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The fascination of microcavity exciton-polaritons (polaritons) rests upon the combination of advanced technological control over both the III-V semiconductor material platform as well as the precise spectroscopic access to polaritonic states, which provide access to the investigation of open questions and complex phenomena due to the inherent nonlinearity and direct spectroscopic observables such as energy-resolved real and Fourier space information, pseudospin and coherence. The focus of this work was to advance the research area of polariton lattice simulators with a particular emphasis on their lasing properties. Following the brief introduction into the fundamental physics of polariton lattices in chapter 2, important aspects of the sample fabrication as well as the Fourier spectroscopy techniques used to investigate various features of these lattices were summarized in chapter 3. Here, the implementation of a spatial light modulator for advanced excitation schemes was presented.
At the foundation of this work is the capability to confine polaritons into micropillars or microtraps resulting in discrete energy levels. By arranging these pillars or traps into various lattice geometries and ensuring coupling between neighbouring sites, polaritonic band structures were engineered. In chapter 4, the formation of a band structure was visualised in detail by investigating ribbons of honeycomb lattices. Here, the transition of the discrete energy levels of a single chain of microtraps to the fully developed band structure of a honeycomb lattice was observed. This study allows to design the size of individual domains in more complicated lattice geometries such that a description using band structures becomes feasible, as it revealed that a width of just six unit cells is sufficient to reproduce all characteristic features of the S band of a honeycomb lattice. In particular in the context of potential technological applications in the realms of lasing, the laser-like, coherent emission from polariton microcavities that can be achieved through the excitation of polariton condensates is intriguing. The condensation process is significantly altered in a lattice potential environment when compared to a planar microcavity. Therefore, an investigation of the polariton condensation process in a lattice with respect to the characteristics of the excitation laser, the exciton-photon detuning as well as the reduced trap distance that represents a key design parameter for polaritonic lattices was performed. Based on the demonstration of polariton condensation into multiple bands, the preferred condensation into a desired band was achieved by selecting the appropriate detuning. Additionally, a decreased condensation threshold in confined systems compared to a planar microcavity was revealed.
In chapter 5, the influence of the peculiar feature of flatbands arising in certain lattice geometries, such as the Lieb and Kagome lattices, on polaritons and polariton condensates was investigated. Deviations from a lattice simulator described by a tight binding model that is solely based on nearest neighbour coupling cause a remaining dispersiveness of the flatbands along certain directions of the Brillouin zone. Therefore, the influence of the reduced trap distance on the dispersiveness of the flatbands was investigated and precise technological control over the flatbands was demonstrated. As next-nearest neighbour coupling is reduced drastically by increasing the distance between the corresponding traps, increasing the reduced trap distance enables to tune the S flatbands of both Lieb and Kagome lattices from dispersive bands to flatbands with a bandwidth on the order of the polariton linewidth. Additionally to technological control over the band structures, the controlled excitation of large condensates, single compact localized state (CLS) condensates as well as the resonant excitation of polaritons in a Lieb flatband were demonstrated. Furthermore, selective condensation into flatbands was realised. This combination of technological and spectroscopic control illustrates the capabilities of polariton lattice simulators and was used to study the coherence of flatband polariton condensates. Here, the ability to tune the dispersiveness from a dispersive band to an almost perfect flatband in combination with the selectivity of the excitation is particularly valuable. By exciting large flatband condensates, the increasing degree of localisation to a CLS with decreasing dispersiveness was demonstrated by measurements of first order spatial coherence. Furthermore, the first order temporal coherence of CLS condensates was increased from τ = 68 ps for a dispersive flatband, a value typically achieved in high-quality microcavity samples, to a remarkable τ = 459 ps in a flatband with a dispersiveness below the polarion linewidth. Corresponding to this drastic increase of the first order coherence time, a decrease of the second order temporal coherence function from g(2)(τ =0) = 1.062 to g(2)(0) = 1.035 was observed. Next to laser-like, coherent emission, polariton condensates can form vortex lattices. In this work, two distinct vortex lattices that can form in polariton condensates in Kagome flatbands were revealed. Furthermore, chiral, superfluid edge transport was realised by breaking the spatial symmetry through a localised excitation spot. This chirality was related to a change in the vortex orientation at the edge of the lattice and thus opens the path towards further investigations of symmetry breaking and chiral superfluid transport in Kagome lattices.
