@phdthesis{Scholz2012,
  author    = {Scholz, Markus Reiner},
  title     = {Spin polarization, circular dichroism, and robustness of topological surface states},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-96686},
  school      = {Universit{\"a}t Potsdam},
  pages     = {153},
  year      = {2012},
  abstract  = {Dreidimensionale topologische Isolatoren sind ein neues Materialsystem, welches dadurch charakterisiert ist, dass es in seinem Inneren isolierend an der Ober {\"a}che jedoch leitend ist. Urs{\"a}chlich f{\"u}r die Leitf{\"a}higkeit an der Ober {\"a}che sind sogenannte topologische Ober- {\"a}chenzust{\"a}nde, welche das Valenzband des Inneren mit dem Leitungsband des Inneren verbinden. An der Ober {\"a}che ist also die Bandl{\"u}cke, welche die isolierende Eigenschaft verursacht, geschlossen. Die vorliegende Arbeit untersucht diese Ober {\"a}chenzust{\"a}nde mittels spin- und winkelauf- gel{\"o}ster Photoemissionsspektroskopie. Es wird gezeigt, dass in den Materialien Bi2Se3 und Bi2Te3, in {\"u}bereinstimmung mit der Literatur, die entscheidenden Charakteristika eines topologischen Ober {\"a}chenzustands vorzu nden sind: Die Ober {\"a}chenzust{\"a}nde dieser Sys- teme durchqueren die Bandl{\"u}cke in ungerader Anzahl, sie sind nicht entartet und weisen folgerichtig eine hohe Spinpolarisation auf. Weiterhin wird durch Aufdampfen diverser Adsorbate gezeigt, dass der Ober {\"a}chenzust{\"a}n- de von Bi2Se3 und Bi2Te3, wie erwartet, extrem robust ist. Ober {\"a}chenzust{\"a}nde topologisch trivialer Systeme erf{\"u}llen diese Eigenschaft nicht; bereits kleine Verunreinigungen k{\"o}n- nen diese Zust{\"a}nde zerst{\"o}ren, bzw. die Ober {\"a}che isolierend machen. Die topologischen Ober {\"a}chenzust{\"a}nde k{\"o}nnen in der vorliegenden Arbeit noch bis zur Detektionsgrenze der experimentellen Messmethode nachgewiesen werden und die Ober {\"a}che bleibt Leitf{\"a}hig. Unter den Adsorbaten be ndet sich auch Eisen, ein bekanntermaßen magnetisches Materi- al. Eine der Grundvoraussetzungen f{\"u}r topologische Isolatoren ist die Zeitumkehrsymme- trie, die Elektronen, welche den topologischen Ober {\"a}chenzustand besetzen, vorschreibt, dass sie eine bestimmte Spinrichtung haben m{\"u}ssen, wenn sie sich beispielsweise nach links bewegen und den entgegengesetzten Spin wenn sie sich nach rechts bewegen. In magnetischen Materialien ist die Zeitumkehrsymmetrie jedoch explizit gebrochen und die gezeigte Robustheit des Ober {\"a}chenzustands gegen magnetische Materialien daher uner- wartet. Die Zeitumkehrsymmetrie sorgt auch daf{\"u}r, dass eine Streuung der Elektronen um 180°, beispielsweise an einem Gitterdefekt oder an einem Phonon strikt verboten ist. Bei einem solchen Streuprozess bleibt die Spinrichtung erhalten, da aber in der Gegenrichtung nur Zust{\"a}nde mit entgegengesetztem Spin vorhanden sind kann das Elektron nicht in diese Richtung gestreut werden. Dieses Prinzip wird anhand der Lebensdauer der durch Pho- toemission angeregten Zust{\"a}nde untersucht. Hierbei wird gezeigt, dass die Kopplung der Elektronen des Ober {\"a}chenzustands von Bi2Te3 an Phononen unerwartet hoch ist und dass sich eine Anisotropie in der Bandstruktur des Selbigen auch in den Lebensdauern der ange- regten Zust{\"a}nde widerspiegelt. Weiterhin wird gezeigt, dass sich die Ein {\"u}sse von magne- tischen und nicht-magnetischen Verunreinigungen auf die Lebensdauern stark voneinander unterscheiden. Im letzten Teil der vorliegenden Arbeit wird untersucht, ob eine Asymmetrie in der Inten- sit{\"a}tsverteilung der winkelaufgel{\"o}sten Photoemissionsspektren, bei Anregung mit zirku- lar polarisiertem Licht, in Bi2Te3 R{\"u}ckschl{\"u}sse auf die Spinpolarisation der Elektronen erlaubt. Bei Variation der Energie des eingestrahlten Lichts wird ein Vorzeichenwechsel der Asymmetrie beobachtet. Daraus l{\"a}sst sich schlussfolgern, dass die Asymmetrie keine R{\"u}ckschl{\"u}sse auf die Spinpolarisation erlaubt.},
  language  = {en}
}
@phdthesis{ThielemannKuehn2017,
  author    = {Thielemann-K{\"u}hn, Nele},
  title     = {Optically induced ferro- and antiferromagnetic dynamics in the rare-earth metal dysprosium},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-402994},
  school      = {Universit{\"a}t Potsdam},
  pages     = {iv, 121},
  year      = {2017},
