Plasmon–Exciton Coupling Enhances Second‐Order Nonlinear Response in Borophene–ZnO Hybrid Structures

ABSTRACT Nonlinear optical processes in low‐dimensional materials are often weak or symmetry forbidden, limiting their use in nanoscale light sources and on‐chip frequency conversion. Here, we show that combining two weakly nonlinear systems—anisotropic borophene and excitonic zinc oxide—yields an enhanced and resonant nonlinear response. In borophene–ZnO heterostructures, cathodoluminescence reveals a two‐orders‐of‐magnitude enhancement in the near‐infrared region and at half its wavelength, due to an enhanced nonlinear absorption process. Strikingly, this enhancement exhibits a remarkable in‐plane anisotropy, owing to the hyperbolic optical response of borophene. Under tunable near‐infrared excitation, a clear second‐harmonic signal emerges with quadratic power dependence and strong resonance near 800 nm. We attribute these observations to nonlinear plasmon–exciton coupling, which reshapes the excitonic response and enables efficient hybrid pathways for frequency conversion. These results establish anisotropic plasmon–exciton hybridization as a route to controlling nonlinear optical responses in low‐dimensional heterostructures.