Abschlussarbeiten

Michael Seidel
Efficient coupling of single epitaxial quantum dots to plasmonic waveguides
Dissertation, Bayreuth (2024) online, ERef, abstract: show

Metallic nanostructures can confine electromagnetic fields far below the optical wavelength, thus circumventing the diffraction limit. Surface plasmon polaritons, the collective oscillations of conduction band electrons coupled to electromagnetic waves, are associated with high amplitudes of the electric field, concentrated in tiny volumes. This local field enhancement can manipulate the light-matter interaction at the nanoscale. In particular, the coupling of plasmonic waveguides to individual quantum emitters offers exciting prospects. For instance, one could envision a nanocircuit in which the presence or absence of a single photon controls the transmission of another photon, i.e., a single-photon transistor. An ideal quantum plasmonic nanocircuit would feature a source of indistinguishable photons that is coupled with near-unity efficiency to a low-loss single-mode waveguide, with an enhanced emission rate due to the Purcell effect. For such an application, excellent single-photon sources are essential. Epitaxially grown semiconductor quantum dots are considered a near-ideal quantum light source due to their brightness, stability, and narrowband excitonic transitions. On the other hand, wet-chemically grown single-crystalline silver nanowires exhibit strong confinement, single-mode operation, and the lowest propagation losses of plasmonic waveguides in the visible and infrared. As promising as the direct combination of near-surface epitaxial quantum dots and silver nanowires seems to be, it faces tremendous challenges that include reduced photon extraction as well as high attenuation of the propagating plasmon, both consequences of the high refractive index environment of the bulky semiconductor host. These and other issues are addressed in this thesis through novel coupling schemes that are numerically modeled, experimentally realized and optically characterized. The thesis begins with a compact theoretical framework that covers the fundamentals of epitaxial quantum dots, the electromagnetics of propagating surface plasmons, and the modeling of emitters in the vicinity of plasmonic waveguides. Two- and three-dimensional finite element models are used to describe guided plasmonic modes at nanowires, as well as the coupling of quantum emitters into such waveguide modes. Quantities such as the effective mode index, the coupling efficiency, and the Purcell factor are introduced. In contrast to most other works, plasmonic waveguides in inhomogeneous environments, given by semiconducting substrates, are also considered. For the experimental characterization, different optical imaging techniques operating at cryogenic temperatures (20 K), including cathodoluminescence spectroscopy, confocal laser scanning, and photoluminescence imaging, are applied. The first presented quantum dot–plasmon coupling scheme is termed intermediate field coupling. It relies on a planar dielectric layer (n = 1.4), acting as a spacer between the semiconductor substrate (n = 3.4) containing the GaAs quantum dot and the plasmonic waveguide. By tuning the film thickness, one can accomplish either efficient quantum dot–waveguide coupling or efficient waveguide propagation. Numerical simulations show that the optimal overall performance is not achieved in the near field but in the intermediate field for a film thickness around 130 nm. The experimental conditions for such an intermediate field coupling are provided by simply spin-coating a low-index dielectric and dispersing silver nanowires. When the lateral distance between the nanowire and the quantum dot becomes sufficiently small (⪅ 100 nm), coupling is demonstrated by launching surface plasmons through quantum dot luminescence. High-resolution cathodoluminescence imaging determines the lateral quantum dot–nanowire positions precisely (< 30 nm), and the experimentally measured coupling efficiency can be explained by a simple interference model that includes reflections of surface plasmons at the nanowire end. Intermediate field coupling can be applied to other types of emitters in high-index environments (e.g. nitrogen-vacancy centers in diamond), does not rely on nanostructuring processes, and is robust against emitter–waveguide displacement, both laterally and in growth-direction. The latter allows the use of deeply buried quantum dots with exceptional quantum optical properties. Hence, intermediate field coupling paves the way to a lifetime-limited, truly nanoscale single-plasmon source. The second coupling scheme is based on the integration of single epitaxial quantum dots into semiconductor mesa structures that are surrounded by a dielectric layer with a lower refractive index. This approach promises increased coupling efficiency due to a reduced distance to the plasmonic waveguide on top, and increased propagation efficiency due to the dielectric that supports more efficiently propagating waveguide modes. Numerical models for a disk-shaped AlGaAs mesa (several hundred nanometers in diameter) on a silver backplane suggest that the mesa can act as a dielectric nanoresonator that either suppresses or enhances the quantum dot emission, which can be taken advantage of to design single-plasmon sources with efficiencies up to 50 %. Although the optimized target structure is feasible with advanced nanostructuring methods, an experimental realization uses a fabrication-wise simpler design based on GaAs substrates. The nanostructure is processed by deterministic integration of the quantum dots via in situ electron beam lithography, planarization of the etched topography, and dispersion of colloidal silver nanowires. Even though coupling of a mesa–waveguide hybrid structure is observed, both coupling and propagation efficiency are affected by imperfect planarization and consequential nanowire bending. Unlike intermediate field coupling, the mesa-based coupling approach is technologically demanding and relies on precise and deterministic fabrication methods. However, the nanoresonator-enhanced coupling scheme offers unprecedented efficient single-plasmon generation and efficient propagation through plasmonic waveguides and, therefore, opens up a path towards a scalable plasmonic quantum circuitry.

