Principle and application of two-color attoclock

Abstract

Time-resolving ultrafast electron dynamics driven by lights in atoms and molecules is one of the central topics in both science and technology, as a prerequisite for controlling the electron dynamics and creating unprecedented functions to serve for human. Photoionization by intense lasers is the fundamental interaction between lights and atoms. For strong-field ionization, the amplitude and the phase of the ionized electron wave packet could give direct insights of electron dynamics in the classically forbidden, sub-Coulomb-barrier region. Electron tunneling through the suppressed Coulomb potential and traveling in continuum leave fingerprints not only on the amplitude of the electron wave packet, but also on its phase distribution. Recently, benefiting from that the laser pulse duration approaches to the natural time scale of intra-atomic electron dynamics, one was allowed to image the amplitude and the phase of an electron wave function on the attosecond scale using new metrologies combined with attosecond light pulses, such as attosecond streaking camera and the technique of reconstruction of attosecond beating by interference of two-photon transition. Alternatively, attoclock is based on the angle-time mapping principle in one-color circularly polarized femtosecond light fields, without resorting to the attosecond light pulses. It is originally introduced as a powerful method to address the issue of tunneling time delays with the attosecond time resolution in strong-field physics. In recent years, due to the flexible manipuility and the rich diversity of the two-color femtosecond light fields, the two-color fields have gradually become an important method to control the ultrafast electron dynamics driven by intense lasers. The two-color attoclock combined with attoclock and two-color fields, then attracts the increasing attention, and hence becomes the research front of strong-field physics. In this review, we will introduce two kinds of the two-color attoclock. First, based on the co-rotating two-color light fields, we demonstrate a novel time-resolved photoelectron interferometry, i.e., double-hand attoclock. Using this technique, one can extract the phase information and the amplitude information of the ionized electron wavepackets by strong-field ionization with intense light pulses. From the extracted time-resolved distributions of electron phase and amplitude, we reveal the electron sub-barrier dynamics with attosecond time resolution. As to the second two-color attoclock, we study the old and controversial question, i.e., tunneling time delay. Introducing a linearly polarized second harmonic field to calibrate the attoclock constructed by a circularly polarized fundamental-frequency light field, we demonstrate an improved attoclock. The improved attoclock can measure the photoelectron offset angle more accurately, which corresponds to tunneling time delay. Theoretically, we also demonstrate that the instantaneous tunneling picture and the Wigner delay picture for tunneling, which seem contradictory with each other, can be unified within the same theoretical framework, i.e., strong-field approximation theory. For the result of the improved attoclock, the two tunneling pictures can both agree with the experiment. To conclude, the two-color attoclock possesses not only very high time resolution but also good space resolution. Therefore, the two-color attclock has a very exciting potential in molecular orbital imaging, tunneling ionization of molecules and so on.