  Coherent supercontinuum radiation in the extreme-ultraviolet (XUV) range is
indispensable for synthesizing attosecond light pulses and for exploring
transient atomic structures. Here, we report the striking observations of
coherent XUV supercontinuum (XSC) extended from below to far above the
ionization threshold, which exhibits completely different temporal and spatial
properties comparing to the conventional rescattering induced high harmonic
generation (HHG). We demonstrate that the strong-field created coherence among
bound orbitals strongly distort the atomic transition energies during the
pulse, leading to coherent emission spanning tens of electron-volts, in
contrast to the line emission via free-induction decay occurring after the
pulse. The supposed non-radiating bound dark states contribute as well by
emitting dressed energy through dark-to-bright emission mechanism. All the
processes modulated at sub-cycle time scale jointly form this new-type coherent
XSC. This work achieves the strong-field attosecond control of the exotic
atomic radiation dynamics and provides the means of simultaneous generation of
separated attosecond sources, i.e., XSC and HHG, with potential advancing
attosecond interferometry.
