Coherent XUV super continuum emission from atomic bound states

TL;DR

This paper reports the discovery of a new type of coherent XUV supercontinuum emission from atomic bound states, which differs from traditional high harmonic generation and enables advanced attosecond control and sources.

Contribution

It reveals a novel mechanism of coherent XUV supercontinuum emission involving bound-state coherence and dark-to-bright state transitions, expanding understanding of atomic radiation under strong fields.

Findings

Extended XUV supercontinuum from below to above ionization threshold

Demonstration of bound-state coherence distorting atomic transition energies

Identification of dark-to-bright emission mechanisms contributing to supercontinuum

Abstract

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.