  abstract  = {Approaching physical limits in speed and size of today's magnetic storage and processing technologies demands new concepts for controlling magnetization and moves researches on optically induced magnetic dynamics. Studies on photoinduced magnetization dynamics and their underlying mechanisms have been primarily performed on ferromagnetic metals. Ferromagnetic dynamics bases on transfer of the conserved angular momentum connected with atomic magnetic moments out of the parallel aligned magnetic system into other degrees of freedom. In this thesis the so far rarely studied response of antiferromagnetic order to ultra-short optical laser pulses in a metal is investigated. The experiments were performed at the FemtoSpex slicing facility at the storage ring BESSY II, an unique source for ultra-short elliptically polarized x-ray pulses. Laser-induced changes of the 4f-magnetic order parameter in ferro- and antiferromagnetic dysprosium (Dy), were studied by x-ray methods, which yield directly comparable quantities. The discovered fundamental differences in the temporal and spatial behavior of ferro- and antiferrmagnetic dynamics are assinged to an additional channel for angular momentum transfer, which reduces the antiferromagnetic order by redistributing angular momentum within the non-parallel aligned magnetic system, and hence conserves the zero net magnetization. It is shown that antiferromagnetic dynamics proceeds considerably faster and more energy-efficient than demagnetization in ferromagnets. By probing antiferromagnetic order in time and space, it is found to be affected along the whole sample depth of an in situ grown 73 nm tick Dy film. Interatomic transfer of angular momentum via fast diffusion of laser-excited 5d electrons is held responsible for the out-most long-ranging effect. Ultrafast ferromagnetic dynamics can be expected to base on the same origin, which however leads to demagnetization only in regions close to interfaces caused by super-diffusive spin transport. Dynamics due to local scattering processes of excited but less mobile electrons, occur in both magnetic alignments only in directly excited regions of the sample and on slower pisosecond timescales. The thesis provides fundamental insights into photoinduced magnetic dynamics by directly comparing ferro- and antiferromagnetic dynamics in the same material and by consideration of the laser-induced magnetic depth profile.},
  language  = {en}
}
@phdthesis{Solopow2019,
  author    = {Solopow, Sergej},
  title     = {Wavelength dependent demagnetization dynamics in Co2MnGa Heusler-alloy},
  doi       = {10.25932/publishup-42786},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427860},
  school      = {Universit{\"a}t Potsdam},
  pages     = {91},
  year      = {2019},
  abstract  = {In dieser Arbeit haben wir ultraschnelle Entmagnetisierung an einer Heusler-Legierung untersucht. Es handelt sich um ein Halbmetall, das sich in einer ferromagnetischen Phase befindet. Die Besonderheit dieses Materials besteht im Aufbau einer Bandstruktur. Diese bildet Zustandsdichten, in der die Majorit{\"a}tselektronen eine metallische B{\"a}nderbildung aufweisen und die Minorit{\"a}tselektronen eine Bandl{\"u}cke in der N{\"a}he des Fermi-Niveaus aufweisen, das dem Aufbau eines Halbleiters entspricht. Mit Hilfe der Pump-Probe-Experimente haben wir zeitaufgel{\"o}ste Messungen durchgef{\"u}hrt. F{\"u}r das Pumpen wurden ultrakurze Laserpulse mit einer Pulsdauer von 100 fs benutzt. Wir haben dabei zwei verschiedene Wellenl{\"a}ngen mit 400 nm und 1240 nm benutzt, um den Effekt der Prim{\"a}ranregung und der Bandl{\"u}cke in den Minorit{\"a}tszust{\"a}nden zu untersuchen. Dabei wurde zum ersten Mal OPA (Optical Parametrical Amplifier) f{\"u}r die Erzeugung der langwelligen Pulse an der FEMTOSPEX-Beamline getestet und erfolgreich bei den Experimenten verwendet. Wir haben Wellenl{\"a}ngen bedingte Unterschiede in der Entmagnetisierungszeit gemessen. Mit der Erh{\"o}hung der Photonenenergie ist der Prozess der Entmagnetisierung deutlich schneller als bei einer niedrigeren Photonenenergie. Wir verkn{\"u}pften diese Ergebnisse mit der Existenz der Energiel{\"u}cke f{\"u}r Minorit{\"a}tselektronen. Mit Hilfe lokaler Elliot-Yafet-Streuprozesse k{\"o}nnen die beobachteten Zeiten gut erkl{\"a}rt werden. Wir haben in dieser Arbeit auch eine neue Probe-Methode f{\"u}r die Magnetisierung angewandt und somit experimentell deren Effektivit{\"a}t, n{\"a}mlich XMCD in Refletiongeometry, best{\"a}tigen k{\"o}nnen. Statische Experimente liefern somit deutliche Indizien daf{\"u}r, dass eine magnetische von einer rein elektronischen Antwort des Systems getrennt werden kann. Unter der Voraussetzung, dass die Photonenenergie der R{\"o}ntgenstrahlung auf die L3 Kante des entsprechenden Elements eingestellt, ein geeigneter Einfallswinkel gew{\"a}hlt und die zirkulare Polarisation fixiert wird, ist es m{\"o}glich, diese Methode zur Analyse magnetischer und elektronischer Respons anzuwenden.},
  language  = {en}
}
@phdthesis{Arora2018,
  author    = {Arora, Ashima},
