Abstract
Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (Eg ≥ 3.0 eV) and large NLO effect (dij ≥ 0.5 × AgGaS2) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa3S6O satisfy the property-balance demand (Eg: 3.21-3.27 eV and dij: 0.9-1.0 × AgGaS2) as promising PM NLO crystals through gathering their property advantages between LaMGa3O7 and LaMGa3S7 by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S7O and GaS3O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.
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