Recently, researchers at Peking University together with their collaborators from USA and Japan, have made progress in the attosecond physics. In the context of atomic ionization by a few-cycle attosecond pulse in the presence of a weak IR field, they propose a scheme of probing interactions inside an atom by "electron hologram". Their work has recently been published by the prestigious journal of Phys. Rev. Lett. (Ming-Hui Xu, Liang-You Peng*, Zheng Zhang, Qihuang Gong*, Xiao-Min Tong*, E. A. Pronin, and A. F. Starace*, Phys. Rev. Lett. 107, 183001 (2011),http://prl.aps.org/abstract/PRL/v107/i18/e183001) (download).

This work demonstrates the connection between the particle-like and the wave-like (dual) nature of electrons interacting with an atom from which they are ionized. The establishment of this connection indicates the promise of using laser-driven electrons to create holographic "images" of atomic and molecular ionic potentials. They investigate the use of an extreme ultraviolet (XUV), ultrashort laser pulse (of attosecond duration) to ionize an electron in the presence of an intense infrared (IR) laser field. Owing to the ultrashort duration of the attosecond XUV pulse, the ionized electrons have a wide spread in energy, including some with very low energies that are driven by the IR laser electric field to scatter from the residual ion. They demonstrate that there exist many different particle-like electron trajectories that scatter one or more times from the atomic potential. The wave-like interference of these different particle-like trajectories creates a highly structured electron ionization spectrum that carries information on the interactions of the ionized electron with the residual ion. Such interference structures can be regarded as an "electron hologram" in that the scattered electrons "scan" the target ion and the directly-ionized electrons (that do not undergo scattering) serve as a "reference" beam. This idea of "electron hologram" may be soon taken up by the experimentalist.