Photoluminescence Properties of Strongly Coupled Gold Plasmonic Nanoarray and Molecular Aggregates with Altered Photostability

Abstract Photoluminescence properties are important for the application of exciton‐polaritons. Photoluminescence from lower polariton is usually observed, whereas that from upper polariton is weak. In this study, the emission angle‐resolved photoluminescence properties of the strongly coupled gold plasmonic nanoarray cavities and molecular aggregates of squaraine are investigated. Angle‐resolved photoluminescence shows similar dispersion as that of reflectance, with strong PL from both upper and lower polariton branches. This is attributed to the direct relaxation of high‐energy excited states to upper and lower polariton branches, followed by radiative relaxation. Weak exciton‐vibration coupling and inefficient polariton‐polariton interactions may account for the weak inter‐branch, intra‐branch, and polariton‐exciton reservoir scattering. It is further found that the photobleaching rate of J‐aggregate excitons is suppressed and accelerated under strong coupling for short and long irradiation times, respectively. This is due to the combined effects of strong coupling‐induced polariton wavefunction delocalization and the effect of hot electrons in plasmonic nanoarrays, with the latter accelerating the photobleaching. This study provides insights for a controllable modulation of the photophysical properties of molecular excitons in strong coupling conditions.