Attosecond Rabi Oscillations Revealed in EUV-Driven High Harmonic Spectroscopy

The prominent progress in the development of high-intensity extreme ultraviolet (EUV) coherent light sources enables the exploration of a novel regime of nonlinear optical interactions and attosecond metrology driven by short-wavelength pulses [1]–[3]. Here, we theoretically demonstrate the presence of ultrafast resonant two-state dynamics in EUV nonlinear optics phenomena, where the driving-laser photon energies are close to atomic transitions and the intense field induces ultrafast ionization. We observe, for the first time, a distinctive multi-peak harmonic signature of attosecond Rabi oscillations in the high-harmonic emission from helium driven by high-intensity EUV pulses. The periodic population exchange between the 1s and 2p orbitals of the helium atom triggers a temporal modulation of the dipole emission that leads to a multi-peak harmonic structure in the frequency domain. Interestingly, the period of such Rabi oscillations is of femtosecond-to-attosecond time scales. To the best of our knowledge, these are the fastest resonant dynamics ever reported.