  title     = {Optical and electric field control of magnetism},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-421479},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ii, 126},
  year      = {2018},
  abstract  = {Future magnetic recording industry needs a high-density data storage technology. However, switching the magnetization of small bits requires high magnetic fields that cause excessive heat dissipation. Therefore, controlling magnetism without applying external magnetic field is an important research topic for potential applications in data storage devices with low power consumption. Among the different approaches being investigated, two of them stand out, namely i) all-optical helicity dependent switching (AO-HDS) and ii) ferroelectric control of magnetism. This thesis aims to contribute towards a better understanding of the physical processes behinds these effects as well as reporting new and exciting possibility for the optical and/or electric control of magnetic properties. Hence, the thesis contains two differentiated chapters of results; the first devoted to AO-HDS on TbFe alloys and the second to the electric field control of magnetism in an archetypal Fe/BaTiO3 system. In the first part, the scalability of the AO-HDS to small laser spot-sizes of few microns in the ferrimagnetic TbFe alloy is investigated by spatially resolving the magnetic contrast with photo-emission electron microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD). The results show that the AO-HDS is a local effect within the laser spot size that occurs in the ring-shaped region in the vicinity of thermal demagnetization. Within the ring region, the helicity dependent switching occurs via thermally activated domain wall motion. Further, the thesis reports on a novel effect of thickness dependent inversion of the switching orientation. It addresses some of the important questions like the role of laser heating and the microscopic mechanism driving AO-HDS. The second part of the thesis focuses on the electric field control of magnetism in an artificial multiferroic heterostructure. The sample consists of an Fe wedge with thickness varying between 0:5 nm and 3 nm, deposited on top of a ferroelectric and ferroelastic BaTiO3 [001]-oriented single crystal substrate. Here, the magnetic contrast is imaged via PEEM and XMCD as a function of out-of-plane voltage. The results show the evidence of the electric field control of superparamagnetism mediated by a ferroelastic modification of the magnetic anisotropy. The changes in the magnetoelastic anisotropy drive the transition from the superparamagnetic to superferromagnetic state at localized sample positions.},
  language  = {en}
}
@phdthesis{Roland2017,
  author    = {Roland, Steffen},
  title     = {Charge carrier recombination and open circuit voltage in organic solar cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-397721},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VI, 145},
  year      = {2017},
  abstract  = {Tremendous progress in the development of thin film solar cell techniques has been made over the last decade. The field of organic solar cells is constantly developing, new material classes like Perowskite solar cells are emerging and different types of hybrid organic/inorganic material combinations are being investigated for their physical properties and their applicability in thin film electronics. Besides typical single-junction architectures for solar cells, multi-junction concepts are also being investigated as they enable the overcoming of theoretical limitations of a single-junction. In multi-junction devices each sub-cell operates in different wavelength regimes and should exhibit optimized band-gap energies. It is exactly this tunability of the band-gap energy that renders organic solar cell materials interesting candidates for multi-junction applications. Nevertheless, only few attempts have been made to combine inorganic and organic solar cells in series connected multi-junction architectures. Even though a great diversity of organic solar cells exists nowadays, their open circuit voltage is usually low compared to the band-gap of the active layer. Hence, organic low band-gap solar cells in particular show low open circuit voltages and the key factors that determine the voltage losses are not yet fully understood. Besides open circuit voltage losses the recombination of charges in organic solar cells is also a prevailing research topic, especially with respect to the influence of trap states. The exploratory focus of this work is therefore set, on the one hand, on the development of hybrid organic/inorganic multi-junctions and, on the other hand, on gaining a deeper understanding of the open circuit