RESUMEN
We propose an attosecond extreme ultraviolet pump IR-probe photoionization protocol that employs pairs of counterrotating consecutive harmonics and angularly resolved photoelectron detection, thereby providing a direct measurement of ionization phases. The present method, which we call circular holographic ionization-phase meter, gives also access to the phase of photoemission amplitudes of even-parity continuum states from a single time-delay measurement since the relative phase of one- and two-photon ionization pathways is imprinted in the photoemission anisotropy. The method is illustrated with ab initio simulations of photoionization via autoionizing resonances in helium. The rapid phase excursion in the transition amplitude to both the dipole-allowed (2s2p)^{1}P^{o} and the dipole-forbidden (2p^{2})^{1}D^{e} states are faithfully reproduced.
RESUMEN
The reconstruction of the full temporal dipole response of a strongly driven time-dependent system from a single absorption spectrum is demonstrated, only requiring that a sufficiently short pulse is employed to initialize the coherent excitation of the system. We apply this finding to the time-domain observation of Rabi cycling between doubly excited atomic states in the few-femtosecond regime. This allows us to pinpoint the breakdown of few-level quantum dynamics at the critical laser intensity near 2 TW/cm^{2} in doubly excited helium. The present approach unlocks single-shot real-time-resolved signal reconstruction across timescales down to attoseconds for nonequilibrium states of matter. In contrast to conventional pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The potential future applications of this technique range from testing fundamental quantum dynamics in strong fields to measuring and controlling ultrafast chemical and biological reaction processes when applied to traditional transient-absorption spectroscopy.