Preface
xi
and, thus, energy exchange between states. Thus, a natural motivation for timeresolving investigations and the consideration of temporal characteristics is given.
It seems that who wants to understand nature needs to understand resonances, and
excitations of the respective system related to those resonances, and the dynamically
resulting conglomerates which give birth to a complex world full of interactions,
energy exchange, and transformations. They all resemble different states of one
and the same overall entity, the different facets of which can unfold a plethora of
impressions and wonders, described by the approximative laws of physics, which
define rules based on observations and a sense for logic, which originates from the
aspiration of order and predictability.
Besides the daily routine of modern-day scientists driven by paper publishing,
project acquisition, and device/method optimisation, the exploratory and philosophical side of research deserves some attention as well. I allow myself to tap a bit the
philosophy part related to my activities as a Ph.D./Dr. rer. nat. in the consideration
of the subjects of nature in the lines above, as one hardly devotes the academic
work time to a detached view on the own research. A tiny segment of this work
may be philosophical before our attention is diverted towards elaborate summaries
of scientific work within the scope of a Habilitation project. I believe that this may
be a good approach (in addition to promoting the fascinating technologies typically
obtained through scientific work) to substantially motivate the next generation of
young scientists to commit themselves to often very narrow-focused research, and to
trigger the sustainable interest of, in particular, the youth in knowledge-based topics
and science. All the other times, enough words about scientific achievements and a
bit of science fiction are spent in review articles, papers, and press releases.
Let us now briefly think about excitations, such as excitons and exciton complexes
in materials. In quasi-perfect crystals with suppressed decay channels, their lifetime
can be even noticeable for us, and formation of multi-particle electronically bound
species is facilitated, analog to molecule formation with atoms. Or, in crystalline
quantum materials, their binding energies can be high enough to enable the study of
their properties in the presence of a big bath of lattice vibrations, with which they
can couple to form even other entities such as polarons. Or, at low temperatures
with reduced decoherence rates, reversible energy exchange with other oscillators
can shape their behaviour drastically, as observed in the strong-coupling regime of a
matter polarisation wave (coherent excitons) with the propagating electromagnetic
mode (light in matter) or with photonic modes of high-quality optical microcavities,
giving rise to hybrid light–matter quasi-particles (exciton–polaritons).
Individually considered, matter excitations such as in 2D crystals with their zoo
of excitonic complexes across opposing valleys and spin–orbit split bands can act
as a distant analog for the elementary particles of our world, from which compound
particles are formed, e.g. protons and neutrons composed of the quarks—excitations
of their quantum field described in the frame of quantum chromodynamics. In simpler
terms, the well-known Coulomb-bound electron–hole pair referred to as exciton is
the popular analog of a hydrogen-like (or more precisely, positronium-like) system
within solids. All this focuses on the particle nature, whereas we do not want to forget
that every excitation resembles a quant occupying a quantum-mechanical mode of the
xi
and, thus, energy exchange between states. Thus, a natural motivation for timeresolving investigations and the consideration of temporal characteristics is given.
It seems that who wants to understand nature needs to understand resonances, and
excitations of the respective system related to those resonances, and the dynamically
resulting conglomerates which give birth to a complex world full of interactions,
energy exchange, and transformations. They all resemble different states of one
and the same overall entity, the different facets of which can unfold a plethora of
impressions and wonders, described by the approximative laws of physics, which
define rules based on observations and a sense for logic, which originates from the
aspiration of order and predictability.
Besides the daily routine of modern-day scientists driven by paper publishing,
project acquisition, and device/method optimisation, the exploratory and philosophical side of research deserves some attention as well. I allow myself to tap a bit the
philosophy part related to my activities as a Ph.D./Dr. rer. nat. in the consideration
of the subjects of nature in the lines above, as one hardly devotes the academic
work time to a detached view on the own research. A tiny segment of this work
may be philosophical before our attention is diverted towards elaborate summaries
of scientific work within the scope of a Habilitation project. I believe that this may
be a good approach (in addition to promoting the fascinating technologies typically
obtained through scientific work) to substantially motivate the next generation of
young scientists to commit themselves to often very narrow-focused research, and to
trigger the sustainable interest of, in particular, the youth in knowledge-based topics
and science. All the other times, enough words about scientific achievements and a
bit of science fiction are spent in review articles, papers, and press releases.
Let us now briefly think about excitations, such as excitons and exciton complexes
in materials. In quasi-perfect crystals with suppressed decay channels, their lifetime
can be even noticeable for us, and formation of multi-particle electronically bound
species is facilitated, analog to molecule formation with atoms. Or, in crystalline
quantum materials, their binding energies can be high enough to enable the study of
their properties in the presence of a big bath of lattice vibrations, with which they
can couple to form even other entities such as polarons. Or, at low temperatures
with reduced decoherence rates, reversible energy exchange with other oscillators
can shape their behaviour drastically, as observed in the strong-coupling regime of a
matter polarisation wave (coherent excitons) with the propagating electromagnetic
mode (light in matter) or with photonic modes of high-quality optical microcavities,
giving rise to hybrid light–matter quasi-particles (exciton–polaritons).
Individually considered, matter excitations such as in 2D crystals with their zoo
of excitonic complexes across opposing valleys and spin–orbit split bands can act
as a distant analog for the elementary particles of our world, from which compound
particles are formed, e.g. protons and neutrons composed of the quarks—excitations
of their quantum field described in the frame of quantum chromodynamics. In simpler
terms, the well-known Coulomb-bound electron–hole pair referred to as exciton is
the popular analog of a hydrogen-like (or more precisely, positronium-like) system
within solids. All this focuses on the particle nature, whereas we do not want to forget
that every excitation resembles a quant occupying a quantum-mechanical mode of the