Electron-transparent samples were prepared by depositing through sputtering a silver thin film (43 nm) on a Si3N4 membrane (30 nm) suspended on an 80 μm by 80 μm window in a Si support. Grooves completely penetrating the Ag film and partially penetrating the Si3N4 membrane were produced by focused Ga ion beam milling. The samples were mounted in a double-tilt TEM holder.

The 55-fs-long, 1.57-eV, 300-kHz repetition rate optical pulses from a Ti:Sapphire regenerative amplifier were used as both signal and reference fields. The third harmonic of these pulses was used to produce electron pulses via photoemission from a LaB6 cathode. Experiments were conducted in a modified JEOL-2100 TEM, as described in (48).

In all of our experiments, the full width at half maximum of the laser beams was 25 μm. For the local holography experiment, SPPs were selectively excited by aligning the laser polarization with the normal-to-edge direction; this was achieved by maximizing the PINEM response from a single pulse in the region of the sample where excitation only from one nanocavity was present. Energy-filtered imaging was performed using a Gatan GIF Quantum electron energy loss spectrometer.

In our experimental setup, the light beam propagated at an angle of 4.5° ± 1° with respect to the direction of the electron beam, which was oriented along the optical axis of the microscope. For the local interference experiment, the sample was tilted by 12° with respect to electron direction while maintaining the normal to the surface within the plane defined by the light and electron beam directions. This allowed us to minimize the contribution from the semi-infinite field reflected by the flat sample surface, as, for these conditions, modification produced on the electron wave function by the incident and reflected beams was exactly canceled (35). The presence of this tilt resulted in retardation effects, which were manifested as a tilt of the plasmon wavefront with respect to the edge of the groove sources. The images presented in Figs. 2 and 4 are projections of the electron distribution on the plane perpendicular to the optical axis of the microscope. Geometrical considerations accounting for the retardation effects and the calculation of the corresponding corrections to the phase and group velocities are presented in note S1.

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