Figure 4: Excitation energy-dependent imaging versus energy-space imaging.

(a) Finite-element simulation of the excitation energy dependence of the photoinduced SPP field strength (log₁₀ |E_z| integrated over the volume surrounding the nanowire) in a ϕ=0° normal incidence excitation geometry. The simulated nanowire length and radius are 2 μm and 40 nm, respectively. Representative spatial field distributions of the various odd-order SPP resonances are shown below (|E_z| in the plane 10 nm below the wire), with electron counts plotted using the same linear colour scale. The vertical scale bar in the image of the m=1 SPP mode corresponds to 500 nm and holds for all images. (b) Selectively photoexciting only one of the SPP modes of this wire (here m=5) in a single-wavelength PINEM experiment instead (at ω₅, ϕ=0°) quantizes the energy exchange of its field distribution with the probing electrons, as shown in this conceptual PINEM energy-space map. The corresponding electron energy gain spectrum is depicted on the top. Electron counts in both the map and the spectrum are plotted on a linear scale. Though not shown here, a similar series of quantized features is present on the energy loss side of the ZLP in both the energy-space map and the spectrum.