Resolving the phase of Fano resonance wave packets with photoelectron frequency-resolved optical gating

Abstract The creation of structured electronic wave packets (EWPs) energetically close to Fano resonances has been achieved with ultrafast extreme ultraviolet coherent light sources. However, direct real-time observations of EWP evolution and full reconstructions of the quantum properties of EWPs, including both amplitude and phase, are lacking. Here we introduce and demonstrate a comprehensive approach for the direct measurement and complete characterization of structured EWPs created within a prototypical Fano resonance. Because of its analogy with frequency-resolved optical gating (FROG), we named the method photoelectron FROG. The correlated EWP is initiated by a carefully engineered extreme UV pump pulse. A weak near-infrared laser field, serving as a probe pulse, samples the evolution of the EWPs in the time domain, as well as in the frequency domain. The amplitude and phase of the EWPs are obtained via a time-dependent reconstruction algorithm based on a short-time Fourier transformation. Given the excellent agreement between our experimental results and time-dependent reconstructions, we expect this method to be broadly applicable to the study of ultrafast processes, especially electronic ones, in complex systems, as well as the coherent control of such systems on their fundamental timescales.