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Series in Quantum Electronics
edited by
Henry Baltes, Peter Günter, Ursula Keller,
Fritz K. Kneubühl †, Walter Lukosz,
Hans Melchior, Markus W. Sigrist
Vol. 49
Florian Schapper
Attosecond Structure of High-Order
Harmonics
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
(
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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