Room Temperature Plexcitonic Optical Chirality Based on Spontaneous Symmetry Breaking Plasmonic Nanoparticles

Abstract The Plasmon‐exciton strong coupling systems have attracted growing interests due to their powerful ability in manipulating the atomic energy levels, generating coherent quantum eigenstates, and offering promising applications in quantum communications, catalysis and beyond. Chirality divides plasmon into two intriguing states that enable lots of new physical effects and applications. However, the strong coupling between chiral plasmon and excitons remains largely unexplored. This study demonstrates the room‐temperature strong coupling from one chiral plasmon‐exciton system composed of J‐aggregates and nominally‐achiral gold nanocubes (AuNCs). Through establishing the single‐particle polarization scattering measurement system, this work observes distinct Rabi splitting and anti‐crossing behavior in both the scattering and g‐factor spectra, which are verified by the numerical simulations with the half‐hollow Gaussian beam excitations. The in‐depth multi‐dipole theory analysis further reveals that the im‐perfect geometry of AuNCs induces crossed in‐plane dipoles and their coherent superposition results in strong chiroptical effect, while the strong‐coupling changes the dipole strength and phase, and leads to a Rabi‐like splitting in g‐factor spectra. Furthermore, this work develops a quasi‐static multipole theory to better understand the chiral plexcitonic behavior. These findings provide deeper insights into the plexcitonic optical chirality, and will advance the development of next‐generation chiral optical devices.