Techniques

Our experimental tool of choice is presently attosecond transient absorption spectroscopy (ATAS). ATAS uses attosecond extreme ultraviolet (XUV) pulses to probe the change in absorbance induced by an ultrashort optical pump pulse. With a time-resolution on the order of femtoseconds or more, the method is also known as XUV transient absorption spectroscopy. Due to the high probe photon energy, the method yields element-specificity through access to characteristic atomic core levels. 

In ATAS a short near-infrared (NIR) pulse acts as a pump and the resulting changes in XUV absorbance are probed by a single attosecond pulse or an attosecond pulse train. If the mechanism behind the absorption change is driven by the field of the pump pulse, the method offers true attosecond resolution – if the dynamics follows the pump pulse intensity instead, resolution is limited by the pump pulse duration.

As is the case with most techniques in attosecond science, ATAS was first applied to study gas phase systems (see, e.g., [1]). Only shortly after, the method was also demonstrated with solid samples. As the atomic nuclei are too heavy to show dynamics on attosecond scales, we primarily study the response of the electrons in the material. When extending our measurements to longer time spans, we can observe how the electrons interact with the lattice and how they dissipate their energy in a material.

[1] M. Holler, F. Schapper, L. Gallmann, and U. Keller, “Attosecond Electron Wave-Packet Interference Observed by Transient Absorption,” Phys. Rev. Lett. 106, 123601 (2011)