Electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) is a powerful technique to probe optical and electronic excitations with a sub-nanometer spatial resolution [1, 2]. However, probing spontaneous losses, this technique does not provide time-domain access to ultrafast processes and is restricted in its spectral resolution even for the most advanced electron sources available (10-100 meV).

Here, recent developments in the field of ultrafast transmission electron microscopy (UTEM) promise to overcome these limitations by probing laser-excited optical modes with femtosecond electron pulses. Specifically, in a stroboscopic laser-pump/electron probe scheme, an optically excited sample is measured with photo-emitted ultrashort electron wave-packets [3] and scanning their relative time delay gives access the involved ultrafast dynamics. Such an instrument thus combines the spatial resolution of a conventional TEM (nm) with the unrivaled spectral and temporal resolutions provided by ultrafast lasers (resp. sub-meV and hundreds of femtoseconds), and therefore offers unique capabilities to probe optical fields at the nanoscale.

In this presentation, I will demonstrate how this technique – usually referred to as photon-induced near-field electron microscopy (PINEM) – can be used to analyse the modal structure of the optical response of individual plasmonic nano-resonators (gold and silver nano-triangles) directly at the nanoscale. We will present our boundary element method (BEM)-based data analysis which enable us to extract from optical near-field maps, the magnitude and relative phase of each plasmonic modes excited by a femtosecond pump laser. Thanks to this method, we will analyse the influence of the laser polarization, wavelength and incidence angle on the population of each modes. Particularly, we will show that the total optical near-field excited by the laser, result from the interference pattern of a large number of plasmonic modes which can be controlled by tuning the laser properties. Finally, we will theoretically and experimentally demonstrate that the plasmonic optical near-field can be coherently manipulated by pumping the system with two phase-locked optical pulses of different wavelength, which enables to create a complex beating pattern between two different plasmonic modes in the same single nano-resonator.