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Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 Podcast
Modeling light-field-controlled electron motion in atoms and solids

Modeling light-field-controlled electron motion in atoms and solids

Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 vor 14 Jahren

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Recent advancements in laser technology are quickly moving the
frontiers of research: quantum dynamics can now be investigated in
more detail, on new timescales, with an unprecedented level of
control. These new possibilities offer a new ground for the
theoretical study of fundamental processes; at the same time, a
proper understanding of phenomena involved is necessary to explain
measurements, and to indicate directions for further experiments.
This Thesis deals with the theoretical investigation of particular
cases of light-matter interaction, in atoms and in dielectrics.
Regimes considered here have just become a subject of intensive
investigation: they are acquiring more and more relevance as
technological advancements make them experimentally accessible. In
the first part of the Thesis I consider a process as fundamental as
the single-photon ionization of atoms: my modeling will include an
ultrashort pulse (full width half maximum ~ 100 as = 10^-16 s)
exciting an electron to the continuum, and a strong few-cycle
near-infrared laser field. This configuration is suitable to
reproduce recent streaking experiments on atoms. I developed a
numerical tool to simulate these dynamics in three dimensions: the
process is quite elaborate and requires an adequate description of
multi-electron atoms. With proper approximations I was able to
calculate photoelectron spectra using just a few dipole matrix
elements, which were obtained with the aid of our external
collaborators, from refined atomic structure calculations. The
results of our relatively simple tool are in very good agreement
with more sophisticated numerical calculations. In addition to
that, I discuss my contribution to the theoretical support of a
fundamental experiment [I]: both simulations and measurements
indicate a delay between two different channel of photoemission in
neon. A careful investigation of the limit of validity of
approximations employed reveals that the Coulomb-Volkov
approximation is not suitable to describe fine details of the
interaction with the laser pulse. I also report on our analysis of
experimental data from angle-resolved attosecond streaking. The
second part of the Thesis is devoted to the investigation of
inter-band excitations in dielectrics; driving this process with a
high degree of control is on the edge of current technology. The
ultrafast creation of charge carriers in an insulator is
intriguing: dielectric properties of the medium change drastically,
revealing features of the peculiar electron dynamics in such a
situation. I have simulated this process solving the time dependent
Schroedinger equation for a single electron in a one-dimensional
lattice and analyzed how the charge Q displaced during the
interaction with the pulse depends on laser parameters. These
calculations reproduce to a good extent the behavior observed in
the experiment. Both the theory and the experiment point out a
strong dependence of Q on laser parameters: this promises a high
degree of control, and at the same time suggests the possibility of
a solid-state device to characterize an optical pulse. I also study
in detail the modification occurring in the electric response of
the sample to the electric field. The purpose of this analysis is
to identify some features directly related to dynamics of newly
created charge carriers. During my investigation of electron
dynamics during an excitation process, I have often faced the
difficulty to identify quantities which might resemble eigenstates
of the time-dependent Hamiltonian. Similar field-dressed states
would describe the distortion due to the field, of eigenstates of
the field-free Hamiltonian. A proper definition of field-dressed
states would allow a correct interpretation of the wavefunction in
terms of instantaneous excited population, which is otherwise
impossible to define.