Arguably the most influential concept in solid-state physics of the recent decades is the idea of topological order that has also provided a new degree of freedom to control the propagation of light. Therefore, in chapter 6, the interplay of topologically non-trivial band structures with polaritons, polariton condensates and lasing was emphasised. Firstly, a two-dimensional exciton-polariton topological insulator based on a honeycomb lattice was realised. Here, a topologically non-trivial band gap was opened at the Dirac points through a combination of TE-TM splitting of the photonic mode and Zeeman splitting of the excitonic mode. While the band gap is too small compared to the linewidth to be observed in the linear regime, the excitation of polariton condensates allowed to observe the characteristic, topologically protected, chiral edge modes that are robust against scattering at defects as well as lattice corners. This result represents a valuable step towards the investigation of non-linear and non-Hermitian topological physics, based on the inherent gain and loss of microcavities as well as the ability of polaritons to interact with each other. Apart from fundamental interest, the field of topological photonics is driven by the search of potential technological applications, where one direction is to advance the development of lasers. In this work, the starting point towards studying topological lasing was the Su-Schrieffer-Heeger (SSH) model, since it combines a simple and well-understood geometry with a large topological gap. The coherence properties of the topological edge defect of an SSH chain was studied in detail, revealing a promising degree of second order temporal coherence of g(2)(0) = 1.07 for a microlaser with a diameter of only d = 3.5 µm. In the context of topological lasing, the idea of using a propagating, topologically protected mode to ensure coherent coupling of laser arrays is particularly promising. Here, a topologically non-trivial interface mode between the two distinct domains of the crystalline topological insulator (CTI) was realised. After establishing selective lasing from this mode, the coherence properties were studied and coherence of a full, hexagonal interface comprised of 30 vertical-cavity surface-emitting lasers (VCSELs) was demonstrated. This result thus represents the first demonstration of a topological insulator VCSEL array, combining the compact size and convenient light collection of vertically emitting lasers with an in-plane topological protection.
Finally, in chapter 7, an approach towards engineering the band structures of Lieb and honeycomb lattices by unbalancing the eigenenergies of the sites within each unit cell was presented. For Lieb lattices, this technique opens up a path towards controlling the coupling of a flatband to dispersive bands and could enable a detailed study of the influence of this coupling on the polariton flatband states. In an unbalanced honeycomb lattice, a quantum valley Hall boundary mode between two distinct, unbalanced honeycomb domains with permuted sites in the unit cells was demonstrated. This boundary mode could serve as the foundation for the realisation of a polariton quantum valley Hall effect with a truly topologically protected spin based on vortex charges. Modifying polariton lattices by unbalancing the eigenenergies of the sites that comprise a unit cell was thus identified as an additional, promising path for the future development of polariton lattice simulators.
Thermoplastic polymers have a history of decades of safe and effective use in the clinic as implantable medical devices. In recent years additive manufacturing (AM) saw increased clinical interest for the fabrication of customizable and implantable medical devices and training models using the patients’ own radiological data. However, approval from the various regulatory bodies remains a significant hurdle. A possible solution is to fabricate the AM scaffolds using materials and techniques with a clinical safety record, e.g. melt processing of polymers. Melt Electrowriting (MEW) is a novel, high resolution AM technique which uses thermoplastic polymers. MEW produces scaffolds with microscale fibers and precise fiber placement, allowing the control of the scaffold microarchitecture. Additionally, MEW can process medical-grade thermoplastic polymers, without the use of solvents paving the way for the production of medical devices for clinical applications. This pathway is investigated in this thesis, where the layout is designed to resemble the journey of a medical device produced via MEW from conception to early in vivo experiments. To do so, first, a brief history of the development of medical implants and the regenerative capability of the human body is given in Chapter 1. In Chapter 2, a review of the use of thermoplastic polymers in medicine, with a focus on poly(ε-caprolactone) (PCL), is illustrated, as this is the polymer used in the rest of the thesis. This review is followed by a comparison of the state of the art, regarding in vivo and clinical experiments, of three polymer melt AM technologies: melt-extrusion, selective laser sintering and MEW. The first two techniques already saw successful translation to the bedside, producing patient-specific, regulatory-approved AM implants. To follow in the footsteps of these two technologies, the MEW device parameters need to be optimized. The MEW process parameters and their interplay are further discussed in Chapter 3 focusing on the importance of a steady mass flow rate of the polymer during printing. MEW reaches a balance between polymer flow, the stabilizing electric field and moving