All-optical attoclock for imaging tunnelling wavepackets

Whether or not an electron wavepacket accumulates a time delay when tunnelling out of an atom is still under debate. Improved all-optical characterization of the tunnelling dynamics by combining one- and two-colour driving fields may shed light on this question. Recent measurements of time delays during tunnelling of cold atoms through an optically created potential barrier have fuelled an ongoing debate about possible time delays during light-induced tunnelling of an electron from an atom. Yet, such a delay-whether it is present or not-is only one quantity characterizing the tunnelling wavepacket, whilst the underlying dynamics are richer. Here we show how to complement photo-electron detection in laser-induced tunnelling by measuring the light emitted by the tunnelling electron-the so-called Brunel radiation. Using a combination of single- and two-colour driving fields, we identify the all-optical signatures of the reshaping of the tunnelling wavepacket as it emerges from the tunnelling barrier and moves away from the core. This reshaping includes not only an effective time delay but also the time-reversal asymmetry of the ionization process, which we describe theoretically and observe experimentally. We show how both delay and reshaping are mapped onto the polarization properties of the Brunel radiation, with different harmonics behaving as different hands of a clock moving at different speeds. The all-optical detection may also allow time-resolved measurements of optical tunnelling in condensed matter systems on the attosecond time scale.