Probing Coherent Surface Plasmon Polariton Propagation Using Ultrabroadband Spectral Interferometry

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

Suppression of Radiative Damping and Enhancement of Second Harmonic Generation in Bull’s Eye Nanoresonators

We report a drastic increase of the damping time of plasmonic eigenmodes in resonant bull’s eye (BE) nanoresonators to more than 35 fs. This is achieved by tailoring the groove depth of the resonator and by coupling the confined plasmonic field in the aperture to an extended resonator mode such that spatial coherence is preserved over distances of more than 10 μm. Experimentally, this is demonstrated by probing the plasmon dynamics at the field level using broadband spectral interferometry. The nanoresonator allows us to efficiently concentrate the incident field inside the central aperture of the BE and to tailor its local optical nonlinearity by varying the aperture geometry. By replacing the central circular hole with an annular ring structure, we obtain 50-times higher second harmonic generation efficiency, allowing us to demonstrate the efficient concentration of long-lived plasmonic modes inside nanoapertures by interferometric frequency-resolved autocorrelation. Such a light concentration in a nanoresonator with high quality factor has high potential for sensing and coherent control of light-matter interactions on the nanoscale