Enhancement of Second-Order Optical Nonlinearity in a Lutetium Selenite by Monodentate Anion Partial Substitution

The systematic modification of the molecular structure of nonlinear optical (NLO)-active materials is a very attractive approach to the improvement of NLO performance. In this study, one lutetium selenite compound Lu(SeO3)(HSeO3)(H2O)-(H2O) (1) was synthesized by hydrothermal means, while a further two lutetium selenites Lu3F(SeO3)4 (2) and Lu(SeO3)(NO3)(H2O) (3) were successfully synthesized by anion partial substitution (APS) of the parent structure 1 under hydrothermal conditions. Specifically, substitution of the [HSeO3]− anions in the noncentrosymmetric 1 by F− or [NO3]− anions with differing denticity leads to the formation of polar 2 and centrosymmetric 3. Our study reveals that the denticity of the secondary functional anions has a significant influence on the coordination environments of the rare-earth-metal cation Lu3+ and consequently the molecular configuration and NLO performance of the resultant multidimensional selenites. In contrast to 1, which displays a weak second-harmonic generation (SHG) response (0.1 x KH2PO4 (KDP)), 2 exhibits a greatly improved NLO performance, including a strong SHG signal (2.5 x KDP, the highest value among rare-earth-metal-based NLO selenites), a wide band gap (3.57 eV) and optical transparency window (0.35-10.3 μm), high thermal stability (∼550 °C), and a large laser damage threshold (36 x AgGaS2). These results suggest that 2, as the first example of a fluorinated lutetium selenite, is a strong NLO candidate crystal spanning a region from the near-ultraviolet to the mid-infrared. These APS studies highlight a new feasible approach toward high-performance NLO crystals.This research was financially supported by the National Natural Science Foundation of China (nos. 51432006 and 51172100), the Ministry of Education of China for the Changjiang Innovation Research Team (no. IRT13R24), the Ministry of Education and the State Administration of Foreign Experts Affairs for the 111 Project (no. B13025), the Innovation Program of Shanghai Municipal Education Commission, and the National and Shanghai Postdoctoral Program for Innovative Talents (nos. BX201800216 and 2018192). M.G.H. and C.Z. thank the Australian Research Council for support (DP170100411). The authors thank G.Z. and B.X.L. at FJIRSM for help with the LDT measurements