collector to produce reproducible, high-resolution scaffolds. An imbalance creates phenomena like fiber pulsing or arcing which result in defective scaffolds and potential printer damage. Chapter 4 shows the use of X-ray microtomography (µCT) as a non-destructive method to characterize the pore-related features: total porosity and the pore size distribution. MEW scaffolds are three-dimensional (3D) constructs but have long been treated in the literature as two-dimensional (2D) ones and characterized mainly by microscopy, including stereo- and scanning electron microscopy, where pore size was simply reported as the distance between the fibers in a single layer. These methods, together with the trend of producing scaffolds with symmetrical pores in the 0/90° and 0/60/120° laydown patterns, disregarded the lateral connections between pores and the potential of MEW to be used for more complex 3D structures, mimicking the extracellular matrix. Here we characterized scaffolds in the aforementioned symmetrical laydown patterns, along with the more complex 0/45/90/135° and 0/30/60/90/120/150° ones. A 2D pore size estimation was done first using stereomicroscopy, followed by and compared to µCT scanning. The scaffolds with symmetrical laydown patterns resulted in the predominance of one pore size, while those with more complex patterns had a broader distribution, which could be better shown by µCT scans. Moreover, in the symmetrical scaffolds, the size of 3D pores was not able to reach the value of the fiber spacing due to a flattening effect of the scaffold, where the thickness of the scaffold was less than the fiber spacing, further restricting the pore size distribution in such scaffolds. This method could be used for quality assurance of fabricated scaffolds prior to use in in vitro or in vivo experiments and would be important for a clinical translation. Chapter 5 illustrates a proof of principle subcutaneous implantation in vivo experiment. MEW scaffolds were already featured in small animal in vivo experiments, but to date, no analysis of the foreign body reaction (FBR) to such implants was performed. FBR is an immune reaction to implanted foreign materials, including medical devices, aimed at protecting the host from potential adverse effects and can interfere with the function of some medical implants. Medical-grade PCL was used to melt electrowrite scaffolds with 50 and 60 µm fiber spacing for the 0/90° and 0/60/120° laydown patterns, respectively. These implants were implanted subcutaneously in immunocompetent, outbred mice, with appropriate controls, and explanted after 2, 4, 7 and 14 days. A thorough characterization of the scaffolds before implantation was done, followed by a full histopathological analysis of the FBR to the implants after excision. The scaffolds, irrespective of their pore geometry, induced an extensive FBR in the form of accumulation of foreign body giant cells around the fiber walls, in a manner that almost occluded available pore spaces with little to no neovascularization. This reaction was not induced by the material itself, as the same reaction failed to develop in the PCL solid film controls. A discussion of the results was given with special regard to the literature available on flat surgical meshes, as well as other hydrogel-based porous scaffolds with similar pore sizes. Finally, a general summary of the thesis in Chapter 6 recapitulates the most important points with a focus on future directions for MEW.
This work investigates the correlations between spin states and the light emission properties of organic light-emitting diodes (OLEDs), which are based on the principle of thermally activated delayed fluorescence. The spin-spin interactions responsible for this mechanism are investigated in this work using methods based on spin-sensitive electron paramagnetic resonance (EPR). In particular, this method has been applied to electrically driven OLEDs. The magnetic resonance has been detected by electroluminescence, giving this method its name: electroluminescence detected magnetic resonance (ELDMR).
Initial investigations on a novel deep blue TADF emitter were performed. Furthermore, the ELDMR method was used in this work to directly detect the spin states in the OLED. These measurements were further underlined by time-resolved experiments such as transient electro- and photoluminescence.
Die vorliegende Arbeit beschäftigt sich mit der volumenbildgebenden Untersuchung von mechanischen Ermüdungsprozessen in Titan-Zahnimplantaten. Im Vordergrund steht die Entwicklung einer neuen Messmethode der In-situ-Mikrotomografie am Synchrotron.
Zahnimplantate werden beim Gebrauch mechanisch wiederholt belastet (Wechsellast). Nach vielen zyklischen Belastungen können aufgrund von mikroplastische Verformungen Ermüdungsschäden auftreten. Diese können im Extremfall zum Versagen und Verlust eines Implantats führen. Die Computertomographie ist eine sehr geeignete zerstörungsfrei Prüfmethode, um Zahnimplantate zu untersuchen. Diese Arbeit erweitert die bisherige CT-Methode insofern, dass In-situ-Beobachtungen bei mechanischer Belastung möglich sind.
Die in dieser Arbeit untersuchten Zahnimplantate weisen an der Implantat-Abutment-Grenzfläche bei eintretender Ermüdung einen Mikrospalt auf. Dieser wird als Indikator für einsetzende Fatigue- Prozesse benutzt. Der in der Synchrotron CT verfügbare Inlinephasenkontrast ermöglicht eine verbesserte Bestimmung der Mikrospaltgröße. Da die schnellen Bewegungen der Ermüdungsprüfung mittels Standard-CT-Verfahren schwer zu erfassen sind, war die stroboskopische Aufnahmemethode das zielführende Messverfahren, um in-situ-Prüfung zu ermöglichen.