voltage and the recombination processes of organic solar cells. In the first part of this thesis, the development of a hybrid organic/inorganic triple-junction will be discussed which showed at that time (Jan. 2015) a record power conversion efficiency of 11.7\%. The inorganic sub-cells of these devices consist of hydrogenated amorphous silicon and were delivered by the Competence Center Thin-Film and Nanotechnology for Photovoltaics in Berlin. Different recombination contacts and organic sub-cells were tested in conjunction with these inorganic sub-cells on the basis of optical modeling predictions for the optimal layer thicknesses to finally reach record efficiencies for this type of solar cells. In the second part, organic model systems will be investigated to gain a better understanding of the fundamental loss mechanisms that limit the open circuit voltage of organic solar cells. First, bilayer systems with different orientation of the donor and acceptor molecules were investigated to study the influence of the donor/acceptor orientation on non-radiative voltage loss. Secondly, three different bulk heterojunction solar cells all comprising the same amount of fluorination and the same polymer backbone in the donor component were examined to study the influence of long range electrostatics on the open circuit voltage. Thirdly, the device performance of two bulk heterojunction solar cells was compared which consisted of the same donor polymer but used different fullerene acceptor molecules. By this means, the influence of changing the energetics of the acceptor component on the open circuit voltage was investigated and a full analysis of the charge carrier dynamics was presented to unravel the reasons for the worse performance of the solar cell with the higher open circuit voltage. In the third part, a new recombination model for organic solar cells will be introduced and its applicability shown for a typical low band-gap cell. This model sheds new light on the recombination process in organic solar cells in a broader context as it re-evaluates the recombination pathway of charge carriers in devices which show the presence of trap states. Thereby it addresses a current research topic and helps to resolve alleged discrepancies which can arise from the interpretation of data derived by different measurement techniques.},
  language  = {en}
}
@phdthesis{Mitzscherling2015,
  author    = {Mitzscherling, Steffen},
  title     = {Polyelectrolyte multilayers for plasmonics and picosecond ultrasonics},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-80833},
  school      = {Universit{\"a}t Potsdam},
  pages     = {93},
  year      = {2015},
  abstract  = {This thesis investigates the application of polyelectrolyte multilayers in plasmonics and picosecond acoustics. The observed samples were fabricated by the spin-assisted layer-by-layer deposition technique that allowed a precise tuning of layer thickness in the range of few nanometers. The first field of interest deals with the interaction of light-induced localized surface plasmons (LSP) of rod-shaped gold nanoparticles with the particles' environment. The environment consists of an air phase and a phase of polyelectrolytes, whose ratio affects the spectral position of the LSP resonance. Measured UV-VIS spectra showed the shift of the LSP absorption peak as a function of the cover layer thickness of the particles. The data are modeled using an average dielectric function instead of the dielectric functions of air and polyelectrolytes. In addition using a measured dielectric function of the gold nanoparticles, the position of the LSP absorption peak could be simulated with good agreement to the data. The analytic model helps to understand the optical properties of metal nanoparticles in an inhomogeneous environment. The second part of this work discusses the applicability of PAzo/PAH and dye-doped PSS/PAH polyelectrolyte multilayers as transducers to generate hypersound pulses. The generated strain pulses were detected by time-domain Brillouin scattering (TDBS) using a pump-probe laser setup. Transducer layers made of polyelectrolytes were compared qualitatively to common aluminum transducers in terms of measured TDBS signal amplitude, degradation due to laser excitation, and sample preparation. The measurements proved that fast and easy prepared polyelectrolyte transducers provided stronger TDBS signals than the aluminum transducer. AFM topography measurements showed a degradation of the polyelectrolyte structures, especially for the PAzo/PAH sample. To quantify the induced strain, optical barriers were introduced to separate the transducer material from the medium of the hypersound propagation. Difficulties in the sample preparation prohibited a reliable quantification. But the experiments showed that a coating with transparent polyelectrolytes increases the efficiency of aluminum transducers and modifies the excited phonon distribution. The adoption of polyelectrolytes to the scientific field of picosecond acoustics enables a cheap and fast fabrication of transducer layers on most surfaces. In contrast to aluminum layers the polyelectrolytes are transparent over a wide spectral range. Thus, the strain modulation can be probed from surface and back.},
