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Coherent manipulation of atoms by strong electromagnetic radiation can be used to engineer new optical properties of matter. The technique of using a strong control field to manipulate the transmission of a weak probe field, such as it is done in Autler-Townes splitting, has recently been extended to control the transmission of x rays.
If a strong femtosecond pulse is used to control the transmission of an isolated attosecond extreme ultraviolet pulse, then basic assumptions of traditional quantum optics break down. The rotating wave approximation becomes invalid, and the sub-cycle timing of the attosecond pulse within the optical cycle of the control pulse influences the transmission dramatically. Furthermore the transmitted radiation is modulated over a wide frequency spectrum when the Rabi frequency exceeds the frequency of the strong control field. This effect is observed in a transient absorption experiment with Xenon atoms and can be explained in the framework of a few-level quantum model.
The transmitted spectrum also reveals information about the valence electron motion in the remaining ion if the control pulse is strong enough to induce strong field ionization. Attosecond transient absorption, that has already been used to observe the valence electron motion in singly charged ions, could be used to monitor the dynamics in doubly charged ions. This observation could give useful information to understand the role of electron correlation in strong field double ionization by circularly polarized fields, a topic that is currently heavily debated.
