Focusing properties of attosecond pulses

Abstract: The discovery of high-order harmonic generation (HHG) has made it possible to see the electron dynamics on their natural time scale, attoseconds. In the HHG process, a femtosecond infrared (IR) laser pulse is converted into a train of attosecond pulses in the extreme ultraviolet (XUV), but with a very low conversion efficiency. The low intensity has to be used efficiently and that means keeping the photons compact in both space and time at the interaction with the matteryou want to study. In this thesis, the focusing properties have been measured experimentally of high-order harmonics generated in Ar using a multi terawatt laser system in a loose focusing geometry. A few micrometre focus is obtained using focusing optics with a high demagnification factor. The waist size and position of the individual harmonics have been measured using a knifeedgetechnique and the SWORD (spectral wavefront optical reconstruction by di↵raction) technique, while varying generation parameters. The measurements show that the harmonic focus positions are spread out along the propagation axis, which consequently decreases the peak intensity and increases the pulse duration due to the local narrowed bandwidth. However, there is a way to improve the focus by choosing the right generation parameters.