  language  = {en}
}
@phdthesis{Kurpiers2019,
  author    = {Kurpiers, Jona},
  title     = {Probing the pathways of free charge generation and recombination in organic solar cells},
  doi       = {10.25932/publishup-42909},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-429099},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VI, 128, xxi},
  year      = {2019},
  abstract  = {Organic semiconductors are a promising class of materials. Their special properties are the particularly good absorption, low weight and easy processing into thin films. Therefore, intense research has been devoted to the realization of thin film organic solar cells (OPVs). Because of the low dielectric constant of organic semiconductors, primary excitations (excitons) are strongly bound and a type II heterojunction needs to be introduced to split these excitations into free charges. Therefore, most organic solar cells consist of at least an electron donor and electron acceptor material. For such donor acceptor systems mainly three states are relevant; the photoexcited exciton on the donor or acceptor material, the charge transfer state at the donor-acceptor interface and the charge separated state of a free electron and hole. The interplay between these states significantly determines the efficiency of organic solar cells. Due to the high absorption and the low charge carrier mobilities, the active layers are usually thin but also, exciton dissociation and free charge formation proceeds rapidely, which makes the study of carrier dynamics highly challenging. Therefore, the focus of this work was first to install new experimental setups for the investigation of the charge carrier dynamics in complete devices with superior sensitivity and time resolution and, second, to apply these methods to prototypical photovoltaic materials to address specific questions in the field of organic and hybrid photovoltaics. Regarding the first goal, a new setup combining transient absorption spectroscopy (TAS) and time delayed collection field (TDCF) was designed and installed in Potsdam. An important part of this work concerned the improvement of the electronic components with respect to time resolution and sensitivity. To this end, a highly sensitive amplifier for driving and detecting the device response in TDCF was developed. This system was then applied to selected organic and hybrid model systems with a particular focus on the understanding of the loss mechanisms that limit the fill factor and short circuit current of organic solar cells. The first model system was a hybrid photovoltaic material comprising inorganic quantum dots decorated with organic ligands. Measurements with TDCF revealed fast free carrier recombination, in part assisted by traps, while bias-assisted charge extraction measurements showed high mobility. The measured parameters then served as input for a successful description of the device performance with an analytical model. With a further improvement of the instrumentation, a second topic was the detailed analysis of non-geminate recombination in a disordered polymer:fullerene blend where an important question was the effect of disorder on the carrier dynamics. The measurements revealed that early time highly mobile charges undergo fast non-geminate recombination at the contacts, causing an apparent field dependence of free charge generation in TDCF experiments if not conducted properly. On the other hand, recombination the later time scale was determined by dispersive recombination in the bulk of the active layer, showing the characteristics of carrier dynamics in an exponential density of state distribution. Importantly, the comparison with steady state recombination data suggested a very weak impact of non-thermalized carriers on the recombination properties of the solar cells under application relevant illumination conditions. Finally, temperature and field dependent studies of free charge generation were performed on three donor-acceptor combinations, with two donor polymers of the same material family blended with two different fullerene acceptor molecules. These particular material