  Electrons photoemitted by extreme ultraviolet attosecond pulses derive
spatially from the first few atomic surface layers and energetically from the
valence band and highest atomic orbitals. As a result, it is possible to probe
the emission dynamics from a narrow two-dimensional region in the presence of
optical fields as well as obtain elemental specific information. However,
combining this with spatially-resolved imaging is a long-standing challenge
because of the large inherent spectral width of attosecond pulses as well as
the difficulty of making them at high repetition rates. Here we demonstrate an
attosecond interferometry experiment on a zinc oxide (ZnO) surface using
spatially and energetically resolved photoelectrons. We combine photoemission
electron microscopy with near-infrared pump - extreme ultraviolet probe laser
spectroscopy and resolve the instantaneous phase of an infrared field with high
spatial resolution. Our results show how the core level states with low binding
energy of ZnO are well suited to perform spatially resolved attosecond
interferometry experiments. We observe a distinct phase shift of the attosecond
beat signal across the laser focus which we attribute to wavefront differences
between the pump and the probe fields at the surface. Our work demonstrates a
clear pathway for attosecond interferometry with high spatial resolution at
atomic scale surface regions opening up for a detailed understanding of
nanometric light-matter interaction.
