Multi-electron correlations in the emission of coherent extreme-ultraviolet light

Electron-electron correlation is at the origin of complexity in multi-electron systems. Since the total response of a complex system is not the sum of its independent constituents, the many-electron problem remains a computational challenge even though understanding correlation mechanisms is essential for a full control of the electric and magnetic properties of matter. In this work, we disentangle the fingerprints of correlated electron dynamics in the strong laser-atom interaction of the simplest multi-electron system. The two electrons of the helium atom interacting with an intense electromagnetic field represent an extraordinary scenario to explore correlations from a fundamental view, since we can perform a numerical integration of the time-dependent Schrödinger equation. Our theoretical results predict a correlation mechanism of back-reaction showing a distinctive signature in high-order harmonic generation. An extension of the high-harmonic spectra towards higher frequencies encodes the information of back-reaction. This finding demonstrates that correlation phenomena can be tracked with high-harmonic spectroscopy, and opens the possibility to experimentally probe complex systems