Realizing Excellent Infrared Nonlinear Optical Performance in Eu-Based Chalcogenides via Rational Cross Substitution Strategy

In recent years, rare-earth-based chalcogenides have gained attention promising materials in the field of infrared nonlinear optical (IR-NLO) applications owing to their exceptional physicochemical properties. However, they frequently encounter challenges such as adverse two-photon absorption and low laser-induced damage thresholds (LIDTs) caused by narrow optical band gaps (<i>E</i>_g), which limit their practical utility. In this study, we started with the centrosymmetric (CS) parent compound EuGa₂S₄ to develop two new noncentrosymmetric (NCS) Eu-based chalcogenides, namely, EuZnSiS₄ and EuCdSiS₄, employing a rational cross-substitution strategy. Despite having identical stoichiometry, both compounds crystallize in distinct NCS orthorhombic space groups [<i>Fdd</i>2 (no. 43) <i>vs Ama</i>2 (no. 40)], as confirmed by single-crystal structure analysis. Their crystal structures feature highly distorted tetrahedral motifs interconnected via corner-sharing, forming unique two-dimensional layers that host Eu²⁺ cations. Furthermore, both compounds exhibit robust phase-matching second-harmonic generation (SHG) intensities of 1.5 × AgGaS₂ for EuZnSiS₄ and 2.8 × AgGaS₂ for EuCdSiS₄ under 2050 nm excitation. They also demonstrate high LIDTs (approximately 14-17 × AgGaS₂), wide <i>E</i>_g (>2.5 eV), and transparency windows extending up to 18.2 μm. Particularly noteworthy, EuCdSiS₄ stands out as a pioneering example in the Eu-based IR-NLO system for successfully combining a broad E_g (>2.56 eV, equivalent to that of AgGaS₂) with a significant SHG effect (>1.0 × AgGaS₂) simultaneously. Structural analyses and theoretical insights underscore that the reasonable combination of asymmetric functional units plays a pivotal role in driving the CS-to-NCS structural transformation and enhancing the NLO and linear optical properties of these Eu-based chalcogenides. This study presents a promising chemical pathway for advancing rare-earth-based functional materials and suggests exciting opportunities for their future applications in IR-NLO technologies.