Tailoring optical response of a hybrid comprising a quantum dimer emitter strongly coupled to a metallic nanoparticle

We study theoretically the optical response of a nanohybrid comprising a symmetric quantum dimer emitter coupled to a metal nanoparticle (MNP). The interactions between the exitonic transitions in the dimer and the plasmons in the MNP lead to interesting effects in the composite's input-output characteristics for the light intensity and the absorption spectrum, which we study in the linear and nonlinear regimes. We find that the exciton-plasmon hybridization leads to optical bistability and hysteresis for the one-exciton transition and enhancement of excitation for the two-exciton transition. The latter leads to a significant decrease in the field strength needed to saturate the system. In the linear regime, the absorption spectrum has a dispersive (Fano-like) line shape. The spectral position and shape of this spectrum depend on the detuning of the dimer's one-exciton resonance relative to the plasmon resonance and the ratio of the exciton-plasmon coupling constant to the exciton dephasing rate. When the applied field intensity to the nonlinear regime is increased, the Fano-like singularities in the absorption spectra are smeared and they disappear due to saturation of the dimer, which leads to the MNP dominating the spectrum. The above effects, for which we provide physical explanations, allow one to tailor the Fano-like shape of the absorption spectrum, by changing either the detuning or the input power.