Field-induced tunnel ionization is one of the mostfundamental strong-field phenomena, initiating a broad range of strong-field processes. The tunneling dynamics leaves its signature in these phenomena, playing a significant role in their temporal properties [1, 2, 3]. Here we apply attosecond electron holography to obtain a deeper insight into the tunneling dynamics. In this scheme, introduced by [4], strong field light-matter interaction leads to a holographic measurement of various electron trajectories. Once the electron tunnel ionizes, it is accelerated by the strong laser field, which dictates its final momentum, serving as a reference beam. Alternatively, the electron may scatter off the parent ion,serving as a signal beam. The photoelectron momentum distribution records the holographic interference between the two trajectories.

In the following experiment we resolve the temporal properties associated with the hologram. Specifically, by adding a weak second harmonic field we apply a sub-cycle perturbation which modulates each electron trajectory according to its ionization time. 

The photoelectron momentum distribution generated by 788 nm (~1.3 × 1014 W/cm2) pulses, interacting with Argon atoms. The result is a spider shaped fringe pattern associated with the two electron trajectories’ interference. Introducing a weak second harmonic field polarized parallel to the fundamental field, modulates the interference pattern. The phase changes across the hologram, indicating a distinct phase difference between neighboring peaks and dips (i.e., regions of constructive and destructive interference, correspondingly). The rapid variations of the phase across the momentum plane are associated directly with the tunneling dynamics. As we add the perturbative field we modify the tunneling probability and hence the electron trajectory’s strength (i.e., probability amplitude), which oscillates with the two-color delay. The oscillation phase is directly associated with the tunneling time.

[1] D. Shafir et al. Nature 485, 343 (2012)        

[2] M. Uiberacker, et al. Nature 446, 627 (2007)

[3] P. Eckle, et al. Science 322, 1525 (2008)    
[4] Y. Huismans et al. Science 331, 61 (2011)