Speaker
Description
Comprehensive knowledge of ultrafast out-of-equilibrium dynamics in materials is essential to identify the microscopic processes that govern their electronic, magnetic and structural properties. Controlling these processes with light may ultimately enable optically driven functional devices that exploit quantum coherence, collective excitations and transient states of matter.
Many of the relevant interactions occur on femtosecond and attosecond timescales, including charge redistribution, electronic screening, spin–orbit coupling and the initial transfer of energy among electronic, spin and lattice degrees of freedom. Resolving these processes on their natural length and timescales requires ultrashort, temporally stable, high-photon-energy pulses that can provide element- and orbital-specific sensitivity to evolving electronic and magnetic states.
In principle, X-ray Free-Electron Lasers (XFELs) provide a platform for such studies. Their high brightness and tunable photon energy enable electronic and magnetic states to be probed through element-specific core-level transitions, while their ultrashort pulse duration provides access to the earliest stages of light-induced dynamics. However, although few-femtosecond and attosecond X-ray pulses are increasingly available at XFELs around the world, their practical application presents a major experimental challenge. That is, for field-resolved experiments, both the X-ray probe pulse duration and the relative arrival time and phase of the driving external laser pulse must be determined on a single-shot basis – with a precision comparable to the timescales under investigation. Otherwise, timing uncertainty between the independent sources can obscure the intrinsic material response and the processes involved by limiting the effective temporal resolution in pump–probe experiments.
Here, we present a method for attosecond characterization of ultrashort X-ray pulses generated at SwissFEL using a self-referenced attosecond streaking approach developed for the new Diavolezza endstation. These measurements permit further exploration and development of advanced XFEL emission modes, which include high intensity attosecond pulses, pulse pairs, pulse trains and temporally coherent X-ray emission made possible by external laser seeding. Importantly, the self-referenced streaking measurements also provide a relative measurement of the time of arrival of the XFEL pulse with respect to the external laser, as well as the relative phase with sub-femtosecond resolution.
Self-referenced streaking provides a practical route towards reliable waveform-resolved experiments at XFELs. The time-resolution in these studies will range from femtosecond to attosecond resolution, scaling with the wavelength of the external optical laser. This capability opens new opportunities to resolve charge transfer, screening, spin dynamics and light-induced changes of quantum states with element specificity and attosecond temporal resolution.
