Researchers from Jilin University and Hainan University in China have made a groundbreaking discovery in the field of attosecond optical metrology, which enables the study of ultrafast electron dynamics on atomic scales. Led by Xiaokai Li, Xiwang Liu, Chuncheng Wang, Shuai Ben, Shengpeng Zhou, Yizhang Yang, Xiaohong Song, Jing Chen, Weifeng Yang, and Dajun Ding, the team has found that Coulomb focusing plays a crucial role in attosecond angular streaking measurements.
This technique is used to probe electron dynamics with attosecond precision by mapping the delay time of electron tunneling to the deflection angle corresponding to the maximum photoelectron yield through ionization in elliptically polarized laser fields.
The researchers used energy-resolved angular streaking measurements and an improved Coulomb-corrected strong-field approximation method to reveal that the measured photoelectron angular offsets are determined jointly by Coulomb interaction in quantum tunneling and the Coulomb focusing effect in classical continuum states. This discovery has significant implications for our understanding of attosecond chemistry and the development of petahertz ultrafast photoelectric signals.
Coulomb Focusing in Attosecond Angular Streaking: A New Perspective on Electron Dynamics
The study of ultrafast electron dynamics is crucial for understanding the fundamental processes in nature, such as molecular bond breaking and formation, chemical reactions, and biological metabolism. Attosecond optical metrology techniques have enabled probing of electron dynamics on the atomic temporal and spatial scales, providing potential pathways for achieving quantum control in attosecond chemistry and processing petahertz ultrafast photoelectric signals.
One of these techniques is attosecond angular streaking, which maps the delay time of electron tunneling to the deflection angle corresponding to the maximum photoelectron yield through ionization in elliptically polarized laser fields. However, a key assumption in attosecond angular streaking is the one-to-one correspondence between the electron deflection angle and the tunneling ionization instant, which remains valid.
The Role of Coulomb Interaction in Attosecond Angular Streaking
Recent research has shown that the measured photoelectron angular offsets are determined jointly by Coulomb interaction in quantum tunneling and the Coulomb focusing effect in classical continuum states. This is in contrast to the commonly assumed one-to-one correspondence between the highest-yield electrons and the peak of the laser electric field.
The study used a combined approach of energy-resolved angular streaking measurements and improved Coulomb-corrected strong-field approximation (ICCSFA) method, which includes the under-barrier Coulombic interaction. The results indicate that the strong-field approximation model, which neglects Coulomb interactions, agrees with the single classical trajectory approximation simulations, showing a one-to-one correspondence between the highest-yield electrons and the peak of the electric field.
However, the simulated results deviate significantly from the experimental results. Only by considering the Coulomb interaction in quantum tunneling and continuum propagation (ICCSFA method) can the energy-dependent electron deflection angles be consistent with the experimental findings. The highest-yield electrons were found to originate from tunneling ionization within a few tens of attoseconds.
Coherent Superposition of Electrons and Coulomb Focusing
The research reveals that the coherent superposition of electrons ionized at different initial times determines the final electron deflection angle. Additionally, statistical analysis of electron trajectory shows that Coulomb focusing alters the counts distribution of trajectories for electrons of different energies. The sub-barrier Coulomb attraction in nonadiabatic tunneling further enhances this effect, revealing the physical mechanism behind the counterintuitive energy dependence observed in angular streaking experiments.
Implications for Attosecond Angular Streaking Measurements
This work indicates that attosecond angular streaking measurements are closely related to the statistical distribution of momentum/energy of electron wave packets generated by quantum tunneling. The Coulomb focusing effect disrupts the one-to-one correspondence between the emission angle of the highest-yield electrons and the tunneling ionization time.
The research provides a crucial avenue for the intuitive interpretation of attosecond angular streaking experiments and offers new tools for decoding the sub-barrier tunneling dynamics in the classically forbidden region. This study has significant implications for our understanding of ultrafast electron dynamics and its applications in attosecond chemistry and petahertz electronics.
Future Directions
The findings of this research open up new avenues for exploring the role of Coulomb interaction in attosecond angular streaking measurements. Further studies can focus on developing more accurate models that incorporate Coulomb interactions, which will enable a deeper understanding of ultrafast electron dynamics.
Additionally, the development of new experimental techniques that can probe the coherent superposition of electrons and Coulomb focusing effects will be crucial for advancing our knowledge in this field. The integration of theoretical models with experimental techniques will provide a powerful toolset for exploring the intricacies of ultrafast electron dynamics.
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