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 × KH2PO4 (KDP)), 2 exhibits a greatly improved NLO performance,
including a strong SHG signal (2.5 × 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 × 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.