Fig. 1: SPRINT.
a, Integrating attosecond transient absorption with XUV–XUV interferometry. A phase-locked XUV–IR (XUV 1 + IR) beam, with a relative delay Δt is focused into helium atoms, inducing a nonlinear interaction. The underlying dynamics are encoded in the transient amplitude and phase of the transmitted light, characterized by 2ωIR oscillations. Interfering the transmitted XUV field (black) with a reference XUV source (magenta) provides access to the transient phase Δϕ (Δt). Manipulating Δt encodes the transient amplitude and phase, mapped into the instantaneous amplitude and phase of the interferogram. b, Illustration of the nonlinear XUV–IR interaction. Harmonic 13 (magenta) is modified by multipath interference. These paths couple adjacent odd harmonics (harmonic 11 and harmonic 15 in orange and purple, respectively) via four-wave-mixing processes, involving two additional IR photons. Harmonic 15 is resonant with the 1s4p excited state. c, In an absorption-only measurement, the dynamics of all the three quantum paths are mapped into the 2ωIR Fourier peak, preventing access to their individual coherent contribution. In contrast, within SPRINT, the reference source interferes with each individual path, mapping its individual contribution onto a distinct Fourier peak. This peak identifies the absorbed XUV photon energy that initiates each path, simultaneously resolving its amplitude and phase evolution.
