Measurement and Control of Attosecond Light Fields

Detta är en avhandling från Department of Physics, Lund University

Sammanfattning: Attosecond pulses are used to study electron dynamics in atoms and molecules. In this thesis, schemes to control the generation of attosecond pulses and pulse-shaping techniques to compress the pulses are presented. Generation of attosecond pulses requires high intensity, which is reached by focusing a pulsed femtosecond laser. The emitted pulses come isolated or in an attosecond pulse train (APT), depending on the duration of the driving field. In several experiments, we have controlled the pulse repetition rate in the APT by adding the second harmonic to the driving field. An APT with one pulse per cycle of the driving field is then generated, instead of a train with two pulses per cycle, which is the case for a one-color field. A rather strong second harmonic changes the shape of the generating field, which leads to a tunable central photon energy of the attosecond pulses. With a short driving field an APT containing few pulses is generated. The spectrum of a short APT shows additional interference structures. In analogy with multi-slit interference, these structures are secondary maxima, positioned in between the principal maxima. The number of secondary maxima is related to the number of pulses in the APT. Attosecond pulses are emitted by a macroscopic medium. How the macroscopic conditions affect the pulse duration has also been studied. Directly after the generation the attosecond pulses have, in general, a relatively long pulse duration. Spectral filtering is important to shape the spectrum and compress the pulses. We have used thin transmission filters and multi-layer XUV-mirrors for filtering. We measured a pulse duration of 130 as, for attosecond pulses generated in Ne and filtered by Zr. Most schemes to characterize attosecond pulses are based on a cross-correlation with an IR field. We have used the RABITT (reconstruction of attosecond beating by interference of two-photon transitions) and the AC-streak camera techniques, capable of measuring different types of APT:s. Finally, attosecond pulses have been used in two application experiments: Momentum shearing interferometry; and the Quantum stroboscope, where electron scattering off the atomic potential was observed.