Fig. 2: Resolving the sub-cycle transient phase in helium.
a, Attosecond transient absorption. Intensity of harmonic 13 in the absence of the reference source. Negative delays correspond to the XUV preceding the IR. The 1.3 fs periodicity corresponds to the 2ωIR oscillations. For visibility, we focus on several oscillation periods where the XUV precedes the IR by ~20 fs. b, Attosecond interferometry. Interferometric signal of harmonic 13 in the absence of nonlinear interaction with helium atoms. The 200 as periodicity corresponds to the optical period of harmonic 13. c, Attosecond transient interferometry, obtained with all three gas sources. The beating pattern between the 1.3 fs and 200 as periodicities encodes the transient phase information. d–f, Fourier analysis of a–c (d–f, respectively) focusing on the low- and high-oscillation frequencies. g, Retrieved transient phase of harmonic 13, presenting a pronounced sub-cycle phase oscillation at 2ωIR. h, Experimentally retrieved temporal evolution of the envelope of the resonant sideband 15 in harmonic 13. i, Reconstructed oscillating waveform of the generated field at several representative values of the delay, as highlighted in h. The waveforms are vertically shifted for visibility.
