Time-Resolved Photoemission Electron Microscopy on a ZnO Surface Using an Extreme Ultraviolet Attosecond Pulse Pair

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 2D 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, this work demonstrates an attosecond interferometry experiment on a zinc oxide (ZnO) surface using spatially and energetically resolved photoelectrons. Photoemission electron microscopy is combined with near-infrared pump - extreme ultraviolet probe laser spectroscopy and the instantaneous phase of an infrared field is resolved with high spatial resolution. Results show how the core level states with low binding energy of ZnO are well suited to perform spatially resolved attosecond interferometry experiments. A distinct phase shift of the attosecond beat signal is observed across the laser focus which is attributed to wavefront differences between the pump and the probe fields at the surface. This 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. By illuminating a clean zinc oxide surface with a few-cycle near-infrared light pulse and a pair of attosecond extreme ultraviolet pulses, an attosecond interferometry experiment is performed for the first time using an electron microscope. The combined spatio-temporal resolution is a significant step towards imaging optical fields and electronic excitations in heterogeneous environments with attosecond-nanometer precision. image