文摘
Surface plasmon polaritons (SPPs) are short-lived evanescent waves that can confine light at the surface of metallic nanostructures and transport energy over mesoscopic distances. They may be used to generate and process information encoded as optical signals to realize nanometer-scale and ultrafast all-optical circuitry. The propagation properties of these SPPs are defined by the geometry and composition of the nanostructure. Due to their short, femtosecond lifetimes, it has so far proven difficult to track this propagation in the time domain and to directly study the effect of the propagation on the shape of a coherent SPP wavepacket. Here, we introduce an ultrabroadband far-field spectral interferometry method, allowing for the reconstruction of the plasmonic field in the time domain, to characterize coherent SPP propagation in metallic nanostructures. Group velocity and dispersion of SPPs are determined with high precision in a broad frequency range in the visible and near-infrared region, and the propagating SPP field is tracked with high time resolution over distances of tens of micrometers. Our results shed new light on the interplay between nanostructure geometry and coherent SPP propagation and hence are important for probing plasmon–matter interactions as well as for implementations of ultrafast plasmonic circuitry.