Sanchayeeta Jana
Fluorescence-detected Two Dimensional Electronic Spectroscopy (F-2DES) of Single Molecules in Solution
Dissertation, Bayreuth (2024) online, ERef, abstract: show

We can get a clearer picture of the microcosm around us by studying individual molecules. Since a molecule is only a few angstroms (10⁻¹⁰ m) in size, we can not observe or study such tiny objects with the naked eye. For this reason, scientists have developed a variety of experimental tools to delve deep into the understanding of the behavior of individual molecules. These tools include optical and electron microscopes capable of imaging molecules. Although the optical microscope has a lower spatial resolution than its electron counterpart, the advantage of optical microscopes is that one can do optical spectroscopy simultaneously. Absorption and emission spectroscopy provide information about the electronic energy levels of the molecule, while infrared spectroscopy provides information about vibrational energy levels, revealing the molecular structure. Ultrafast pump-probe spectroscopy, in which two ultrashort pulses are used to excite and then probe the excited state of the molecule, provides dynamic information on the ultrashort timescale. Here, we present a variant of ultrafast spectroscopy, Two-dimensional Electronic Spectroscopy (2DES) of single molecules to study the structural and dynamical properties of individual molecules. Two-dimensional electronic and infrared spectroscopy are widely used for an ensemble of molecules and have proven to be an important experimental tool for studying ultrafast energy transfer processes and linewidth broadening mechanisms. However, the implementation of this technique at the single molecule level is experimentally challenging. Because 2D spectroscopy is a nonlinear technique, one must saturate the excitation, detect the nonlinear signal contribution from a single molecule, and separate it from the linear signal from the molecule and the background signal from trillions of host molecules. Therefore, an extremely sensitive detection technique combined with powerful signal processing mechanisms is required to measure the 2D spectra of single molecules. We use confocal fluorescence microscopy to detect fluorescence signals from individual molecules. We combine the microscope with four phase-modulated pulses with three controllable interpulse delays to excite and de-excite the molecule. Three of the four pulses create a third-order nonlinearity in the system, and the fourth pulse converts the coherence state to a population state that emits the fluorescence signal, which we then detect with a single photon counting detector. This technique, known as fluorescence-detected two-dimensional electronic spectroscopy (F-2DES), has the advantage of colinear geometry; thus, we can integrate it with an optical microscope. As the molecule interacts with the four laser pulses, there are many different interaction paths according to the Liouville-von Neumann equation. The emitted fluorescence signal contains signal contributions from all the possible pathways, and phase-sensitive detection allows the identification of different interaction pathways. We measure the fluorescence signal in the time domain as a function of the interpulse delay and convert it to the frequency domain spectra by Fourier transformation. To generate the sequences of four phase-modulated pulses, we build a four-arm cascaded interferometer. To test our technique, we measured the 2D spectra of Rubidium (Rb), which has a very narrow absorption line, making the interpretation of the 2D spectra straightforward. We have developed a phase correction algorithm to phase correct the broadband 2D spectra. After successfully testing our setup, we measured 2D spectra of single freely diffusing CF 800 molecules in the dimethyl sulfoxide (DMSO) solution. We analyzed the nonlinear non-rephasing and rephasing spectra of CF 800 molecules and calculated the purely absorptive 2D spectra. We were able to separate the homogeneous broadening from the inhomogeneous broadening in the purely absorptive 2D spectra. The 2D spectra measured at different population times indicated coupling between different vibration levels in the molecules. We needed only about ten million photons from the freely diffusing molecules to measure the 2D spectra; this demonstrates the possibility of measuring the 2D spectra of an immobilized molecule that irreversibly photobleaches after emitting a few million photons. In general, our method provides a wealth of information on linear and nonlinear dynamical processes of single molecules on different time scales, from femtoseconds to a few hundred seconds.

Lucas Ludwig
Entwicklung eines aktiven Holraumresonators zur Detektion von Mikroplastik in hochdissipativen Medien
Masterarbeit, Bayreuth (2023), ERef

Johannes Dachs
Aufbau eines Mikormanipulators zum Transfer von monokristallinen Gold-Flocken
Bachelorarbeit, Bayreuth (2023), ERef

Tina Hammerschmidt
Farbzentren in Diamant als Quantensensor
Bachelorarbeit, Bayreuth (2023), ERef

Julian Alin
Graustufen-Lithografie für plasmonische Wellenleiter
Masterarbeit, Bayreuth (2023), ERef

Kathrin Ständer
Photostabilität von einzelnen Dibenzothiophen-Molekülen bei Anregung mit kurzen Laser-Pulsen
Bachelorarbeit, Bayreuth (2022), ERef

Svenja Hofmann
Entwicklung, Charakterisierung und Anwendung eines 3D-gedruckten Gitterspektrometers
Bachelorarbeit, Bayreuth (2022), ERef

Tim Pfadenhauer
Numerical Optimization of the Coupling Between a Quantum Dot and a Plasmonic Waveguide on a Semiconductor Substrate
Masterarbeit, Bayreuth (2022), ERef

Kilian Wittmann
Abbildung der nichtlinearen Antwort von plasmonischen Nanostrukturen
Masterarbeit, Bayreuth (2022), ERef

Andreas K. Schmid
Winkelaufgelöste Spektroskopie periodischer plasmonischer Nanostrukturen
Masterarbeit, Bayreuth (2022), ERef

Christian Schörner
Crystalline Silver Plasmonic Nanocircuitry for Efficient Coupling with Single Organic Molecules
Dissertation, Bayreuth (2022) online, ERef, abstract: show