combinations were chosen to analyze the influence of the energetic and morphology of the blend on the efficiency of charge generation. To this end, activation energies for photocurrent generation were accurately determined for a wide range of excitation energies. The results prove that the formation of free charge is via thermalized charge transfer states and does not involve hot exciton splitting. Surprisingly, activation energies were of the order of thermal energy at room temperature. This led to the important conclusion that organic solar cells perform well not because of predominate high energy pathways but because the thermalized CT states are weakly bound. In addition, a model is introduced to interconnect the dissociation efficiency of the charge transfer state with its recombination observable with photoluminescence, which rules out a previously proposed two-pool model for free charge formation and recombination. Finally, based on the results, proposals for the further development of organic solar cells are formulated.},
  language  = {en}
}
@phdthesis{Willig2019,
  author    = {Willig, Lisa},
  title     = {Ultrafast magneto-optical studies of remagnetisation dynamics in transition metals},
  doi       = {10.25932/publishup-44194},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-441942},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XIV, 113, XVII},
  year      = {2019},
  abstract  = {Ultrafast magnetisation dynamics have been investigated intensely for two decades. The recovery process after demagnetisation, however, was rarely studied experimentally and discussed in detail. The focus of this work lies on the investigation of the magnetisation on long timescales after laser excitation. It combines two ultrafast time resolved methods to study the relaxation of the magnetic and lattice system after excitation with a high fluence ultrashort laser pulse. The magnetic system is investigated by time resolved measurements of the magneto-optical Kerr effect. The experimental setup has been implemented in the scope of this work. The lattice dynamics were obtained with ultrafast X-ray diffraction. The combination of both techniques leads to a better understanding of the mechanisms involved in magnetisation recovery from a non-equilibrium condition. Three different groups of samples are investigated in this work: Thin Nickel layers capped with nonmagnetic materials, a continuous sample of the ordered L10 phase of Iron Platinum and a sample consisting of Iron Platinum nanoparticles embedded in a carbon matrix. The study of the remagnetisation reveals a general trend for all of the samples: The remagnetisation process can be described by two time dependences. A first exponential recovery that slows down with an increasing amount of energy absorbed in the system until an approximately linear time dependence is observed. This is followed by a second exponential recovery. In case of low fluence excitation, the first recovery is faster than the second. With increasing fluence the first recovery is slowed down and can be described as a linear function. If the pump-induced temperature increase in the sample is sufficiently high, a phase transition to a paramagnetic state is observed. In the remagnetisation process, the transition into the ferromagnetic state is characterised by a distinct transition between the linear and exponential recovery. From the combination of the transient lattice temperature Tp(t) obtained from ultrafast X-ray measurements and magnetisation M(t) gained from magneto-optical measurements we construct the transient magnetisation versus temperature relations M(Tp). If the lattice temperature remains below the Curie temperature the remagnetisation curve M(Tp) is linear and stays below the M(T) curve in equilibrium in the continuous transition metal layers. When the sample is heated above phase transition, the remagnetisation converges towards the static temperature dependence. For the granular Iron Platinum sample the M(Tp) curves for different fluences coincide, i.e. the remagnetisation follows a similar path irrespective of the initial laser-induced temperature jump.},
  language  = {en}
}
@phdthesis{Bojahr2016,
  author    = {Bojahr, Andre},
  title     = {Hypersound interaction studied by time-resolved inelastic light and x-ray scattering},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-93860},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xxiii, 201},
  year      = {2016},
