The generation of ultra-short light pulses is a necessity to track the fastest dynamics in matter. While femtosecond lasers have opened the way to femtochemistry, allowing to follow the movements of atoms in molecules during chemical reactions, many electronic processes inside the atoms are still too fast to be resolved with these facilities, since they occur on a sub-femtosecond - so called attosecond - timescale (1 attosecond = 10^-18 s). Indeed, these lasers cannot provide pulses shorter than the period of the radiation they emit (2.7 fs at 800 nm), and are thus reaching their limits (the shortest reported laser pulses are 3.8 fs long). In order to break through the femtosecond barrier into the attosecond regime, the wavelength of the source must be decreased, as well as the spectral width increased. High order harmonic generation fulfills both requirements, and has recently opened the way to attophysics.

By focusing an intense laser pulse in a gas jet, one can generate odd harmonics of the laser frequency up to very high orders with a similar efficiency. If these harmonics are phase-locked, then the corresponding temporal profile is a train of pulses whose duration is set by the number of superposed harmonics (analogy with mode-locked lasers). In 2001, the first demonstrations of the attosecond structure of the harmonic emission were performed independently by the groups of P. Agostini (Saclay) and F. Krausz (Vienna). Since then, we have thoroughly characterized the attosecond pulses emitted by atoms and molecules and shown that they give insight into the ultrafast electron dynamics in the emission process. During this seminar, I will review our latest studies on atoms and molecules.