Quantum emitters such as atoms, quantum dots or single organic molecules, with a size of a few Angström to a few nanometer, are among the tiniest light sources imaginable. The coupling of a single quantum emitter to an optical waveguide is particularly interesting, as the quantum mechanical state of the emitter can be accessed and modified via optical signals propagating along the waveguide. At room temperature, the interaction of a quantum emitter embedded in a solid with light is typically a weak effect, as the cross-section for absorption and scattering is by many orders of magnitude smaller than a diffraction-limited spot of light. Future applications of the waveguide-emitter system in nanophotonic circuitry, however, require an efficient coupling of the emitter and the waveguide circuit in order to achieve a successful implementation of functionalities, such as the realization of a single-photon transistor. Plasmonic nanostructures from noble metals break the diffraction limit by mixing optical fields with electronic excitations and thus promise to open up regimes of much more efficient light-matter interaction by confining the light to the position of the quantum emitter. In this regard, this thesis considers complex-shaped plasmonic nanocircuits fabricated from crystalline silver in order to realize an efficient emitterwaveguide junction in the visible spectral range. To address this issue, high-quality materials and nanofabrication protocols are required, sensitive experimental setups are designed and operated, and comprehensive numerical modeling is performed to compare theoretical expectations with experimental results. At first, different numerical models are presented, which allow to calculate fundamental properties of plasmonic nanostructures. These include absorption and scattering calculations, eigenmodes of waveguides, propagation parameters, incoupling and collection efficiencies, and the full three-dimensional simulation of dipole-emission near a plasmonic waveguide. Furthermore, the theoretical framework for far-field imaging is briefly reviewed and implemented numerically. The following part is dedicated to high aspect ratio single-crystalline silver flakes, which form the general basis of all plasmonic nano-constructs throughout this thesis. Chemical synthesis protocols are presented that allow to fabricate laterally large (several 10 µm) but thin (< 100 nm) flakes of extraordinarily high quality. This quality is characterized by an elemental composition of pure silver, single-crystallinity across the whole structure and a truly atomically-smooth surface. Therefore, the flakes are ideal platforms for the top-down fabrication of plasmonic nanostructures of nearly arbitrary shape by focused ion beam milling. Furthermore, atomic layer deposition of few-nanometer cover layers is applied to protect the flakes and nanostructures from degradation. Based on these new opportunities of nanofabrication with single-crystalline silver, in the next part, the first successful fabrication and application of a complex-shaped silver plasmonic circuit is presented. Its central waveguide part is given by two silver nanowires separated by only 60 nm. By using silver instead of the conventionally used gold, the circuit can operate in the visible spectral range where a variety of dye molecules with high fluorescence quantum yield exists. A tiny (~20 nm) polystyrene bead doped with a few tens of fluorophores is attached to the circuit and its excitation is found to depend sensitively on the excited near-field close to the structure. Further, a controlled remote excitation of the bead via different coherent superpositions of waveguide modes, as well as the subsequent efficient funneling of the majority of the fluorescence (~63 %) back into the waveguide, is demonstrated. In the following chapter, single organic dye molecules located inside the 60nm gap between two silver nanowires are investigated. A newly constructed optical setup allows to detect for the first time a single-molecule nonlinearity in a plasmonic waveguide. This effect is demonstrated fully remotely, i.e., by applying the necessary optical pulse-sequence via propagating plasmonic modes traveling along the waveguide gap toward the molecule. A strong drop in the single-molecule fluorescence emission by stimulated emission is detected, signaling a nonlinear plasmon-plasmon interaction in the waveguide at the single molecule. Particularly, due to the plasmonic field confinement, the efficiency of triggering the stimulated emission transition is found to be about 30 times higher in the waveguide compared to a diffraction-limited Gaussian focus. Finally, isolated crystalline silver nanowires in a few nanometer distance to an underlying silver flake surface are investigated with regard to the extreme light-confinement in the gap. In particular, along the short nanowire axis, the structure represents a nanoscale metal-insulator-metal Fabry-Pérot cavity for gap-plasmons with a length of 20-30nm and a height of about 2nm only. The resonances of several silver cavities are investigated experimentally, finding a good agreement with numerical scattering simulations. Furthermore, an optical setup is presented that probes the reflected light from the sample with an auto-balanced photodetector, reaching a sensitivity of about 1E-6 at 1 Hz bandwidth. First steps and measurements toward such an ultra-sensitive probing of plasmonic nano-cavities are performed, with the future goal of enhancing the coherent light-matter interaction of single quantum emitters placed in the hot-spot region.

Valentin Dichtl
Anrege-Abfrage-Spektroskopie mit sehr hoher Repetitionsrate
Masterarbeit, Bayreuth (2022), ERef

Julian Obermeier
Ultrafast Nonlinear Spectroscopy of Nanostructures
Dissertation, Bayreuth (2022) online, ERef, abstract: show

