Temporal Aspects of High-Intensity Laser-Matter Interactions

Sammanfattning: The availability of short-pulse, high-intensity lasers has opened doors to new areas in atomic and plasma physics. The short pulses (a few femtoseconds), that are available today, enable unprecedented temporal measurements, while the high peak power accessible (several terawatts) allows physicists to study the interaction between light and relativistic plasmas. The aim of this work was twofold: i) to generate and characterize extreme ultraviolet (XUV) harmonic emission from the interaction between a gas and an intense laser pulse, and ii) to study phenomena occurring in the interaction between laser-produced plasmas and laser pulses with intensities exceeding 10¹⁸ W/cm².

The first part was aimed at improving our understanding of high-order harmonic emission in general and its time–frequency structure in particular. Spectrally, this harmonic emission consists of a comb of frequencies where each tooth in the “comb” corresponds to a pulse with a duration of the order of the laser pulse driving the generation process (typically a few tens of femtoseconds). Temporally, the superposition of several of these harmonics may form a train of attosecond pulses.

In this work different methods of characterizing these XUV pulses have been studied, both experimentally and theoretically. Based on the technique of so-called sideband generation, which is simply the cross-correlation signal between XUV pulses and ultrashort infrared pulses (~ 10 fs) in a photoelectron spectrum, time–frequency characterization of individual harmonics has been demonstrated. Using a similar technique, trains of pulses of attosecond duration have also been observed.

In the second part, the interaction between plasmas and laser pulses at relativistic intensities has been studied. Fascinating phenomena are observed, such as relativistic channelling and the acceleration of electrons to energies of several MeV over distances as short as 1 mm. The work focused mainly on the influence of the laser pulse duration in the laser–plasma interaction.