Optimization of Ultrafast Strong-Field Phenomena

their native, attosecond time scale. Ultrafast electron dynamics is the driving
force behind chemical reactions, it determines the optical response of matter,
and it is the cornerstone of multiple ultrafast nanoscale imaging techniques.
Attosecond phenomena are often driven by strong-field light-matter interaction.
Femtosecond laser pulses with electric fields rivaling those of atomic binding
forces drive complex nonlinear phenomena in atoms, molecules, and solid state.
They include electron excitations, nonlinear frequency up-conversion known
as high-order harmonic generation (HHG), and emission of ultra-energetic
electrons via above-threshold ionization (ATI). These processes have important
roles in ultrafast technologies. For example, HHG is used as a source for
coherent X-ray pulses with durations down to attoseconds, ATI is used for
building electron wave packets for self-interrogation spectroscopy of matter,
and excited Rydberg-states of atoms are prime candidates for multi-qubit
quantum computing.
Control of strong-field attosecond phenomena can be achieved by shaping
the temporal profile of the driving femtosecond pulse in modern light-field
synthesizers. This dissertation is a computational expedition to shaping the
driving laser pulses for optimizing strong-field light-matter interaction in HHG,
ATI, and Rydberg-state preparation in atoms.
We begin this dissertation with a brief review of relevant strong-field attosecond
phenomena with an emphasis on their theoretical modeling. We continue
with an overview of control and optimization of these phenomena both from
an experimental and a computational point of view. Later, we describe in
detail the computational models we have used. The corresponding software is
provided in the online supplementary material.
Our optimization studies deliver experimentally feasible optimization/control
schemes for shaping the driving femtosecond laser pulses to increase the
maximum energy and signal strength of HHG and ATI in atomic gases. We also demonstrate how the optimized processes behind the optimized
HHG and ATI can be understood with a semiclassical three-step model. The
excitation of alkali metals to their Rydberg states is shown to be feasible with
multicolor femtosecond fields, decreasing the excitation time by several orders
of magnitude compared to traditional methods. On the downside, in its current
form the proposed scheme lacks the finesse to populate only a single final state.
We also develop a new finite element simulation suite for studying attosecond
phenomena in nanostructures. Nanostructures shape the spatial profile of the
driving laser field, something existing simulation software cannot easily model.
Our software suite is designed for simulating these systems efficiently, and it
can incorporate the spatial inhomogeneity of the driving field with ease.
We close this dissertation with a summary of our optimization studies and
obtained results. They are discussed in the context of other recent work in the
field, and we also reflect on possible improvements and directions for future
work.