Nanostructures like subwavelength metal nanoparticles or quantum dots build the bridge between the atomic length scale and the macroscopic one. Although they consist of thousands of atoms, they show properties only explainable by quantum mechanical approaches. Nowadays, those systems are widely used in commercially available technologies but are still object of scientific research, even down to the clarification of fundamental physical questions. The formation of strongly coupled systems, formed by two or more nanostructures, sources for entangled photons and the nonlinear response of nanosystems are topics of worldwide research in the field of nanooptics. Especially the later offers many open questions and often it is not clear, if restrictions like, e.g., symmetry restrictions from the macroscopic scale still hold in the nanoscopic universe. We address some of those open questions in this thesis experimentally as well as theoretically. The use of ultrafast pump-probe techniques allow us to reveal temporal dynamics on the femto second timescale, while spectroscopic measurements complete the picture. For theoretical modeling, we combine intuitive analytical models with numerical approaches like optimization algorithms and the Finite Element Method. Our research ranges from isolated gold nanoparticles over complex waveguide-like nanocircuits to layered semiconductor structures with embedded quantum dots. The first two chapters lay a basis of physical framework for the later investigated topics. The field of plasmonics is embedded in the theory of electromagnetism and we give an insight in the field of nonlinear optics. Moreover, we introduce computational electrodynamics with a focus on the Finite Element Method and nanostructures that are of interest in the subsequent work. The following two chapters focus on second-harmonic generation on the nanoscale. We demonstrate theoretically and experimentally, that the current understanding has to be expanded. In contrast to the past understanding, subwavelength particles made of a symmetric material do not necessarily have to possess a geometrical asymmetry to emit second-harmonic light. In fact, it is already sufficient if an optical or plasmonic mode of appropriate symmetry is present. We prove our hypothesis in a complex plasmonic nanocircuit, carefully exclude influences that could weaken our statement and, moreover, explore the nonlinear operation of the nanocircuit to its full extend. The later is described by an intuitive analytical model. The next chapter marks the transition towards time resolved experimental techniques. We investigate the temporal dynamics of a low density, quantum dot based semiconductor saturable absorber mirror. We reveal time constants of the recovery process after saturation and determine characteristic constants of the saturation process itself, offering enough information for a future application as mode locking device in an ultrafast red emitting vertical-external-cavity surface emitting laser. Moreover, we also conduct experiments with varying wavelength, that reveal the structure's behavior in spectral proximity to the desired operation wavelength. The last chapter of the thesis focuses again on plasmonic nanostructures but lays focus on the temporal dynamics of quasi free, hot electrons and their role in nonlinear optical processes. Up to now, this relation, as well as the connection to the bandstructure of the material itself, remains rather unclear. We present our experimental approach, involving a multi-color pump-probe setup. We create hot electrons in the conduction band with a 400nm pump pulse, probe the structure with a delayed infrared probe pulse and investigate the impact on the nonlinear signals. Namely, second-harmonic generation, third-harmonic generation and multi-photon luminescence. We observe partly differing changes in all three of them regarding the sign of the modulation as well as recovery and decay times. In the subsequent measurements, we sacrifice the spectral information for a better signal-to-noise ratio by a Lock-In detection scheme with a single photon counting device. Therefor, we focused on the dominating signal, given by changes in the photo luminescence. We investigate excitation power dependent changes in the pump-probe peak and present first steps towards a theoretical model involving a two-temperature model for the thermalization of the generated hot electrons. Moreover, we present a technique for ultrafast multi-color pulse characterization. We use difference-frequency generation in order to obtain cross-correlation frequency resolved optical gating traces from nonlinear micro crystals and also gold nanorods themselves in the sample plane of the experiment.

Felix Schablitzki
Raster-Kraft-Mikroskopie mit einer Kelvin-Sonde
Bachelorarbeit, Bayreuth (2021), ERef

Julius Hlawatsch
Simulating the Optical Properties of Nanosystems using the Discrete Dipole Approximation
Bachelorarbeit, Bayreuth (2021), ERef

Sophie Meißner
Angle-resolved emission of structured plasmonic waveguides
Bachelorarbeit, Bayreuth (2021), ERef

Michael Stur
Einzelmolekülspektroskopie an plasmonischen Oberflächen
Bachelorarbeit, Bayreuth (2021), ERef

Simon Durst
Excitonic Coupling to Surface Lattice Resonances : Experiment and Simulation
Masterarbeit, Bayreuth (2021), ERef

Niklas Stenger
Numerische Methoden zur Simulation mittels Nanoobjekten modulierter Fluoreszenz von Cadmiumselenid-Nanoemittern
Masterarbeit, Bayreuth (2021), ERef

Fabian Paul
Shaping ultrashort laser pulses for pump-probe spectroscopy
Masterarbeit, Bayreuth (2021), ERef

Sebastian Matt
Synthese und Charakterisierung von flachen Gold-Einkristallen für die Plasmonik
Bachelorarbeit, Bayreuth (2021), ERef

Johannes Klier
Lineare und nichtlineare Spektroskopie an Goldnanostrukturen und Graphen
Masterarbeit, Bayreuth (2020), ERef

Kilian Wittmann
Anregung von Molekülen mittels der dritten Harmonischen von plasmonischen Partikeln
Bachelorarbeit, Bayreuth (2020), ERef

Julian Alin
Spin-on-Glas-Schichten zur Planarisierung nanoskaliger Strukturen
Bachelorarbeit, Bayreuth (2020), ERef

Jonas Jürgen Albert
Aperturlose Nahfeldmikroskopie fluoreszierender Nanoobjekte
Dissertation, Bayreuth (2019) online, ERef, abstract: show

