Tailoring femtosecond LSP resonance and lifetime in a nanoresonator via phase retardation

The manipulation of femtosecond plasmon resonance and lifetime in a nanoantenna is crucial for the realization of integrated and miniaturized plasmonic circuits. Here, we have used FDTD simulations to study the plasmonic resonance and lifetime variation of the far-end (P_A) and near-end (P_B) hotspots of size-gradient nanoresonators. We found that the near-field spectrum of P_A is red-shifted compared to P_B due to the phase retardation effect. By capturing the ultrafast dynamics of both hotspots, we confirm that these phenomena are governed by the transient evolution of the plasmonic field in the forced oscillation regime. Furthermore, the lifetimes of plasmonic hotspots scale directly with their near-field intensities. Meanwhile, the lifetime τ_A is always larger than that of τ_B in the same nanoresonator because of the smaller non-radiative damping of hotspot P_A. Our results provide a basis for the miniaturization of plasmonic nanoresonators.