X-RAY PUMP-PROBE · X-ray pump-probe spectroscopies - new tools to study ultrafast surface dynamics -

X-ray pump-probe spectroscopies - new tools to study ultrafast surface dynamics

Rapid progress in ultrafast X-ray science worldwide, both in high-harmonic generation (HHG) and X-ray free-electron laser (FEL) sources, has paved the way for a completely new generation of experiments investigating ultrafast electronic, magnetic, and structural dynamics in complex materials with ultrahigh time resolution and element specificity. Within the Marie Curie project, ultrafast material science experiments that are based on the use of table-top HHG lightsources have been developed. By the virtue of the short wavelength X-ray pulses produced by HHG lightsources, we could show that HHG is an ideal probe for even the fastest (i.e. electronic) dynamics in matter. Using elemental absorption edges, site-specific magnetic, electronic, structural, and chemical dynamics can be captured, providing unique capabilities for the study of elementary excitations in solids. In the area of ultrafast magnetisation dynamics, ultrafast X-ray pulses from HHG were used for the first time as an element-specific probe of ultrafast demagnetisation (PRX 2, 011005 (2012)) in a ferromagnetic alloy permalloy, which is a specific alloy of Fe and Ni. It was found that the ultrafast demagnetisation of Ni is delayed with respect to Fe by approximately 18 fs, despite the strong exchange coupling that aligns their magnetic moments in thermodynamic equilibrium. The delay is interpreted as due to the finite spin-flip scattering time given by the exchange energy in the material, which essentially measures the timescale of the exchange interaction for the first time (PNAS 109, 4792 (2012)). Therefore, this measurement probes the dynamics of the fundamental quantum mechanical exchange interaction in magnetic materials, while also being relevant to next-generation data storage technologies. In the same context, we investigated the ultrafast dynamics in technologically important magnetic trilayer structures. Here, we succeeded to photo induce an ultrafast increase of the magnetisation in a buried Fe layer (Nature Comm., submitted, (2012)). Only the in the Marie Curie project proposed combination of X-ray pump-probe spectroscopy with magneto-optical Kerr effect did provide the necessary experimental capability to explore these types of physics (PRL 103, 257402, (2009)). In the area of ultrafast dynamics in complex materials, angle-resolved photoemission spectroscopy (ARPES) in the extreme ultraviolet (XUV) regime by the use of HHG opens for the first time the possibility to directly study electron dynamics on a surface at high electron momenta. A very prominent example that requires such a new experimental technique is graphene, with its Dirac cone at the K-point of the Brillouin zone. First preliminary work did show the suitability of the developed technique for the direct study of the hot carrier dynamics in graphene. Both electron and hole dynamics can be monitored as a function of time, and we see the timescales of ultrafast optical-phonon generation and the subsequent relaxation of the carriers by electron and phonon scattering processes (in preparation). In the area of photo-induced phase transitions, we worked on the transition metal dichalcogenide TiSe2 which shows a second-order charge-density wave phase transition at about 200 K. These investigations were carried out in collaboration with Profs Bauer, Kipp, and Dr Rossnagel at the University of Kiel, Germany, and has revealed a photoinduced breakdown of long-range charge order that occurs on a sub 20-fs timescale. This exceptional fast dynamics happens before the in-thermal equilibrium concomitant periodic lattice distortion is perturbed which shows that this transition is completely electronically induced (Nature 471, 490 (2011)).

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