Die 4 kommerziellen Zahnimplantattypen werden neben der In-situ-Fatigue Prüfung auch mittels klassischer Ermüdungsprüfung untersucht und mit der Neuen Messmethode verglichen. Die hier entwickelte In-situ-Fatigue-Prüfstation kann Proben bis zu 345 N tomographisch untersuchen. Neben den experimentellen Untersuchungen wird eine statische FEM-Betrachtung durchgeführt und mit experimentellen Messdaten verglichen. Zuletzt wird mit der entwickelten Messtation Knochenrisse in der Implantat Umgebung untersucht.
The motivation for this work has been contributing a step to the advancement of technology. A next leap in technology would be the realization of a scalable quantum computer. One potential route is via topological quantum computing. A profound understanding of topological materials is thus essential. My work contributes by the investigation of the exemplary topological material HgTe. The focus lies on the understanding of the topological surface states (TSS) and new possibilities to manipulate them appropriately. Traditionally top gate electrodes are used to adjust the carrier density in such semi-conductor materials. We found that the electric field of the top gate can further alter the properties of the HgTe layer. The formation of additional massive Volkov-Pankratov states limits the accessibility of the TSS. The understanding of these states and their interplay with the TSS is necessary to appropriately design devices and to ensure their desired properties. Similarly, I observed the existence and stability of TSSs even without a bandgap in the bulk band structure in the inversion induced Dirac semi-metal phase of compressively strained HgTe. The finding of topological surface states in inversion-induced Dirac semi-metals provides a consistent and simple explanation for the observation reported for \(\text{Cd}_3\text{As}_2\).
These observations have only been possible due to the high quality of the MBE grown HgTe layers and the access of different phases of HgTe via strain engineering. As a starting point I performed Magneto-transport measurements on 67 nm thick tensilely strained HgTe layers grown on a CdTe substrate. We observed multiple transport channels in this three-dimensional topological insulator and successfully identified them. Not only do the expected topological surface states exist, but also additional massive surface states have been observed. These additional massive surface states are formed due to the electrical field applied at the top gate, which is routinely used to vary the carrier density in the HgTe layer. The additional massive surface states are called Volkov-Pankratov states after B. A. Volkov and O. A. Pankratov. They predicted the existence of similar massive surface states at the interface of materials with mutually inverted bands. We first found indications for such massive Volkov-Pankratov states in high-frequency compressibility measurements for very high electron densities in a fruitful collaboration with LPA in Paris. Magneto-transport measurements and \(k \cdot p\) calculations revealed that such Volkov-Pankratov states are also responsible for the observed whole transport. We also found indications for similar massive VPS in the electron regime, which coexist with the topological surface states. The topological surface states exist over the full investigated gate range including a regime of pure topological insulator transport. To increase the variability of the topological surface states we introduced a modulation doping layer in the buffer layer. This modulation doping layer also enabled us to separate and identify the top and bottom topological surface states.
We used the variability of the bulk band structure of HgTe with strain to engineer the band structure of choice using virtual substrates. The virtual substrates enable us to grow compressively strained HgTe layers that do not possess a bandgap, but instead linear crossing points. These layers are predicted to beDirac semi-metals. Indeed I observed also topological surface states and massive Volkov-Pankratov states in the compressively strained Dirac semi-metal phase. The observation of topological surfaces states also in the Dirac semi-metal phase has two consequences: First, it highlights that no bulk bandgap is necessary to observe topological surface states. Second, the observation of TSS also in the Dirac semi-metal phase emphasizes the importance of the underlying band inversion in this phase. I could not find any clear signatures of the predicted disjoint topological surface states, which are typically called Fermi-arcs. The presence of topological surface states and massive Volkov-Pankratov states offer a simple explanation for the observed quantum Hall effect and other two-dimensional transport phenomena in the class of inversion induced Dirac semi-metals, as \(\text{Cd}_3\text{As}_2\). This emphasizes the importance of the inherent bulk band inversion of different topological materials and provides a consistent and elegant explanation for the observed phenomena in these materials. Additionally, it offers a route to design further experiments, devices, and thus the foundation for the induction of superconductivity and thus topological quantum computing.
