A phase-controlled orthogonal two-color (OTC) femtosecond laser pulse is employed to probe the time delay of photoelectron emission in the strong-field ionization of atoms. The OTC field spatiotemporally steers the emission dynamics of the photoelectrons and meanwhile allows us to unambiguously distinguish the main and sideband peaks of the above-threshold ionization spectrum. The relative phase shift between the main and sideband peaks, retrieved from the phase-of-phase of the photoelectron spectrum as a function of the laser phase, gradually decreases with increasing electron energy, and becomes zero for the fast electron which is mainly produced by the rescattering process. Furthermore, a Freeman resonance delay of 140±40 attoseconds between photoelectrons emitted via the 4f and 5p Rydberg states of argon is observed.
FIG. 1. Schematic illustrations of photoemission dynamics of (a) the Freeman resonance via the field-dressed 5p or 4f Rydberg states of Ar, (b) the sideband generation from the adjacent main peaks by absorbing or emitting an additional photon via the virtual states (VS) in the continuum, and (c) the field-driving rescattering process. The green bubble and yellow surface stand for the electron and nuclear potential, respectively. (d) Schematic view of experimental setup
FIG. 2. (a),(e) Measured and (b),(f) normalized 2D spectra of Ee vs ϕL. (c),(g) Measured ϕL-integrated Ee distribution (blue solid curves) and retrieved contrast amplitude A0 (red dashed curves). (d),(h) Retrieved phase-of-phase ϕPP of main (blue solid circles) and sideband peaks (red solid squares). The numerically simulated ϕPP are correspondingly shown as the blue open circles and red open squares. The left and right panels are for electrons emitting to (a)–(d) jϕej < 3° and (e)–(h) 20° < ϕe < 40°, respectively. The horizontal green lines in (d),(h) indicate the energy position of 2UOTC p .
