Phase‐ and Interface‐Engineered Organic DAST Nanowires for Strong Second‐Harmonic Generation

Efficient and tunable nonlinear optical responses in organic nanocrystals are highly desirable for on‐chip nanophotonics. However, synergistically engineering their crystallographic phases and localized light‐matter interactions remains a formidable challenge. Here, we demonstrate a solvent‐regulated self‐assembly strategy for phase‐selective growth of high‐quality 4‐N,N‐dimethylamino‐4′‐N′‐methyl‐stilbazolium tosylate (DAST) nanowires (NWs), enabling precise control over crystal symmetry to tailor second‐order nonlinear optical responses. A thermally induced phase transition converts the initially SHG‐inactive hydrated DAST NWs into an SHG‐active anhydrous phase. To further boost conversion efficiency, an interface‐engineering strategy is implemented by integrating a single DAST nanowire with an Au film. The high surface‐to‐volume ratio of one‐dimensional nanowires facilitates highly efficient interfacial interactions and strain transfer from the substrate. This hybrid system reveals synergistic coupling of localized electric field amplification, tensile‐strain‐enhanced intrinsic χ (2) , and maximized SHG collection efficiency. Consequently, it yields a remarkable 80‐fold SHG enhancement relative to a Si substrate, delivering an SHG efficiency of ∼2.6 × 10 −4 W −1 and an effective nonlinear coefficient of 2028 pmV −1 , outperforming inorganic counterparts by over an order of magnitude. Moreover, the interface‐engineering intrinsically reconstructs SHG polarization symmetry. This work establishes a growth‐, phase‐, and interface‐engineered organic nanocrystal platform for high‐performance nonlinear nanophotonics and on‐chip integrated optoelectronic devices.