Another possible path towards quantum computing has been proposed based on the chiral anomaly. The chiral anomaly is an apparent transport anomaly that manifests itself as an additional magnetic field-driven current in three-dimensional topological semimetals with a linear crossing point in their bulk band structure. I observed the chiral anomaly in compressively strained HgTe samples and performed multiple control experiments to identify the observed reduction of the magnetoresistance with the chiral anomaly. First, the dependence of the so-called negative magnetoresistance on the angle and strength of the magnetic field has been shown to fit the expectation for the chiral anomaly. Second, extrinsic effects as scattering could be excluded as a source for the observed negative MR using samples with different mobilities and thus impurity concentrations. Third, the necessity of the linear crossing point has been shown by shifting the electrochemical potential away from the linear crossing points, which diminished the negative magnetoresistance. Fourth, I could not observe a negative magnetoresistance in the three-dimensional topological insulator phase of HgTe. These observations together prove the existence of the chiral anomaly and verify compressively strained HgTe as Dirac semi-metal. Surprisingly, the chiral anomaly is also present in unstrained HgTe samples, which constitute a semi-metal with a quadratic band touching point. This observation reveals the relevance of the Zeeman effect for the chiral anomaly due to the lifting of the spin-degeneracy in these samples. Additionally to the chiral anomaly, the Dirac semi-metal phase of compressively strained HgTe showed other interesting effects. For low magnetic fields, a strong weak-antilocalization has been observed. Such a strong weak-anti-localization correction in a three-dimensional layer is surprising and interesting. Additionally, non-trivial magnetic field strength and direction dependencies have been observed. These include a strong positive magnetoresistance for high magnetic fields, which could indicate a metal-insulator transition. On a more device-oriented note, the semi-metal phase of unstrained HgTe constitutes the lower limit of the by strain engineering adjustable minimal carrier density of the topological surface states and thus of very high mobility.
To sum up, topological surface states have been observed in the three-dimensional topological insulator phase and the Dirac semi-metal phase of HgTe. The existence and accessibility of topological surface states are thus independent of the existence of a bandgap in the bulk band structure. The topological surface states can be accompanied by massive Volkov-Pankratov states. These VPS are created by electric fields, which are routinely applied to adjust the carrier density in semiconductor devices. The theoretical predicted chiral anomaly has been observed in the Dirac semi-metal phase of HgTe. In contrast to theoretical predictions, no indications for the Fermi-arc called disjoint surface states have been observed, but instead the topological and massive Volkov-Pankratov surface states have been found. These states are thus expected for all inversion-induced topological materials.
Das Ziel dieser Arbeit war die Entwicklung und Weiterentwicklung von Laserlichtquellen basierend auf der Interbandkaskadentechnologie in einem Wellenlängenbereich von ca. 3 bis 6 µm. Der Fokus lag dabei auf der Entwicklung von Kantenemitter-Halbleiterlasern, welche bei verschiedensten Emissionswellenlängen erfolgreich hergestellt werden konnten. Dabei wurde auf jeweilige Herausforderungen eingegangen, welche entweder durch die Herstellung selbst oder der anwendungstechnischen Zielsetzung bedingt war. Im Rahmen dieser Arbeit wurden verschiedene, spektral einzelmodige Halbleiterlaser im angesprochenen Wellenlängenbereich entwickelt und hergestellt. Basierend auf dem jeweiligen Epitaxiematerial und der angestrebten Emissionswellenlänge wurden Simulationen der optischen Lasermode durchgeführt und die grundlegenden für die Herstellung notwendigen Parameter bestimmt und experimentell umgesetzt. Des Weiteren wurden die verwendeten Verfahren für den jeweiligen Herstellungsprozess angepasst und optimiert. Das umfasst die in den ersten Kapiteln beschriebenen Schritte wie optische Lithografie, Elektronenstrahllithografie, reaktives Trockenätzen und verschiedene Arten der Materialdeposition. Mit einer Emissionswellenlänge von 2,8 µm wurde beispielsweise der bislang kurzwelligste bei Raumtemperatur im Dauerstrichbetrieb betriebene einzelmodige Interbandkaskadenlaser hergestellt. Dessen Leistungsmerkmale sind mit Diodenlasern im entsprechenden Emissionsbereich vergleichbar. Somit ergänzt die Interbandkaskadentechnologie bestehende Technologien nahtlos und es ist eine