Abstract
Attosecond control of the optical response of helium atoms to extreme ultraviolet radiation in the presence of moderately strong infrared laser light has been recently demonstrated both by employing attosecond pulse trains (APTs) and single attosecond pulses. In the case of APTs the interference between different transiently bound electron wavepackets excited by consecutive attosecond light bursts in the train was indicated as the predominant mechanism leading to the control. We studied the same physical system with transient absorption spectroscopy using elliptically polarized infrared pulses or APTs with a varying number of pulses down to a single pulse. Our new results are not consistent with this kind of wavepacket interference being the dominant mechanism and show that its role in the control over the photoabsorption probability has to be rediscussed.
Highlights
In this paper we address a few remaining open questions: how does the system behave during the transition from multiple to SAP excitation? How strong is the modulation of the photoabsorption probability affected by the interference mechanisms initiated within the same attosecond pulse compared to the wavepacket interference generated by subsequent attosecond pulses?
In the second section we will present the results obtained by changing the time duration of the driving IR pulses used in the high-order harmonics generation (HHG) process, i.e. by effectively changing the number of pulses in the attosecond pulse trains (APTs)
We studied the optical response of helium atoms excited by an attosecond pulse train and a delayed few-cycle IR pulse in a TA scheme
Summary
The EWP interference picture proposed in [2] to explain the fast oscillation of the ionization probability of He is based on the assumption that two parts of an EWP excited by subsequent attosecond pulses of the train can interfere. A rough estimate of the number of pulses constituting the train in each case can be obtained by calculating the harmonic spectrum generated by the driving IR fields measured with SPIDER. In order to compare the oscillation contrast of different measurements, we calculated the IR-induced absorption Abs(E, τ ) as equal to − T (E, τ ) = T0(E) − T (E, τ ), where T (E, τ ) is the transmitted XUV spectrum with the IR pulse delayed by τ with respect to the APT. EIp = [20 eV, Ip] (red-dashed curves) This analysis shows that, both, the absorption enhancement around τ 0 and the 2ω-oscillations (ω being the driving frequency of the IR field), are not canceled out after the energy integration process. In this case a linear dependence of the oscillation amplitude on the IR pulse intensity was observed independent of the duration of the APT used in the experiment
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