Die vorliegende Arbeit befasst sich mit der Inbetriebnahme und dem optisch erweiterten Aufbau eines Rasterkraftmikroskops, welches Licht-Materie-Wechselwirkung unterhalb der Auflösungsgrenze des Lichts ermöglicht. Mit diesem Setup werden individuelle halbleitende und organische Nanostrukturen untersucht, deren Eigenschaften sie interessant für neue Nanotechnologien machen. Dabei stehen vor allem ausgedehnte Zustände, entweder durch entsprechende Wechselwirkung zwischen den Strukturen oder innerhalb eindimensionaler Nanodrähte im Fokus, da hier effizienter Energietransport möglich ist. Dabei wird das Verhalten von Nanoobjekten mit einer Größe von nur wenigen Nanometern deutlich von deren Geometrie und dem dielektrischen Umfeld beeinflusst. Standardmäßige optische Spektroskopie liefert jedoch keine Auskunft über strukturelle Einzelheiten der Objekte in dieser Größenordnung. Durch die Verbindung von zeitaufgelöster Photoemission und Rasterkraftmikroskopie bietet unser Aufbau räumlich hochauflösende optische Spektroskopie, welche direkt mit Topographie-Informationen korreliert ist. Dabei wird die Möglichkeit einer gezielten Manipulation der Umgebung dazu genutzt, um die Emissionseigenschaften der Nanoobjekte zu ändern und zu untersuchen. Im ersten Teil der Arbeit wird auf die Methode der optischen Nahfeldmikroskopie eingegangen, dessen Herzstück ein Rasterkraftmikroskop darstellt. Hierbei werden verschiedene Betriebsmodi und grundlegende Rauschquellen diskutiert. Im Anschluss wird eine weitere Methode, die Kelvin-Probe-Force-Mikroskopie vorgestellt, welche einen Einblick auf unterschiedliche Austrittsarbeiten der Elektronen an Oberflächen liefert und so helfen kann, erzeugte Ladungsträger und deren Ausbreitung in den Strukturen darzustellen. Bei den drei genannten Rastersonden-Mikroskopie-Methoden dient eine metallische Spitze als Sonde, weshalb sie während einer Messung simultan eingesetzt werden können, um so maximale Information über die Probe zu erhalten. Der nächsten Abschnitt befasst sich mit dem experimentellen Aufbau des optischen Nahfeldmikroskops. Dabei wird das Rasterkraftmikroskop getestet und mit einem invertierten optischen Fluoreszenzmikroskop vereint. Um die notwendige Auflösung zu erhalten, werden Rauschquellen identifiziert und minimiert. Außerdem werden Details zu den wichtigsten verwendeten Messmethoden, beispielsweise Time-Correlated-Single-Photon-Counting bezüglich der Position der Nahfeldsonde, sowie der Kraftsensorkalibration und Mikroskop-Justage erläutert. Aufgrund ihrer Rolle für die Datenanalyse bei der Rasterkraftmikroskopie wird auch eine grundlegende Bildbearbeitung von Topographiemessungen vorgestellt. Im Folgenden wird mit dem Aufbau der Einfluss einer Goldspitze auf die Emissionseigenschaften einzelner CdSe-Nanokristalle gemessen. Um das vorherrschende Quenching des Signals nachvollziehen und erklären zu können, dient ein analytischen Modell, welches zusätzlich durch Finite-Elemente-Simulationen bekräftigt wird. Es zeigt deutlich, wie das Quench-Verhalten von der Distanz zwischen Spitze und Probe, Spitzengeometrie, Polarisation des Emitterdipols und der Wellenlänge abhängt. Aufgrund der Stabilität des Verhaltens kann es zur genauen Justage des Mikroskops verwendet werden. Im weiteren Schritt werden kleine Nanokristallansammlungen untersucht, wobei die hohe räumliche Auflösung die Bestimmung der Emitteranzahl deutlich verbessert. Diese Ansammlungen zeigen ein ähnliches Lumineszenzauslöschung-Verhalten, wobei es mit steigender Anzahl zunehmend zu Abschirmeffekten kommt. Im nächsten Abschnitt werden CdSe-Nanodrähte untersucht, da aufgrund ihrer zweidimensionalen Einengung eine hohe Ladungsträgermobilität entlang ihrer Achse erwartet wird. Durch den Einfluss einer Goldspitze kann gezeigt werden, dass diese Drähte ausgedehnte Exzitonzustände besitzen und damit energieeffizienten Energietransport über Mikrometer gewährleisten können. Jedoch deutet bereits das unterschiedliche optische Verhalten der Drähte ihre Inhomogenität an. Entsprechend können auch ausgedehnte Zustände nur bei einer sehr geringen Anzahl an Drähten beobachtet werden. Damit dieses Verhalten also gezielt eingesetzt werden kann, müssen die Nanodrähte, welche ausgedehnte Zustände besitzen, reproduzierbar hergestellt werden können. Der letzte Teil der Arbeit behandelt das Emissionsverhalten von Polydiacetylen-Nanoröhrchen unter dem Einfluss einer Goldspitze. Diese besitzen aufgrund ihrer selbstorganisierten Anordnung eine hoch geordnete Struktur, womit auch das Verhalten gleicher Nanoröhrchen kaum divergiert. Hierbei existieren zwei Konformationen von Polydiacetylen, wobei die rote stark emittierend und die blaue dunkel ist. Deshalb versprechen die blauen Röhrchen energieeffizient beim Ladungstransport zu sein. Durch eine kontrollierte Manipulation der Umgebung mit der Goldspitze kann eine verstärkte Emission und eine Veränderung des Spektrums beobachtet werden, wobei diese auf spitzenverstärkte Ramanstreuung zurückgeführt werden kann.

