Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) have proved during the past few years to be tremendous tools to study surface plasmons in metallic nanoparticles, thanks to an extremely high spatial resolution combined with a broad spectral range. Despite their apparent close resemblance, qualitative differences between EELS and CL have been theoretically as well as experimentally pinpointed. We demonstrate that these differences are recovered when comparing the full electromagnetic local density of states (EMLDOS) and the radiative EMLDOS. Following the known relation established between EELS and the projection along the electron trajectory of the full EMLDOS, we introduce a formalism based on the Maxwell electric Green tensor to draw a link between CL and the projection along the electron trajectory of the radiative EMLDOS. We discuss in simple terms the differences between EELS (projected full EMLDOS) and CL (projected radiative EMLDOS) through modal decompositions obtained in the quasistatic approximation. Contrary to EELS, CL probes only the radiative modes. Furthermore, CL resonant line shapes may be shifted and asymmetric compared to EELS. The CL asymmetry is due to interferences in the far-field radiation from spectrally and spatially overlapping modes. Our analytical expressions are illustrated through boundary element method numerical simulations.

The local enhancement of few-cycle laser pulses by plasmonic nanostructures opens up for spatiotemporal control of optical interactions on a nanometer and few-femtosecond scale. However, spatially resolved characterization of few-cycle plasmon dynamics poses a major challenge due to the extreme length and time scales involved. In this Letter, we experimentally demonstrate local variations in the dynamics during the few strongest cycles of plasmon-enhanced fields within individual rice-shaped silver nanoparticles. This was done using 5.5 fs laser pulses in an interferometric time-resolved photoemission electron microscopy setup. The experiments are supported by finite-difference time-domain simulations of similar silver structures. The observed differences in the field dynamics across a single particle do not reflect differences in plasmon resonance frequency or dephasing time. They instead arise from a combination of retardation effects and the coherent superposition between multiple plasmon modes of the particle, inherent to a few-cycle pulse excitation. The ability to detect and predict local variations in the few-femtosecond time evolution of multimode coherent plasmon excitations in rationally synthesized nanoparticles can be used in the tailoring of nanostructures for ultrafast and nonlinear plasmonics.