  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 novel 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 fnd 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
signifcant decrease of the field strength needed to saturate the system. In the
linear regime, the absortion 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. Upon
increasing the applied field intensity to the nonlinear regime, the Fano-like
singularities in the absorption spectra are smeared and they disappear due to
the 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.
