Abstract
We show that high harmonic generation driven by an intense near-infrared (IR) laser can be temporally controlled when an attosecond pulse train (APT) is used to ionize the generation medium, thereby replacing tunnel ionization as the first step in the well-known three-step model. New harmonics are formed when the ionization occurs at a well-defined time within the optical cycle of the IR field. The use of APT-created electron wave packets affords new avenues for the study and application of harmonic generation. In the present experiment, this makes it possible to study harmonic generation at IR intensities where tunnel ionization does not give a measurable signal.
Highlights
In this paper, we demonstrate for the first time that it is possible to temporally control HHG by forming the initial EWP with an attosecond pulse train (APT)
We show that the helium harmonics are enhanced periodically as a function of the APT–intense nearinfrared (IR) delay, with a maximum in the efficiency occurring once every half-optical cycle of the IR field
Under our experimental conditions the kinetic energy of the APT-initiated returning EWPs is in a range that is not accessible in EWPs launched by IR tunnel ionization
Summary
We demonstrate for the first time that it is possible to temporally control HHG by forming the initial EWP with an APT. The short duration and high photon energy of the individual pulses in the APT allow the ionization to be fixed to a certain time during the IR cycle, which can be controlled via the relative delay between the two pulses.
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