Christoph Schnupfhagn
Four-wave mixing spectroscopy with shaped laser pulses
Masterarbeit, Bayreuth (2019), ERef

Lucas Ludwig
Modulationsspektroskopie einzelner Nanopartikel
Bachelorarbeit, Bayreuth (2019), ERef

Fabian Paul
Antennen für plasmonische Wellenleiter
Bachelorarbeit, Bayreuth (2018), ERef

Julia Lang
Erzeugung der dritten Harmonischen in Metallfilmen : Einfluss der Dünnfilminterferenz
Bachelorarbeit, Bayreuth (2018), ERef

Fabian Rottmann
Kopplung von Quantenpunkten an plasmonische Wellenleiter
Bachelorarbeit, Bayreuth (2018), ERef

Simon Durst
Angle-resolved transmission spectroscopy of plasmonic nanostructures
Bachelorarbeit, Bayreuth (2018), ERef

Hannah Bleiner
Aufbau eines Praktikumsversuchs zur dopplerfreien Sättigungsspektroskopie von Rubidium
Bachelorarbeit, Bayreuth (2018), ERef

Moritz Heindl
Aufbau und Charakterisierung eines Pulsformers
Masterarbeit, Bayreuth (2018), ERef

Patrick Pietsch
Entwicklung eines Faserinterferometers
Masterarbeit, Bayreuth (2018), ERef

Chiara Bauer
Hochauflösende Spektroskopie von Quantensystemen
Masterarbeit, Bayreuth (2018), ERef

Felix Baier
Resonanzfluoreszentspektroskopie an einzelnen Quantenpunkten
Bachelorarbeit, Bayreuth (2018), ERef

Jannik Kantelhardt
Spektroskopie der photoinduzierten Kraft zwischen plasmonischen Nanostrukturen
Bachelorarbeit, Bayreuth (2018), ERef

Heide-Maria Huber
Umgestaltung des Physik-Anfängerpraktium-Versuchs "Gekoppelte Pendel"
Zulassungsarbeit, Bayreuth (2018), ERef

Anna Ehrsam
Spektroskopie eines Goldnanopartikels auf einem Goldfilm
Bachelorarbeit, (2017), ERef

Sofie Krietenstein
Plasmonische Wellenleiter : Simulation und Experimente
Bachelorarbeit, (2017), ERef

Christian Dicken
Confocal Microscopy and Spectroscopy of Single Nanomagnets and Quantum Emitters
Dissertation, Bayreuth (2017) online, ERef, abstract: show

This thesis is embedded in the realm of single nanoparticle microscopy and spectroscopy. We combine confocal microscopy, ultrafast pulsed lasers and homodyne amplification to make the signature of single nanoparticles and quantum emitters visible. These techniques allow us to observe and manipulate the physical state of quantum dots on picosecond timescales, and we show that the spectral signatures found in our experiments compare well with what we expect from the dynamics of a three-level quantum emitter. These emitters are candidates as nodes of optical networks. Furthermore, we utilize homodyne amplification to analyze the magnetization of single nickel disks and develop a model that is able to predict the optical response of the disks when being embedded in the sample structure. The model allows us to separate the magnetic properties of the nanomagnets from the optical properties of the complete structure.

Julian Obermeier
Nonlinear spectroscopy of propagating surface plasmons
Masterarbeit, Bayreuth (2017), ERef

Simon Streit
Leistungsstabilisierung und Modulation eines durchstimmbaren Lasersystems
Bachelorarbeit, (2016), ERef

Daniela Wolf
Optical Spectroscopy of Graphene and Gold Nanostructures
Dissertation, (2016) online, ERef, abstract: show

This thesis covers two prominent material systems in the still emerging field of nano-optics. On the one hand, we study the optical properties of graphene, from a general point of view as well as under symmetry breaking induced by strain. On the other hand, we make use of the plasmonic properties of gold nanostructures and investigate the generated nonlinear signals on a local scale. With both systems being promising candidates for nanophotonic applications, a spectroscopic investigation is of utmost importance to gain a deeper understanding of the interaction of these systems with light. Graphene’s band structure exhibits a saddle point, which gives rise to a unique optical response in the visible and ultraviolet wavelength regime. Assuming a discrete excitonic state in the vicinity of the saddle point, a classical Fano model based on interference between a continuum of states and the discrete state can be applied. It reproduces very well both lineshape and position of the measured spectrum even down to infrared wavelengths. After a discussion of the general optical properties, we study the influence of strain on graphene. Unlike most other modifications, strain breaks the symmetry of the lattice and the band structure and is hence predicted to induce profound changes in the optical spectrum. In addition to a study of the response considering different substrate materials, we introduce Raman spectroscopy as a reference measurement to reliably quantify the amount of strain applied to the graphene flakes. However, our thorough investigations indicate that the Fano resonance in graphene is very robust towards strain as we do not observe strain-induced changes in the optical spectrum. The second part of this thesis addresses the nonlinear optical properties of gold nanostructures. Whereas most investigations in nonlinear plasmonics focus on the enhancement of nonlinear processes and their spectral characteristics, we consider the spatial origin of the signals which is still under debate. Due to the coherence of the higher harmonics generation process, the third-harmonic emission from different emitting centers of a nanostructure interferes. In analogy to classical diffraction experiments, the interference pattern observed in the Fourier plane is very sensitive towards parameters such as separation and relative phase of the emitting spots. We use this method to accurately determine the high fields inside the nanostructures as the source of third-harmonic generation. Moreover, we show that the emission properties of an elongated plasmonic structure can be switched between a configuration with one emission spot in the center and a configuration with two spatially separated spots by slightly varying the excitation wavelength. Due to the third power dependence of the third-harmonic generation process, the near-fields generated in the vicinity of the particle switch accordingly. While assemblies of nanoparticles are commonly used to shape fields, we show that the local field can also be sculptured around a single, elongated nanostructure by taking higher-order plasmonic modes into account. This approach opens up a new direction for field shaping on the nanoscale.

