Ultrafast reversal of a Fano resonance in a plasmon-exciton system

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 thermal broadening of the quantum-dot transition (to meV values consistent with $\sim$100 K), 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 exciton. It thus represents a new approach to dynamically control the collective optical properties and coherence of coupled nanoparticle systems.

Figure 1

Linear absorption spectrum of an Au-CdSe-Au hybrid nanoparticle system (illustrated in the inset). Absorption spectra, ${\sigma }_{\mathrm{abs}}$, are calculated using the discrete dipole approximation (DDA) and the cavity-quantum-electrodynamics (CQED) and semiclassical (SC) models.

Figure 2

(a) Steady-state absorption spectra calculated using the CQED (dots) and corrected SC (solid) model for various incident intensities. Successive spectra are displaced vertically by $2\times {10}^{-11}{\text{cm}}^2$. (b) Intensity dependence of steady-state absorption cross sections calculated using the CQED, naïve SC, and corrected SC steady-state models, at a photon energy of 2.042 eV. Arrows indicate the intensities at which spectra are plotted in panel (a).

Figure 3

(a) Absorption spectra calculated using the CQED (dots) and corrected SC (solid) models, for ultrafast pulsed excitation. Successive spectra, corresponding to varying fluences, are displaced vertically by $2\times {10}^{-11}{\text{cm}}^2$. (b) Fluence dependence of the absorption cross section for ultrafast pulsed excitation, calculated using the CQED and SC models, at a photon energy of 2.042 eV. (c) Data from panel (b) plotted as a function of pulse area. Arrows indicate the fluences at which spectra are plotted in panel (a).

Figure 4

Time evolution of the QD-plasmon system under pulsed excitation, calculated using the CQED model for two fluences. (a), (d) Pulse's electric field and QD population. (b), (e) QD's dipole and phase lag relative to the pulse. (c), (f) Plasmon's dipole and phase lag. Dashed lines indicate times at which a phase jump occurs.