  High-order harmonic generation driven by intense extreme ultraviolet (EUV)
fields merges quantum optics and attosecond science, giving rise to an
appealing route for the generation of coherent EUV and soft X-ray light for
high-resolution imaging and spectroscopies. We theoretically investigate
ultrafast resonant dynamics during the interaction of He atoms with strong
extreme ultraviolet pulses. At high driving intensities, we identify record
fast attosecond Rabi oscillations imprinting observable signatures in the high
harmonic spectrum. At field strengths suppressing the Coulomb potential barrier
for all the bounded states, we demonstrate the survival of the attosecond
two-level dynamics for several Rabi cycles. Consequently, this intense EUV
laser-atom interaction reveals a new strong-field scenario where the resonant
coupling of two-level bound-bound transitions prevails, contrasting with the
dominance of bound-continuum transitions in the conventional strong-field
infrared regimes. These findings set an interesting perspective for extreme
attosecond nonlinear optics with intense short-wavelength fields.
