Plasmon–Exciton\nStrong Coupling in Colloidal\nAu Nanocubes with Layered Molecular J‑Aggregates

Strong coupling between light and matter forms hybrid\nstates, such\nas exciton–polaritons, which are crucial for advancements in\nquantum science and technology. Plasmonic metal nanoparticles, with\ntheir ultrasmall mode volumes, are effective for generating these\nstates, but the coupling strength is often limited by surface saturation\nof excitonic materials. Additionally, cubic nanoparticles, which can\ngenerate strong local fields, have not been systematically explored.\nThis study investigates strong coupling in Au nanocubes (AuNCs) coupled\nwith J-aggregates, observing spectral splitting in both extinction\nand scattering spectra. Our findings suggest that smaller AuNCs, with\nhigher-quality resonances and reduced mode volumes, achieve stronger\ncoupling. Furthermore, a layer-by-layer (LBL) coating of J-aggregates\non AuNCs results in a ∼21% increase in coupling strength. Simulations\nreveal the mechanism behind the enhanced coupling and confirm that\nthe layering method effectively increases coupling, surpassing the\nlimitations of the finite surface area of nanoparticles.