Abstract text (incl. figure legends and references)

Surface plasmons (SPs) are resonances occurring at interfaces of media with opposite sign of the dielectric functions. They are characterized by strongly enhanced and localized electric fields, which can be exploited in several applications, e.g., Surface Enhanced Raman Spectroscopy or plasmonic waveguides.

To investigate SPs on nanostructures at highest spatial and spectral resolution, electron energy-loss spectroscopy is frequently employed in combination with scanning transmission electron microscopy (so-called plasmon mapping) [1], which provides spatial resolved loss probability maps that reveal the character (dipolar, quadrupolar, etc.) of different SP modes. While the loss probability is proportional to the longitudinal (i.e., parallel to e-beam) field component, transverse field components (e.g., containing magnetic contributions) cannot be probed. To overcome this limitation, momentum transfer by transverse field components to the beam electrons must be measured with spectral and spatial resolution.

Here, one option is to record energy-filtered diffraction patterns in the far field at different probe positions (see lower row in Fig. 1 a) [2]. From the overall intensity in the discs, a spatial resolved loss probability map can be extracted (see Fig. 1 a). The shift of the center of mass of the intensity, on the other hand, corresponds to the transverse momentum transfer to the beam electrons, i.e., to the transverse electromagnetic field (see Fig. 1 b). In this mode, the filtered spectral region is determined by a slit aperture in the energy dispersive plane and is hence limited to ~1eV.

An alternative approach is to acquire ω-q maps with one spectral and one momentum dimension recorded at different probe positions. Here, the sampled direction in q-space is determined by a slit aperture in a diffraction plane. By rotating the sample with respect to the aperture, different directions of momentum transfer can be probed serially (see upper rows in Fig. 2 for two different q-space directions). Integrating the intensity along the q-dimension at each scanning position, leads again to spatially resolved loss probability maps (see lower row in Fig. 2).

Our proof-of-concept experiments show that both setups are suitable to measure transverse electromagnetic field components of SPs. Here, the highest spectral resolution can be obtained by acquiring ω-q maps. Recording energy-filtered diffraction patterns, on the other hand, provides parallel acquisition of the momentum transfer in all q-space directions.

[1] J. Nelayah, et al., Nat. Phys. 3, (2007).

[2] J. Krehl et al., Nat. Comm. 9, (2018).

[3] This project was funded by the European Research Council (ERC) under the Horizon 2020 research and innovation program of the European Union (grant agreement no. 715620).

Fig. 1: Transverse momentum transfer/ electric field components reconstructed from energy-filtered diffraction discs of an Al nanorod. a) Loss probability map (upper image) and diffraction discs (lower row) recorded at probe position 1-4. b) Transverse electric field components derived from the center of mass of the intensity (orange diamonds) in the diffraction discs shown in a).

Fig. 2: Transverse momentum transfer and loss probability maps reconstructed from ω-q mapping of an Al split ring resonator. a) Lateral momentum transfer along two different directions in q-space (indicated by the arrows). b) Corresponding loss probability maps.