Controlling the Orientation‐Dependent Second Harmonic Generation in Hybrid Germanium Perovskite Crystals

Abstract Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface‐dependent anisotropic SHG with the precise design of (101) and (2 0) MAGeI 3 facets. Based on the SHG measurement, the (101) MAGeI 3 single crystal exhibits larger SHG (1.3×(2 0) MAGeI 3 ). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI 3 compared with the (2 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X‐ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature‐dependent steady‐state and time‐resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI 3 single crystal, revealing the higher defect states. Additionally, powder X‐ray diffraction patterns show the (101) MAGeI 3 possesses larger in‐plane polar units [GeI 3 ] − density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (2 0) facet, and the larger built‐in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite‐based nonlinear optical materials.