Characterisation and Applications of Ultrashort Extreme Ultraviolet Pulses

Abstract: When a strong laser field interacts with free atoms, harmonics up to very high orders are created. This extreme ultraviolet (XUV) radiation has interesting properties, such as spatial and temporal coherence, short pulse duration and high brightness. This thesis concentrates on the temporal aspects of this radiation. A method to characterise the pulse duration and time-dependent frequency (chirp) of such harmonic pulses has been developed. To perform the characterisation, the energy and amplitude of a photoelectron peak, generated through ionization of a rare gas after absorption of one XUV and one infrared laser photon, is measured as a function of the delay between the two pulses. In addition, by manipulating the chirp of the driving laser, positive or negative chirp could be induced on the harmonic radiation. By creating two sources of mutually coherent harmonics and analysing the interference in the far field, the first-order autocorrelation function has been measured. By overlapping the two sources, a high order autocorrelation trace of the laser was recorded. An XUV monochromator capable of extracting single harmonic frequencies without stretching them in time has been designed. Experiments have been performed on a prototype working in the infrared wavelength region. High-order harmonics have been used to perform XUV interferometric measurements. The beams were divided in the infrared before the generation of harmonics. The thickness of aluminium filters and the electron density of a laser-produced plasma were determined. In another application, harmonics have been used to perform lifetime measurements of Rydberg states in acetylene.