  When a two-level quantum dot and a plasmonic metal nanoantenna are resonantly
coupled by the electromagnetic near field, the system can exhibit a Fano
resonance, resulting in a transparency dip in the optical spectrum of the
coupled system. We calculate the nonlinear response of such a system, for
illumination both by continuous-wave and ultrafast pulsed lasers, using both a
cavity quantum electrodynamics and a semiclassical coupled-oscillator model.
For the experimentally relevant case of meV thermal broadening of the
quantum-dot transition, we predict that femtosecond pulsed illumination can
lead to a reversal of the Fano resonance, with the induced transparency
changing into a superscattering spike in the spectrum. This ultrafast reversal
is due to a transient change in the phase relationship between the dipoles of
the plasmon and the quantum dot. It thus represents a new approach to
dynamically control the collective optical properties and coherence of coupled
nanoparticle systems.
