Third‐Order Nonlinear Optical Response‐Driven Upconversion Phosphors

A series of silicate materials with noncentrosymmetric structures, namely, Na3ScSi3O9 (NSSO), NSSO:Ce3+, NSSO:Eu2+, and NSSO:Tb3+, have been successfully synthesized by a microwave‐assisted solid‐state reaction. The materials reveal interesting second‐ and third‐harmonic generation and two‐photon absorption (2PA) properties attributable to the unusual connectivity and unique coordination environments around the cations, as evidenced by detailed structural analysis and maximum entropy method calculations. Upon irradiation with 980 nm laser light, the activator‐doped NSSO materials show an unusual upconversion emission property without using Yb3+ sensitizer ions, because of nonlinear absorption processes, as evidenced from 2PA spectroscopy results. The materials also show good chemical stability compared to NaYF4:Er3+, Yb3+. The findings of this work can stimulate the creation of a new scientific field devoted to upconversion luminescence in solid‐state materials. A two‐photon absorption‐driven upconversion process in NSSO:Ce3+, NSSO:Eu2+, and NSSO:Tb3+ compounds (NSSO = Na3ScSi3O9) are studied by nonlinear spectroscopic techniques to provide new insights into the design of chemically stable upconversion phosphors for bio‐imaging applications without using sensitizer ions.