lückenlose Wellenlängenabdeckung bis in den mittleren Infrarotbereich möglich. Je nach Herstellungsprozess wurde außerdem auf die verteilte Rückkopplung eingegangen und die Leistungsfähigkeit des verwendeten Metallgitterkonzeptes anhand von Messungen an spektral einzelmodigen Bauteile aufgezeigt. Es wurden aber auch die je nach Zielsetzung unterschiedlichen Herausforderungen aufgezeigt und diskutiert. Für eine Anwendung wurden spezielle Laserchips mit zwei einzelmodigen Emissionswellenlängen bei 3928 nm und 4009 nm entwickelt. Die beiden Wellenlängen sind für die Detektion von Schwefeldioxid und Schwefelwasserstoff geeignet, welche zur Überwachung und Optimierung der Schwefelgewinnung durch das Claus-Verfahren notwendig sind. Bei der Umsetzung wurden auf einzelnen Chips zwei Laseremitter in einem Abstand von 70 µm platziert und mit je einem Metallgitter versehen. Das verwendete Epitaxiematerial war so konzipiert, dass es optimal für beide Zielwellenlängen verwendet werden kann. Die geforderten Eigenschaften wurden erfüllt und die Bauteile konnten erfolgreich hergestellt werden. Die Emissionseigenschaften und das spektrale Verhalten wurde bei beiden Zielwellenlängen bestimmt. Einzeln betrachtet erfüllen beide Emitter die notwendigen Eigenschaften um für spektroskopische Anwendungen eingesetzt werden zu können. Ergänzend wurde zum einen das Abstimmverhalten der Emissionswellenlänge in Abhängigkeit der Modulationsfrequenz des Betriebsstromes untersucht und zusätzlich die thermische Abhängigkeit der Betriebsparameter beider Kanäle zueinander bestimmt. Diese Abhängigkeit ist für eine simultane Messung mit beiden Kanälen notwendig. Das Konzept mit mehreren Stegwellenleitern pro Laserchip wurde in einem weiteren Fall noch stärker ausgearbeitet. Denn je nach Komplexität eines Gasgemisches sind zur Bestimmung der einzelnen Komponenten mehr Messpunkte bzw. Wellenlängen notwendig. Im zweiten Fall ist die Analyse der Kohlenwasserstoffe Methan, Ethan, Propan, Butan, Iso-Butan, Pentan und Iso-Pentan von Interesse, welche als Hauptbestandteile von Erdgas z.B. in Erdgasaufbereitungsanlagen oder zur Bestimmung des Heizwertes analysiert werden müssen. Die genannten Kohlenwasserstoffe zeigen ein starkes Absorptionsverhalten im Wellenlängenbereich von 3,3 bis 3,5 µm. Auf dem entsprechend angepassten Interbandkaskadenmaterial wurden Bauteile mit neun Wellenleitern pro Laserchip hergestellt. Mithilfe der neun einzelmodigen Emissionskanäle konnte ein Bereich von bis zu 190 nm (21 meV, 167 cm-1) adressiert werden. Außerdem wurde der sich mit zunehmender Wellenlänge ändernde Schichtaufbau und dessen Einfluss auf die Bauteileigenschaften diskutiert. Die Leistungsdaten der langwelligsten Epitaxie waren im Vergleich deutlich schwächer. Um diesen Nachteil zu kompensieren, wurde eine spezielle Wellenleitergeometrie mit doppeltem Steg genutzt. Die Eigenschaften des Konzeptes wurden zuerst mittels Simulation untersucht und ein entsprechendes Herstellungsverfahren entwickelt. Mit der Simulation als Grundlage wurden die verschiedenen Prozessparameter über mehrere Prozessläufe iterativ optimiert und somit die Performance der Laser verbessert. Auch mit diesem Verfahren konnte ausreichende Kopplung an das Metallgitter erzielt werden. Abschließend wurden mit diesem Herstellungsverfahren einzelmodige Laser im Wellenlängenbereich von 5,9 bis über 6 Mikrometern realisiert. Diese Laser emittierten im Dauerstrichbetrieb bei einer maximalen Betriebstemperatur von -2 °C. Insgesamt wurde anhand der im Rahmen dieser Arbeit entwickelten Bauteilen und de ren Charakterisierung gezeigt, dass diese die Anforderungen von TLAS Anwendungen erfüllen. Jedoch konnte nur auf einen Teil der Möglichkeiten eingegangen werden, den die Interbandkaskadentechnologie bietet, denn die angesprochenen Einsatzgebiete stellen nur einzelne grundlegende Möglichkeiten dieser Technologie mit Schwerpunkt auf laserbasierte Lichtquellen dar. Zusammenfassend kann allerdings gesagt werden, dass sich die Interbandkaskadentechnologie etabliert hat. Gerade durch die gezeigten Leistungsdaten bei den Wellenlängen um 2,9 µm, 3,4 µm und 4,0 µm im Dauerstrichbetrieb bei Raumtemperatur wird ersichtlich, dass im Bereich der Sensorik die ICL Technologie in Bezug auf niedriger Strom- bzw. Leistungsaufnahme quasi konkurrenzlos ist. Sicherlich werden die Anwendungsgebiete in Zukunft noch vielfältiger. Denn es sind auf jeden Fall weitere Fortschritte in Richtung höherer Emissionswellenlängen, deutlich höherer Betriebstemperaturen, verbreiterte Emissionsbereiche oder gänzlich andere Bauteil Konzepte wie z.B. für Frequenzkämme bzw. Terahertz Anwendungen zu erwarten. Diese Entwicklung betrifft nicht nur den Einsatz als Lichtquelle, denn auch Interbandkaskadendetektoren bzw. Solarzellen wurden schon realisiert und werden weiterentwickelt.