Marvin Berger
Optical spectroscopy of single units of semiconductor nanocrystal aggregates
Masterarbeit, (2016), ERef

Laura Meißner
Charakterisierung von ein- und zweidimensionalen plasmonischen Wellenleitern
Masterarbeit, (2016), ERef

Jessica Koller
Ortsaufgelöste Spektroskopie dünner Farbstofffilme zur Kopplung an plasmonische Oberflächen
Masterarbeit, (2016), ERef

Matthias Brandstetter
Apertureless Scanning Nearfield Optical Microscopy with Ultra-high Temporal Resolution
Dissertation, Bayreuth (2016) online, ERef, abstract: show

By combining an apertureless scanning nearfield optical microscope (aSNOM) with a pump probe scheme, we create a novel experimental tool called pump probe apertureless scanning near field optical microscope (ppaSNOM), that combines a temporal resolution of 1ps with a spatial resolution of 20nm. This alloptical technique far below the diffraction limit of light allows to study ultrafast processes on the nano scale. As a proof of principle system we choose the mechanical oscillations exhibited by gold nano discs that are impulsively heated through a short pump pulse. First we provide the theoretical foundation needed to understand the optical and mechanical properties of gold nano particles. In particular we use a FEM solver to predict the mechanical properties as well as the field distributions of gold nano discs promising high signal contrast for the experiment operating at 800nm. Furthermore the absorption and scattering cross section calculated with the T-Matrix are used to derive the ideal sample structure. Before aSNOM and farfield pump probe scheme are combined, we characterize them separately. The aSNOM is an interferometric technique that collects light scattered of a dielectric AFM tip. It allows for the simultaneous acquistion of the sample topography, magnitude and phase of the z-component of the electrical nearfield with a spatial resolution of 20nm. The aSNOM measurements of a gold disc with 100nm radius and 50nm height reveal a dipolar plasmon resonance which agrees very well with FEM simulations. In a next step we apply the pump probe measurement scheme to gold nano discs. The impulsive heating of a gold nano disc through a short pump pulse starts mechanical oscillations in the disc. As the volume changes periodically, the optical properties are modulated by the mechanical mode. In this experiment we measure the transient transmission signal of an individual gold nano disc. Changing the delay between pump and probe reveals an oscillatory delay trace as expected. The data analyzation reveals a mechanical oscillation frequency of 10GHz which we can attribute to the first order breathing mode in agreement with FEM simulations. In a last step we combine the aSNOM with the pump probe scheme in order to create a tool with 1ps temporal and 20nm spatial resolution. As a proof of principle measurement we are looking for a pump induced perturbation of the nearfield signal. We use FEM calculations to simulate the 2D distribution of the nearfield perturbation which reveals a dipolar shape. The measurements of several gold discs show no differential nearfield response. Instead we see a ring like structure in the differential nearfield signal that overlaps with the AFM topography. We conclude that the ring structure is an AFM artefact and that our signal is buried in the noise floor. We use the measurement data to estimate an upper limit for the relative pump induced perturbation. The results agree with T-Matrix simulations which suggest that an increase in relative sensitivity by a factor of 10¯² is needed. By reaching the shot noise limit with our ppaSNOM and modifying certain aspects, the detection of timeresolved nearfield signals seems feasible. Some ideas for possible modifications to the ppaSNOM, such as an increase in collection efficiency of the objective or the replacement of the dielectric AFM tip by a metallic tip, are given in the end. Due to time constraints an implementation of the modifications was not possible.

Max Theisen
Aufbau und Anwendung eines Plusformers für ultrakurze Laserpulse
Bachelorarbeit, (2016), ERef

Johannes Weißert
Dispersionsrelation von Oberflächenplasmonen
Zulassungsarbeit, (2016), ERef

Michael Seidel
Entwurf, Herstellung und Charakterisierung eines plasmonischen Wellenleiters
Bachelorarbeit, Bayreuth (2016), ERef

Marco Klement
A fiber interferometer for near-field microscopy
Masterarbeit, (2016), ERef

Christoph Schnupfhagn
Interferenz nichtlinearer Emission von Gold-Nanostrukturen
Bachelorarbeit, (2016), ERef

Jonas Jürgen Albert
Entwicklung und Aufbau eines optischen Nahfeldmikroskops
Masterarbeit, Bayreuth (2015), ERef

Alexander Neufeld
Characterization of two synchronized laser oscillators
Masterarbeit, (2015), ERef

Daniel Sommermann
Entwicklung von Goldspitzen für ein Rastersondenmikroskop
Bachelorarbeit, Bayreuth (2015), ERef

Thorsten Schumacher
Optical Nanoantennas for Ultrafast Nonlinear Spectroscopy of Individual Nanosystems
Dissertation, Bayreuth (2014) online, ERef, abstract: show

