In condensed-matter physics, investigating the electronic structure and its response to external excitation is critical to understand fundamental material properties. For example, angle-resolved photoemission spectroscopy (ARPES) is a versatile tool for observing directly the electronic band structure with energy and momentum information. Recent advances yielded insights to a broad range of quantum materials, e.g. high-temperature superconductors, topological materials, two-dimensional materials, and heterostructures.

In order to carry out a time-resolved study on ultrafast time-scale, the co-called “pump-probe” technique is essential. In this scheme, two laser pulses interact with the material: the pump and the probe pulse. The pump pulse triggers an effect (excitation) within the material. The probe pulse analyses how that effect evolved a few femtoseconds or picoseconds after the trigger. Especially, for time-resolved ARPES experiments scientists want to trigger changes in the electronic band-structure of a complex material by absorbing an ultraviolet or visible pump photon. Subsequently, the band structure is recorded using a series of ARPES measurements for different moments in time. The ARPES probe measurement itself requires UV or EUV probe photons to achieve photo-emission of the electrons at different angles. As a result, the series of ARPES probe measurements gives a stroboscope-like “movie”, giving deep insights on how that complex material behaves on the ultrafast time-scale.

Most advanced experiments require sources that provide simultaneous optical or THz excitation (pump) and EUV/XUV ARPES probing. Typically, 21 eV are photon energy for EUV/XUV probing is implemented in order to access the full momentum space (k-space) of most common materials. Nowadays, laser sources can provide this flexibility from EUV/XUV to mid-IR and even THz. Driving a brilliant EUV high-harmonic generation (HHG) source such as our Moonlander HHG with a White Dwarf HE OPCPA laser system allows even for tuning of the EUV/XUV probing wavelength.