Verlustarmer Ladungsträgertransport ist für die Realisierung effizienter und kleiner elektronischer Bauteile von großem Interesse. Dies hilft entstehende Wärme zu minimieren und den Energieverbrauch gleichzeitig zu reduzieren. Einzelne Streuprozesse, die den Verlust bei Ladungsträgertransport bestimmen, laufen jedoch auf Längenskalen von Nano- bis Mikrometern ab. Um diese detailliert untersuchen zu können, bedarf es Messmethoden mit hoher zeitlicher oder örtlicher Auflösung. Für Letztere gibt es wenige etablierte Experimente, häufig basierend auf der Rastertunnelmikroskopie, welche jedoch verschiedenen Einschränkungen unterliegen. Um die Möglichkeiten der Detektion von Ladungsträgertransport auf Distanzen der mittleren freien Weglänge und damit im ballistischen Regime zu verbessern, wurde im Rahmen dieser Dissertation die Molekulare Nanosonde charakterisiert und etabliert. Diese Messmethode nutzt ein einzelnes Molekül als Detektor für Ladungsträger, welche mit der Sondenspitze des Rastertunnelmikroskops (RTM) wenige Nanometer entfernt vom Molekül in das untersuchte Substrat injiziert werden. Die hohe Auflösung des RTM in Kombination mit der geringen Ausdehnung des molekularen Detektors ermöglicht dabei atomare Kontrolle von Transportpfaden über wenige Nanometer. Der erste Teil dieser Arbeit widmet sich der Charakterisierung der Molekularen Nanosonde. Hierfür werden zunächst die elektronischen Eigenschaften dreier Phthalocyanine mittels Rastertunnelspektroskpie untersucht, welche im Folgenden zur Charakterisierung des Moleküls als Detektor Anwendung finden. Die anschließende Analyse der Potentiallandschaft der Tautomerisation von H2Pc und HPc zeigt, dass die NH- Streckschwinung einem effizienten Schaltprozess zu Grunde liegt. Darauf basierend wird der Einfluss der Umgebung anhand von einzelnen Adatomen sowie des Substrats selbst auf den molekularen Schalter analysiert. In beiden Fällen zeigt sich eine signifikante Änderung der Potentiallandschaft der Tautomerisation. Anschließend wird der Einfluss geometrischer Eigenschaften des Moleküls selbst untersucht, wobei sich eine Entkopplung vom Substrat auf Grund von dreidimensionalen tert-Butyl-Substituenten ergibt. Zusätzlich zeigt sich bei dem Vergleich von Naphthalocyanin zu Phthalocyanin der Einfluss lateraler Ausdehnung auf die Detektionsfläche, was einen nicht-punktförmigen Detektor bestätigt. Im letzten Abschnitt werden zwei Anwendungen der Molekularen Nanosonde präsentiert. Zunächst wird mit Phthalocyanin auf Ag(111) demonstriert, dass die Interferenz von ballistischen Ladungsträgern auf Distanzen von wenigen Nanometern mit dieser Technik detektierbar ist. Im zweiten Teil zeigt sich, dass der ballistische Transport auf einer Pd(110)-Oberfläche durch die anisotrope Reihenstruktur auf atomarer Skala moduliert wird.
Spin-Orbit Torques and Galvanomagnetic Effects Generated by the 3D Topological Insulator HgTe
(2021)
Nature shows us only the tail of the lion. But I have no doubt that the lion belongs with it even if he cannot reveal himself all at once. Albert Einstein
In my dissertation, I addressed the question of whether the 3D topological insulator mercury telluride (3D TI HgTe) is a suitable material for spintronics applications. This question was addressed by investigating the SOTs generated by the 3D TI HgTe in an adjacent ferromagnet (Permalloy) by using the ferromagnetic resonance technique (SOT-FMR).
In the first part of the dissertation, the reader was introduced to the mathematical description of the SOTs of a hybrid system consisting of a topological insulator (TI) and a ferromagnet (FM). Furthermore, the sample preparation and the measurement setup for the SOT-FMR measurements were discussed. Our SOT-FMR measurements showed that at low temperatures (T = 4.2 K) the out-of-plane component of the torque is dominant. At room temperature, both in-plane and out-of-plane components of the torque could be observed. From the symmetry of the mixing voltage (Figs. 3.14 and 3.15) we could conclude that the 3D TI HgTe may be efficient for the generation of spin torques in the permalloy [1]. The investigations reported here showed that the SOT efficiencies generated by the 3D TI HgTe are comparable with other existent topological insulators (see Fig. 3.17). We also discussed in detail the parasitic effects (such as thermovoltages) that can contribute to the correct interpretation of the spin torque efficiencies.
