Giant Mid‐Infrared Second‐Harmonic Generation Response in a Densely‐Stacked Van Der Waals Transition‐Metal Oxychloride

Second‐harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid‐infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO‐active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2Cl2. This exhibits the strongest powder SHG response (2.1 × KTiOPO4 (KTP) @ 2100 nm) for a transition‐metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2Cl2 is the first SHG‐active transition‐metal oxyhalide effective in the IR. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally‐aligned [MoO4Cl2] modules within the two‐dimensional van der Waals layers are responsible for the giant SHG response.