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Attosecond metrology and coherent control at seeded free electron lasers

The group has carried out several studies at the FERMI seeded free-electron laser in Italy, exploiting the unique capabilities of these laser sources in terms of tunability, spectral coherence and phase control. Thanks to these properties, we have achieved several milestones in the field of ultrafast spectroscopy in the extreme ultraviolet, starting with the first demonstration of coherent control of photoionisation in this spectral range, the generation and complete shaping of attosecond pulse trains, and the implementation of an attosecond timing tool for FEL experiments.

In particular, in 2020 the group demonstrated a new approach to the temporal characterisation of attosecond pulse trains that overcomes the lack of synchronisation between the harmonic comb and the infrared field. This is a problem common to all free-electron lasers and has limited the temporal resolution achievable at these facilities to a few tens of femtoseconds. In our novel approach, the synchronisation is replaced by a correlated analysis of the photoelectron spectra measured on a single shot by combining the extreme ultraviolet and infrared fields. Using the same approach, we have also shown that the relative delay between the fields can be reconstructed to an accuracy of one atomic unit (24 as), improving by two orders of magnitude the temporal resolution achievable at these facilities.

We are currently continuing these activities by focusing on the demonstration of attosecond control of photoionisation in atoms, molecules and ions. In addition, we plan to demonstrate a completely new approach to the temporal characterisation of the group delay dispersion of multi-harmonic groups by exploiting non-linear mixing processes (four-wave mixing) in solids.

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