a, Schematic representation of the experimental geometry. Ultrashort 200 keV electron pulses (propagating along the z axis) impinge on a Ag/Si3N4 thin film perforated by a nanofabricated hole in the Ag layer (diameter ~0.8 μm). Light pulses propagate with their wavevector in the y–z plane forming an angle δ = 4.5° with the electron beam direction. The sample can rotate around the x axis by an angle α. An electron energy–momentum analyser allows us to measure the transverse profile of the electron wavefunction in both real and reciprocal space. b, Schematic representation of the non-local holographic method used to image SPPs radially propagating at the Ag/Si3N4 interface away from the hole. c, Experimentally measured spatial distribution of the inelastically scattered electrons following the electron–plasmon interaction. The image is acquired with electrons and light wavepackets in temporal coincidence. The sample is tilted by an angle α = δ to achieve normal light incidence. The image reveals the spiral phase pattern typical of a chiral plasmon generated by illumination with an elliptically polarized light field (scale bar, 2 μm). d, Calculated real-space electron intensity distribution using the semi-analytical theory detailed in the text and the Supplementary Information (scale bar, 2 μm). e, Simulated phase map of the z component of the total electric field at the Ag/Si3N4 interface obtained from FDTD simulations (scale bar, 1 μm).