which makes balancing the d_ij and LIDT challenging in an IR NLO material. The topological feature (fractal dimension) of the electron localization function (ELF) map (distribution of covalency and ionicity) was evaluated for a series of NLO materials, and, phenomenologically, the fine mixing of covalency and ionicity will benefit a balanced d_ij and LIDT. Chemical bonds with different interaction strengths were introduced simultaneously to mix the covalency and iconicity finely, and three new IR NLO sulfides, A₂Ba₃Li₆Ga₂₈S₄₉ (A = K, 1; Rb, 2; Cs, 3), were obtained. They exhibit a strong NLO efficiency (1.9–2.1 × AgGaS₂ at 1064 nm and 0.5–0.6 × AgGaS₂ at 1910 nm) and high LIDTs (16.7–18.0 × AgGaS₂), which fulfill the criteria of being promising IR NLO candidates. This study provides a new method for designing high-performance IR NLO materials based on the topological features of the ELF.