Although the results reported here provide several indications that the 3D TI HgTe might be efficient in exerting spin-torques in adjacent ferromagnets [2], the reader was repeatedly made aware that parasitic effects might contaminate the correct writing and reading of the information in the ferromagnet. These effects should be taken into consideration when interpreting results in the published literature claiming high spin-orbit torque efficiencies [2–4]. The drawbacks of the SOT-FMR measurement method led to a further development of our measurement concept, in which the ferromagnet on top of the 3D TI HgTe was replaced by a
spin-valve structure. In contrast with our measurements, in this measurement setup, the current flowing through the HgTe is known and changes in the spin-valve resistance can be read via the GMR effect.
Moreover, the SOT-FMR experiments required the application of an in-plane magnetic field up to 300 mT to define the magnetization direction in the ferromagnet. Motivated by this fact, we investigated the influence of an in-plane magnetic field in the magnetoresistance of the 3D TI HgTe. The surprising results of these measurements are described in the second part of the dissertation. Although the TI studied here is non-magnetic, its transversal MR (Rxy) showed an oscillating behavior that depended on the angle between the in-plane magnetic field and the electrical current. This effect is a typical property of ferromagnetic materials and is called planar Hall effect (PHE) [5, 6]. Moreover, it was also shown that the PHE amplitude (Rxy) and the longitudinal resistance (Rxx) oscillate as a function of the in-plane magnetic field amplitude for a wide range of carrier densities of the topological insulator.
The PHE was already described in another TI material (Bi2−xSbxTe3) [7]. The authors suggested as a possible mechanism the scattering of the electron off impurities that are polarized by an in-plane magnetic field. We critically discussed this and other theoretical proposed mechanisms existent in the literature [8, 9].
In this thesis, we attempted to explain the origin of the PHE in the 3D TI HgTe by anisotropies in the band structure of this material. The k.p calculations based on 6-orbitals were able to demonstrate that an interplay between Rashba, Dresselhaus, and in-plane magnetic field deforms the Fermi contours of the camel back band of the 3D TI HgTe, which could lead to anisotropies in its conductivity. However, the magnetic fields needed to experimentally observe this effect are as
high as 40 T, i.e., one order of magnitude higher than reported in our experiments. Additionally, calculations of the DoS to assess if there is a difference in the states for Bin parallel and Bin perpendicular to the current were, so far, inconclusive. Moreover, the complicated dependence of Rashba in the p-conducting
regime of HgTe [10] makes it not straightforward the inclusion of this term in the band structure calculations.
Despite the extensive efforts to understand the origin of the galvanomagnetic effects in the 3D TI HgTe, we could not determine a clear mechanism for the origin of the PHE and the MR oscillations studied in this thesis. However, our work clarifies and excludes a few mechanisms reported in the literature as the origin of these effects in the 3D TI HgTe. The major challenge, which still needs to be overcome, is to find a model that simultaneously explains the PHE, the gate dependence, and the oscillations in the magnetoresistance of the 3D TI HgTe as a function of the in-plane magnetic field.
To conclude, the author would like to express her hope to have brought the reader closer to the complexity of the questions addressed in this thesis and to have initiated them into the art of properly conducting electrical transport measurements on topological insulators with in-plane magnetic fields.
This thesis aims to investigate the form-phase diagram of aqueous solutions of the triblock copolymer Pluronic P123 focusing on its high-temperature phases. P123 is based on polyethylene as well as polypropylene oxide blocks and shows a variety of di erent temperaturedependent micelle morphologies or even lyotropic liquid crystal phases in aqueous solutions. Besides the already well-studied spherical aggregates at intermediate temperatures, the size and internal structure of both worm-like and lamellar micelles, which appear near the cloud point, is determined using light, neutron and X-ray scattering. By combining the results of time-resolved dynamic light as well as small-angle neutron and X-ray scattering experiments, the underlying structural changes and kinetics of the sphere-to-worm transition were studied supporting the random fusion process, which is proposed in literature. For temperatures near the cloud point, it was observed that aqueous P123 solutions below the critical crystallization concentration gelate after several hours, which is linked to the presence and structure of polymeric surface layers on the sample container walls as shown by neutron re ectometry
measurements. Using a hierarchical model for the lamellar micelles including their periodicity as well as domain and overall size, it is possible to unify the existing results in literature and propose a direct connection between the near-surface and bulk properties of P123 solutions at temperatures near the cloud point.
This work consists of two parts. On the one hand, it describes simulation and
measurement of the effect of contaminations of the detector gas on the performance
of particle detectors, with special focus on Micromegas detectors. On the other
hand, it includes the setup of a production site for the finalization of drift panels
which are going to be used in the ATLAS NSW. The first part augments these
two parts to give an introduction into the theoretical foundations of gaseous particle
detectors.