One-Side Capping in Two-Dimensional WO₃-Type Materials Leading to Strong Second-Harmonic Response

Nonlinear optical (NLO) crystals are of great importance in laser and photonic technologies. However, it remains a huge challenge to rationally design a NLO crystal with a giant second-harmonic generation (SHG) response due to the unpredictability of inorganic structures. Herein, we report two new non-centrosymmetric molybdenyl iodates, Rb(MoO2)2O(IO4) (RMIO) and Cs(MoO2)2O(IO4) (CMIO), derived from the novel one-side capping of tetragonal MoO3 layers by the group grafting method. The property measurements show that they can exhibit the largest SHG effects [32 and 30 × KH2PO4 (KDP) at 1064 nm and 1.6 and 1.3 × AgGaS2 (AGS) at 2090 nm] among all the reported compounds with two-dimensional WO3-type layers. Systematic studies indicate that well-aligned arrangement and high density of IO4 units on one side of the molybdenum layer have the main contribution to their giant SHG responses. This work will facilitate the development of NLO materials with giant SHG responses.