Boosting SHG in InP Nanomembranes: The Role of the Wurtzite‐Zincblende Polytypic Interface

III–V semiconductors, particularly InP, have demonstrated significant potential for seamless integration into nanophotonic systems. However, conventional zincblende InP crystals exhibit inherently weak second‐harmonic generation (SHG) due to limited second‐order nonlinear susceptibility, presenting a major challenge for developing effective nonlinear optical components in integrated photonic circuits. Here, it is demonstrated that large‐area biphasic wurtzite‐zincblende (WZ–ZB) interfaces can remarkably enhance SHG in InP nanomembranes, yielding up to a 200‐fold intensity increase in the short‐wave infrared range. This enhancement stems from the type‐II band alignment at the polytypic interface, which spatially separates photogenerated carriers, prolonging carrier lifetimes and sustaining a strong nonlinear polarization source. The abrupt phase boundary and the nanomembrane's large surface‐to‐volume ratio introduce additional symmetry‐breaking at the interface, which further amplifies the SHG signal. The polarization dependence of SHG and the resulting SHG‐induced photoluminescence exhibit anisotropic features. The results establish crystalline phase engineering in WZ–ZB InP homostructures as a highly effective strategy for tailoring nonlinear optical processes, offering new pathways for high‐efficiency wavelength conversion and integrated on‐chip nonlinear photonic circuits.