Generation and Application of Attosecond Pulse Trains

Per Johnsson,
Lund Institute of Technology

Attosecond pulse trains (APTs) generated from high-order harmonics are a flexible attosecond source, since the amplitude and relative phase of the spectral components (the harmonics) can be tailored, giving control over both the duration and the carrier frequency of the pulses. When an APT interacts with a gas of atoms, a train of electron wave packets (EWPs) are generated through single-photon ionization. These wave packets are temporally localized with approximately the same duration as the attosecond pulses, and their properties are directly dependent on those of the XUV pulses. The flexibility offered by EWPs generated from APTs should make them a useful tool for the study and control of atomic and molecular strong-field processes, such as harmonic generation and multiple ionization.

At the Lund Laser Centre, we have demonstrated the generation, compression and delivery on target of attosecond pulses using external amplitude and phase control. Using this technique, it is possible to deliver on-target pulses as short as 160 as. I will present recent experiments performed using the APTs available at the Lund Laser Centre. In particular, I will describe two experiments where we study the dynamics of attosecond EWPs in a strong laser field.

In the first experiment, we use a tailored APT to create EWPs from argon and study the energy exchange between the EWPs and a strong IR field, as a function of the delay between the two fields. In this study a magnetic bottle electron spectrometer is used,collecting electrons emitted within a 2p solid angle. At the zero crossings of the laser field, a significant energy (~20 eV) is transferred from the IR field to the electrons resulting in dramatically enhanced above-threshold-ionization in conditions where the IR field alone does not induce any significant ionization of the medium. We also see a clear effect on the delay-dependence of the photoelectron spectrum when we change the initial properties of the EWPs by stretching the attosecond pulses.

In a second experiment, we instead use a velocity map imaging spectrometer in order to access the angular distribution of the emitted photoelectrons. When the EWPs are created using an APT synthesized from odd harmonics of the driving laser field, two EWPs per IRcycle are generated. By measuring the angular distributions, we are able to observeinterference between consecutive EWPs. This interference depends on the phase difference between consecutive EWPs induced by the IR field, but also bears information on the symmetry of the ground-state wave function of the target atoms.