(a) Experimental set-up: few-cycle near infrared (NIR) and isolated extreme ultraviolet (XUV) attosecond pulses with variable delay are focused onto a gold nanotaper. High-energy electrons are emitted via XUV photoionization and subsequently accelerated in the local near fields. The delay-dependent final kinetic energy is measured using a time-of-flight (TOF) spectrometer. The nanotaper can be replaced by a gas target. (b,c) Maximal normalized local field strengths of the component parallel to the taper axis (b) along the laser propagation direction and (c) perpendicular to it as obtained from FDTD simulations. The green arrow shows the maximum electron detection angle. The blue lineout illustrates the XUV photoemission area. (d,e) Response function of the probed Ey-component (axis of laser polarization and TOF spectrometer) for a representative point (as indicated in b) at the nanotaper, showing the (d) absolute value and (e) phase dependence on the wavelength. The response of the shank is close to an infinite cylinder with a diameter of 200 nm calculated using Mie theory (black dashed line). Slight position-dependent oscillations occur due to a plasmon launched at the tip apex (Supplementary Note 4 and 5).