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Series in Quantum Electronics
Henry Baltes, Peter Günter, Ursula Keller,
Fritz K. Kneubühl †, Walter Lukosz,
Hans Melchior, Markus W. Sigrist
Attosecond Structure of High-Order
1st edition 2010. XXII, 122 pages, € 64,00.
ISBN-10: 3-86628-305-9, ISBN-13: 978-3-86628-305-3
High-order harmonics are a unique tool to investigate dynamics in atomic and molecular systems. They are generated in a coherent up-conversion process when an intense, ultrashort laser pulse interacts with a target medium. The emitted harmonic radiation spans a spectrum from the ultraviolet down to the extreme ultraviolet and soft X-ray region, enabling pulse durations in the attosecond (<![if !vml]><![endif]> s) regime.
In this thesis, titanium-sapphire based laser amplifier systems are used to generate intense, ultrashort laser pulses which subsequently interact with rare gas targets to generate high-order harmonics. In a first step, experimental conditions have been found to spectrally and spatially resolve the interference in the harmonic emission from different electron trajectories, or quantum paths. These quantum-path interferences reveal themselves as a modulation of the harmonic yield with intensity. The excursion times of the electrons are found to vary on an attosecond time-scale with intensity.
Furthermore, the temporal structure of high-order harmonics is studied in this thesis. When a multi-cycle laser pulse interacts with a gas atom under suitable experimental conditions, a pulse with attosecond duration is emitted each half-cycle of the driving laser field, forming an attosecond pulse train (APT). The average duration of the pulses in an APT is determined with the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions)-technique. Part of this thesis is the design and implementation of a beamline suitable for versatile XUV-IR pump-probe experiments with attosecond resolution.
While an APT can be routinely generated with a multi-cycle laser pulse, the generation of an isolated attosecond pulse is more challenging. It requires a temporal gate that restricts the harmonic emission to a single half-cycle of the driving laser field. In this thesis, a new method based on polarization gating is proposed. We suggest a polarization-shaping by using two cross-polarized laser pulses, one with a small amount of group-delay dispersion added, to generate a tunable gate in time. With this method, we were able to generate continuous harmonic spectra starting with 12-fs laser pulses, indicating the generation of an isolated attosecond pulse.
Florian Schapper received his diploma degree in physics from the FU Berlin in 2005. He joined the Institute for Quantum Electronics at ETH Zurich the same year. His research focused on few-cycle pulse generation by filamentation, high-order harmonic generation in noble gases and the generation and characterization of attosecond pulses.
Keywords: harmonics, high-order harmonic generation, attosecond pulses, dynamics in atomic systems, dynamics in molecular systems
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