New method effectively improves the angular-temporal resolution of attoclock technology
image: Figure 1: Schematic diagram of the experimental setup. The laser propagates along the z axis and the polarization plane of two-color pulses lies in the x-y plane. The upper diagram shows the transformation of the temporal waveform from linearly polarized laser pulses to the counter-rotating two-color elliptically polarized pulses, of which the relative phase delay, the relative intensity ratio, and the ellipticities can be controlled.
Credit: Ultrafast Science
The advancement of laser manipulating and shaping technology enables the precise control of strong field electron dynamics on a subcycle time scale which helps to resolve this critical issue. Two-color fields, which are the coherent overlap of the strong driving laser fields and its second harmonic field acting as a perturbation, provide an easy-to-implement method to shape the tunnelling electron wavepacket and control the coherent radiation. Circularly or elliptically polarized fields provide a time-resolved approach to access the quantum dynamics through mapping subcycle tunnelling bursts to different emission angles, which is termed as an attoclock. Incorporating the advantages of two-color fields and circularly polarized fields, two-color circularly polarized laser fields offer additional control to the attoclock measurement of tunnelling dynamics and the generation of circularly polarized high harmonics and THz wave.
Recently, the research team from the Institute of Atomic and Molecular Physics at Jilin University and the research team from the National University of Defense Technology have proposed a novel method for precisely measuring and controlling the dynamics of electrons in strong fields. This method employs a dual-elliptically polarized laser field in conjunction with the Classical Trajectory Monte Carlo (CTMC) method and utilizes Phase of Phase (POP) attosecond spectroscopy. By using counter-rotating elliptically polarized fields, the range of POP changes achieved is three times that obtained with traditional co-rotating elliptically polarized fields (see Fig. 2(b) and (d)). This discovery significantly enhances the resolution of time measurements in attosecond technology. The findings have been published in the journal of Ultrafast Science under the title "Strong Field Ionization Dynamics Resolved by Two-color Elliptical Phase-of-phase Spectroscopy".
In this research, a specialized multi-dimensional controllable dual-elliptical polarization laser field was constructed(Fig. 1), enabling precise control over various laser field parameters in experiments. Utilizing a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) setup, 3D momentum imaging of ion-electron coincidence measurements was performed on argon atoms ionized in the dual-elliptical polarization laser fields. A clear correlation between the observed time delays, emission angles, and electron energies was noted (see Fig. 3(d)), demonstrating high consistency between theoretical and experimental results. This alignment improved upon previous discrepancies between theory and experiment, deepening the understanding of atomic strong field tunnel ionization dynamics.
The research results underscored the accuracy and advantages of the triple-frequency component extraction of the Phase of Phase (POP) spectra produced by the proposed method. Compared to traditional attosecond techniques based on elliptically polarized light, this experiment significantly enhanced the temporal resolution of strong field tunnel ionization (see Fig. 3(c)). Moreover, at high two-color laser intensity ratios, the methodology effectively studied the re-scattering dynamics of high-energy electrons, marking the first experimental observation of phase flip signals induced by high-energy electron recollision, previously only predicted by theoretical simulations (noted in Fig. 2(b) and (d) in the region above 18 eV).
This study demonstrates that two-color elliptical Phase of Phase (POP) spectroscopy is a promising method for enhancing attosecond time resolution and holds significant value for in-situ measurements in strong laser fields and research into attosecond tunnel ionization dynamics.