Strong Plasmon–Exciton Coupling Tuned by Corner Etching of Gold Nanocubes and Nanotriangles

ABSTRACT Strong plasmon–exciton coupling occurs when a quantum emitter coherently interacts with the confined field of a plasmonic cavity, forming hybrid polariton states. Here, we investigate how nanoparticle curvature affects this strong coupling by assembling gold nanocubes (AuNCs) and nanotriangles (AuNTs) with cyanine dye J‐aggregates. Controlled cysteamine etching systematically modifies nanoparticle curvature, enabling direct evaluation of morphology‐dependent coupling. Spectral analyses yield vacuum Rabi splittings of 115 meV for AuNCs and 189 meV for AuNTs. As corners round, coupling strength decreases for AuNCs but remains unaffected for AuNTs, mirroring their plasmonic electric field distributions. Comparing coupling strengths with damping rates reveals that AuNCs operate in the intermediate coupling regime due to large plasmon damping, while AuNTs robustly achieve deep strong coupling across all curvatures. Evaluations of mode volume, quality factor, and figure of merit (FOM) further establish AuNTs as superior cavities. While the FOMs for both geometries are relatively insensitive to local curvature, AuNTs consistently exhibit FOMs nearly twice those of AuNCs. These results demonstrate that overall particle shape, rather than corner sharpness, dictates plasmon–exciton strong‐coupling efficiency. This study advances the design of plasmonic cavities for enhanced light–matter interactions by identifying key structural and optical parameters.