Angularly Resolved RABBITT at 2ω: Attosecond Pulses Measurement & Attosecond Signature in the 1s3p Resonance of Helium

Summary form only given. The Reconstruction of Attosecond Harmonic Beating By Interference of Two-photon Transition (RABBITT) method is nowadays routinely used in attosecond physics to retrieve the temporal profile of an attosecond pulse train (APT) generated by high harmonic generation. It is employed to measure the ionization/Wigner time [4] of an escaping electron that scatters in the potential of its parent atom or molecule. The RABBITT signal is obtained by measuring the time-resolved photoelectron spectrum of a given target that interacts with an APT (composed of odd harmonics of the fundamental laser frequency (ω
₀
) dressed by a femtosecond pulse at the frequency ω
₀
. In the spectral domain, several quantum paths lead to the same electron kinetic energy (Fig. 1.(c)) these paths interfere leading to an oscillation of the electron yield at twice the fundamental frequency with a phase that depends on the (i) phase of the harmonic, (ii) the phase of the IR transition (so called continuum-continuum phase) and (iii) the atomic or molecular phase that corresponds to the transition induced by the XUV. In the temporal domain, one attosecond pulse appears for each half optical cycle of the dressing pulse as shown in Fig. 1(a). In this work, we present a variant of the RABBITT method where the dressing field is upconverted to 2coo instead of ob. In the spectral domain illustrated by Fig. 1(d) the interplay between the components is more complex. Several interference channels overlap. The resulting attosecond oscillation depends on more parameters than the parent method. In the temporal domain, only one attosecond pulse is present per optical cycle as shown in Fig. 1(b). This implies that all the attosecond pulses are dressed by an electric field in the same direction in the laboratory frame, which breaks the inversion symmetry present in the original method. We show that this RABBITT at 20) method can be employed to characterize the temporal profile of the attosecond pulse train. This method can also be used in its intra-band spectrally resolved version to measure the attosecond phase shift printed by a dense helium medium at the vicinity of the 1s3p resonance. This last shows that non -local effects can be imprinted into the attosecond oscillation phase shift that depends on the target gas density, enabling attosecond control of angular electron wavepackets.