Attosecond Optical and Electronic Wave Packets

When a low-frequency laser pulse is focused to a high intensity in a gas, the electric field of the laser may become comparable to, or even exceed, the electric field between the electrons and the nucleus in the atom. Under such conditions, through a process known as high-order harmonic generation, bursts of extreme ultraviolet radiation may be emitted, with durations in the attosecond domain (1 as = 10^{-18} s), which is the time-scale of electronic processes. In the work presented in this thesis, attosecond pulse trains (APTs) have been generated in the laboratory. These APTs have further been characterized and finally used in a number of applications.

The first series of experiments was focused on the generation, control and characterization of high-order harmonics on the femtosecond time-scale, corresponding to the duration of the driving pulse. The time-frequency structure of individual harmonics does not significantly affect the properties of the individual attosecond pulses, but is, however, important for the overall structure of the APT, which is also the subject of some theoretical investigations included in this work.

In the second series of experiments, the production and measurement of attosecond pulses in an APT were successfully performed. In addition, external phase control of the attosecond pulses was demonstrated, by means of metallic filters, leading to post-compression of the pulses down to a duration of 170 as, which was, at that time, the shortest pulse duration ever reported.

Finally, the APTs were applied to inject electron wave packets (EWPs), through single-photon ionization, into an external low-frequency laser field. By using the pulses in the APT to obtain precise timing of the ionization, control of the ejected EWPs, and even of the ionization process itself, by the external field, was demonstrated. It has also been shown that, making use of the external control offered by the APTs, it is possible to perform interference experiments on continuum EWPs, in a way very similar to that of traditional interference experiments with photons.