  abstract  = {This publications-based thesis summarizes my contribution to the scientific field of ultrafast structural dynamics. It consists of 16 publications, about the generation, detection and coupling of coherent gigahertz longitudinal acoustic phonons, also called hypersonic waves. To generate such high frequency phonons, femtosecond near infrared laser pulses were used to heat nanostructures composed of perovskite oxides on an ultrashort timescale. As a consequence the heated regions of such a nanostructure expand and a high frequency acoustic phonon pulse is generated. To detect such coherent acoustic sound pulses I use ultrafast variants of optical Brillouin and x-ray scattering. Here an incident optical or x-ray photon is scattered by the excited sound wave in the sample. The scattered light intensity measures the occupation of the phonon modes. The central part of this work is the investigation of coherent high amplitude phonon wave packets which can behave nonlinearly, quite similar to shallow water waves which show a steepening of wave fronts or solitons well known as tsunamis. Due to the high amplitude of the acoustic wave packets in the solid, the acoustic properties can change significantly in the vicinity of the sound pulse. This may lead to a shape change of the pulse. I have observed by time-resolved Brillouin scattering, that a single cycle hypersound pulse shows a wavefront steepening. I excited hypersound pulses with strain amplitudes until 1\% which I have calibrated by ultrafast x-ray diffraction (UXRD). On the basis of this first experiment we developed the idea of the nonlinear mixing of narrowband phonon wave packets which we call "nonlinear phononics" in analogy with the nonlinear optics, which summarizes a kaleidoscope of surprising optical phenomena showing up at very high electric fields. Such phenomena are for instance Second Harmonic Generation, four-wave-mixing or solitons. But in case of excited coherent phonons the wave packets have usually very broad spectra which make it nearly impossible to look at elementary scattering processes between phonons with certain momentum and energy. For that purpose I tested different techniques to excite narrowband phonon wave packets which mainly consist of phonons with a certain momentum and frequency. To this end epitaxially grown metal films on a dielectric substrate were excited with a train of laser pulses. These excitation pulses drive the metal film to oscillate with the frequency given by their inverse temporal displacement and send a hypersonic wave of this frequency into the substrate. The monochromaticity of these wave packets was proven by ultrafast optical Brillouin and x-ray scattering. Using the excitation of such narrowband phonon wave packets I was able to observe the Second Harmonic Generation (SHG) of coherent phonons as a first example of nonlinear wave mixing of nanometric phonon wave packets.},
  language  = {en}
}
@phdthesis{Maerten2015,
  author    = {Maerten, Lena},
  title     = {Spectroscopic perspectives on ultrafast coupling phenomena in perovskite oxides},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-77623},
  school      = {Universit{\"a}t Potsdam},
  year      = {2015},
  abstract  = {In this thesis, I study ultrafast dynamics in perovskite oxides using time resolved broadband spectroscopy. I focus on the observation of coherent phonon propagation by time resolved Brillouin scattering: following the excition of metal transducer films with a femtosecond infrared pump pulse, coherent phonon dynamics in the GHz frequency range are triggered. Their propagation is monitored using a delayed white light probe pulse. The technique is illustrated on various thin films and multilayered samples. I apply the technique to investigate the linear and nonlinear acoustic response in bulk SrTiO_3, which displays a ferroelastic phase transition from a cubic to a tetragonal structural phase at T_a=105 K. In the linear regime, I observe a coupling of the observed acoustic phonon mode to the softening optic modes describing the phase transition. In the nonlinear regime, I find a giant slowing down of the sound velocity in the low temperature phase that is only observable for a strain amplitude exceeding the tetragonality of the material. It is attributed to a coupling of the high frequency phonons to ferroelastic domain walls in the material. I propose a new mechanism for the coupling of strain waves to the domain walls that is only effective for high amplitude strain. A detailed study of the phonon attenuation across a wide temperature range shows that the phonon attenuation at low temperatures is influenced by the domain configuration, which is determined by interface strain. Preliminary measurements on magnetic-ferroelectric multilayers reveal that the excitation fluence needs to be carefully controlled when dynamics at phase transitions are studied.},
  language  = {en}
}