Phase matching achieved by isomorphous substitution in IR nonlinear optical material Ba2SnSSi2O7 with an undiscovered [SnO4S] functional motif

Abstract

In order to enable nonlinear optical (NLO) crystals to output laser power effectively in devices, phase matching (PM) characteristics are very important. However, as a formidable challenge, the interrelationship between macroscopic phase matchability and microscopic crystal structure is still not well investigated. In this study, a PM fresnoite oxychalcogenide Ba₂SnSSi₂O₇ was successfully designed and synthesized via isomorphous substitution of non-phase matching (NPM) parent Ba₂TiOSi₂O₇. Moreover, Ba₂SnSSi₂O₇ exhibits excellent comprehensive properties in IR-NLO oxychalcogenides, which is the first example that breaks through the incompatibility between a large band gap (E_g > 4.0 eV) and a strong SHG efficiency (d_eff > 0.5 × AgGaS₂). In addition, the hetero-ligand [SnO₄S] asymmetric building unit in the structure is an undiscovered bifunctional NLO-active group, which is conducive to the development of novel NLO oxychalcogenides with enhanced d_eff and enlarged birefringence (Δn). This study not only enriches the fresnoite-type material library but also provides a new paradigm for designing high-performance PM NLO materials.