Optical control of electron emission at the attosecond timescale

Coherent control of electron dynamics in matter is a growing research field in ultrafast science, which has been mainly driven over the last two decades by major advances in laser technology. Recently, the advent of extreme-ultraviolet (EUV) light pulses in the attosecond time scale (1as = 10^-18s) has opened up new avenues for experimentalists to manipulate the electronic dynamics with unprecedented precision. In this work, we demonstrate that an asymmetric electron emission from atomic targets can be generated and controlled by combining an attosecond pulse train (APT) and a weak IR field (10¹¹ W/cm2). Electron wave-packets are formed by ionizing argon gas with such APT in the presence of the IR field. Consequently, a mix of energy-degenerate even and odd parity states is fed into the continuum by one- and two-photon transitions. These interfere, leading to an asymmetric electron emission along the polarization vector. At some appropriate time delay between the APT and IR fields, the even and odd angular continuum wave function resulting from one- and two-photon transitions, respectively, add constructively on one side (up) of the polarization vector direction and destructively on the other side (down), thus creating a strong up-down asymmetry in the angular emission of the photoelectrons. The direction of the emission can be controlled by varying the time delay between the two pulses.
In addition, we show that such asymmetric emission is also related to the properties of the APT. The temporal analysis of the modulated electron emission, based on an accurate description of the atomic physics of the photoionization process, then provides a way to measure the temporal profile of the attosecond pulse.