Optical Spectra of Plasmon-Exciton Core-Shell Nanoparticles: A Heuristic Quantum Approach

Light-matter coupling in plasmonic nanocavitieshas beenwidely studied in the past years. Yet, for core-shell particles,popular electromagnetic models that use the classical Lorentz oscillatorto describe the shell predict extinction spectra with three maxima,if the plasmon and the shell absorption are in resonance. In contrast,experiments exhibit only two peaks, as also expected from simple quantummodels of hybrid states. In order to reconcile the convenient andwidely used classical electromagnetic description with experimentaldata, we connect it to the quantum world by conceiving a heuristicquantum model. Our model is based on the permittivity of a two-levelsystem in a classical electric field derived from the optical Blochequations. The light-matter coupling is included via the collectivevacuum Rabi frequency & omega;(0). Using our model, we obtainexcellent agreement with a series of experimental extinction spectraof particles with various coupling strengths due to a systematic sizevariation. The suppression of the third maximum, which mainly stemsfrom the absorption in the shell, can be interpreted as a vacuum inducedpower broadening, which may occur in lossy (plasmonic) cavities belowthe strong-coupling regime.