(a) Electron spectrum under combined illumination with XUV and NIR light, measured using a time-of-flight spectrometer. For the polycrystalline nanowire, the energy landscape under the Fermi edge results from averaging over different crystal structures and orientations, and eventually a contribution from surface contamination. (b) Integrated electron emission maps from the low-energy region of a dominated by strong-field NIR photoemission and (c) for the high-energy region of a caused by linear XUV photoemission. The laser beam propagates in the z direction. The NIR photoemission is strongly enhanced at the apex of the nanotaper, where field enhancement at the apex supports non-linear NIR photoemission processes. The XUV photoemission represents the convolution of the taper geometry with the XUV beam in the focus. The solid black line serves as a guide to the eye showing the outline of the nanotaper, while the solid white line illustrates approximately the geometry of the part of sample that is probed in the experiment. Calculations show that relative amplitude and phase of the electrical field is approximately homogenous over the probed surface region and accordingly averaging preserves results on relative phase and amplitude of the field (Supplementary Note 6; Supplementary Figs 6–8).