High-harmonic generation: manipulating electron wavepackets with laser light

Rodrigo López-Martens,

Abstract: More than ten years ago, it was realized that the broadband emission harmonic that occurs when an atom is ionized in an intense infrared laser field can be a potential source of attosecond pulses. The characteristic plateau region of the harmonic spectrum spans from the ultraviolet into the soft x-ray region, thus in principle providing enough bandwidth to produce pulses as short as a few tens of attoseconds. However, the duration of the harmonic emission is not as short as its bandwidth would allow. Therefore, controlling the amplitude and phase of the harnessed harmonic radiation is the greatest obstacle to produce attosecond pulses.

Whereas the goal behind the development of femtosecond sources was the need to measure the dynamics of molecules and solids, the goal that drives attosecond science is the control over electronic processes. Electrons are much lighter than atoms and can move much faster, so attosecond pulses gives us a tool for studying the motion of electrons. One can imagine “pump-probe” experiments where a single attosecond pulse is used to directly excite electrons, and then the electrons are precisely directed within an atom or a molecule by the field oscillations of an intense femtosecond pulse. Such experiments require mastering the art of generating isolated attosecond pulses with phase-locked few-cycle laser pulses. Technologically more accessible and brighter in terms of photon numbers, attosecond pulse trains generated by 30-odd femtosecond laser pulses offer an alternative approach into attosecond science.

I will describe the progress we made at the Lund Institute of Technology in Sweden in designing and characterizing a controllable source of attosecond pulse trains. I will show that the arsenal of techniques developed for producing and controlling femtosecond pulses in the optical domain can be successfully applied in the XUV domain attosecond pulses usually roam in. Using these pulse trains as an injection mechanism of electron wavepackets via atomic ionization, we study the interaction between these electron wavepackets and a strong infrared laser field. Our results show that attosecond pulse trains can prove to be a useful and intuitive tool for studying and controlling a number of strong-field processes with attosecond precision.