Plasmon–Exciton Interactions in a Core–Shell Geometry: From Enhanced Absorption to Strong Coupling

We present a detailed Mie theory, finite-difference time-domain, and quasi-static study of plasmon–exciton interactions in a spherical core–shell geometry. In particular, we report absorption, scattering, and extinction cross sections of a hybrid core–shell system and identify several important interaction regimes that are determined by the electromagnetic field enhancement and the oscillator strength of electronic excitations. We assign these regimes to enhanced-absorption, exciton-induced transparency and strong coupling, depending on the nature of the observed spectra of the coupled plasmon–exciton resonances. We also show the relevance of performing single-particle absorption or extinction measurements in addition to scattering to validate the interaction regime. Furthermore, at relatively high, yet realistic oscillator strengths we observe emergence of a third mode, which is not predicted by a classical coupled harmonic oscillator model and is attributed to the geometrical resonance of the structure as a whole.