Die Arbeit befasst sich mit der ultraschnellen nichtlinearen Dynamik verschiedener Prozesse in individuellen metallischen und halbleitenden Nanostrukturen, ohne die Mittlung über Ensembles. Nanoobjekte mit einer Größe von nur wenigen Nanometern zeigen außergewöhnliche lineare wie auch nichtlineare optische Eigenschaften. Die zeitabhänginge Abweichung von linearer Licht-Materie Wechselwirkung wird mittels ultraschneller nichtlinearer Spektroskopie untersucht, bei einer Zeitauflösung von weniger als einer Pikosekunde. In der Erforschung einzelner Nanoobjekte, wie Quantenpunkte, Moleküle oder Nanopartikel, ist das bereits schwache nichtlineare Signal von makroskopischer Materie weiter verringert. Optische Nanoantennen, bestehend aus plasmonischen Nanoobjekten, erhöhen lokal die Licht-Materie Wechselwirkung und bieten ein neues Hilfsmittel um zuvor unzugängliche Größen des Nanokosmos zu untersuchen. Die Entwicklung und Anwendung solcher Antennen zur Verstärkung ultraschneller nichtlinearer Signale von einzelnen Nanoobjekten soll erstmalig umgesetzt werden und erfordert hochsensitive experimentelle Methoden und eine gezielte Modellierung und Optimierung wobei numerischen Lösungsverfahren und Modellbildung zum Einsatz kommen. Im ersten Teil der Arbeit wird auf unsere Methode der hochsensitiven ’zeitabhängigen differenziellen Transmissions-Spektroskopie’ eingegangen, gefolgt von den Erweiterungen für zeitaufgelösten ’Einzelphotonen Photolumineszenz-Spektroskopie’ und ’Dunkelfeld-Spektroskopie’. Weiterhin bieten wir einen Überblick über die entwickelten und angewandten numerischen Modelle, welche als Basis unserer theoretischen Arbeit dienen. Im Besonderen wird ein Modell zur Vorhersage der polarisationsabhängigen Emission höherer Harmonischer von komplexen Nanostrukturen vorgestellt und diskutiert. Der nächsten Abschnitt befasst sich mit der erstmaligen Realisierung einer optischen Nanoantenne zur Verstärkung eines extrem schwachen nichtlinearen Signals. Zu diesem Zweck verwenden wir die zeitabhängige Modulation der optischen Eigenschaften eines einzelnen Gold-Nanopartikels, verursacht durch dessen mechanische Oszillationen. Die Antenne wird durch eine zweite, größere Nanostruktur realisiert und befindet sich im Abstand von nur wenigen Nanometern zum untersuchten Nanopartikel.Die Wechselwirkung zwischen beiden Nanoobjekten und die angestrebte Antennenverstärkung kann im Rahmen der Plasmonhybrisierung verstanden werden. Dabei wird das schwache, nichtlineare Signal des Nanopartikels auf das starke Trägersignal der Antenne moduliert. Wir bieten eine detaillierte Einführung in die theoretische Modellierung und experimentelle Analyse. Die gute Übereinstimmung bestätigt die Analogie zu bekannten Radiofrequenzantennen die bei niedereren Frequenzen arbeiten. Im Weiteren ermöglicht unsere hochsensible Methode zum ersten Mal die spektral aufgelöste Untersuchung von ultraschnellen Ladungsträgerdynamiken innerhalb quantisierter Zustände eines einzelnen CdSe Nanodrahtes. Wir messen das anregungsinduzierte Bleichen unterschiedlicher Exzitonenübergänge und erhalten Einsicht in zuvor versteckte Prozesse und Größen wie zum Beispiel die zeitabhängige Population verschiedener Zustände. Die beobachteten Phänomene spielen sich auf unterschiedlichen Zeitskalen ab und werden im Einzelnen diskutiert. Weiterhin finden wir Hinweise für Reabsorptionsprozesse von emittierten Photonen. Zuletzt untersuchen und diskutieren wir die Wechselwirkung zwischen einem einzelnen CdSe Nanodraht und einer plasmonischen Antenne. Der letzte Teil der Arbeit bietet eine allgemeine Diskussion von optischen Nanoantennen. Zu diesem Zweck verwenden wir einen Punkt-Dipol Ansatz auf Basis der ’Discrete Dipole Approximation’, um im Weiteren besonderen Wert auf die elementaren Wechselwirkungsmechanismen zwischen Nanopartikel und Antenne zu legen. Weiterhin erlaubt uns der stark reduzierte Rechenaufwand riesige, zuvor unzugängliche Parameterräume zu analysieren. Wir verwenden die Methode und diskutieren die relevanten Eigenschaften einer optischen Nanoantenne mit maximaler Effizienz. Mittels der Implementierung eines genetischen Algorithmus bieten wir einen ersten Schritt zum Auffinden optimaler Mehr-Teilchen Antennengeometrien.

Christoph Soldner
Hochgeschwindigkeitsdatenerfassung einer Zeilenkamera für ein Laserspektrometer
Bachelorarbeit, (2014), ERef

Julian Obermeier
Implementierung eines optisch- parametrischen Oszillators zur Anregungsspektroskopie
Bachelorarbeit, Bayreuth (2014), ERef

Christian Schörner
Schaltbare Wechselwirkung zwischen photochromen Dithienylethen und plasmonischen Nanostrukturen
Bachelorarbeit, (2014), ERef