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Defect engineering is promising to tailor the physical properties of two-dimensional (2D) semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors have not been clarified. In this work, we thoroughly studied the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2S4 via versatile spectroscopic tools in combination with theoretical calculations. It has been demonstrated that the Zn-In anti-structural defects induce the formation of a series of donor and acceptor levels inside the bandgap, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair (DAP) in Znln2S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2S4 was directly revealed by back focal plane (BFP) imagining, where it presents obviously in-plane dipole orientation within a dozen layers thickness of Znln2S4. These unique features of defects in Znln2S4 including extrinsic absorption, rich recombination paths, gate tunability and in-plane dipole orientation will definitely benefit to the advanced orientation-functional optoelectronic applications.

Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics. Here we report that light-induced halide segregation is strain-activated in MAPb(I1−xBrx)3 with Br concentration below approximately 50%, while it is intrinsic for Br concentration over approximately 50%. Free-standing single crystals of CH3NH3Pb(I0.65Br0.35)3 (35%Br) do not show halide segregation until uniaxial pressure is applied. Besides, 35%Br single crystals grown on lattice-mismatched substrates (e.g. single-crystal CaF2) show inhomogeneous segregation due to heterogenous strain distribution. Through scanning probe microscopy, the above findings are successfully translated to polycrystalline thin films. For 35%Br thin films, halide segregation selectively occurs at grain boundaries due to localized strain at the boundaries; yet for 65%Br films, halide segregation occurs in the whole layer. We close by demonstrating that only the strain-activated halide segregation (35%Br/45%Br thin films) could be suppressed if the strain is properly released via additives (e.g. KI) or ideal substrates (e.g. SiO2)., Mixed-halide perovskites are of interest for photovoltaic devices, but light-induced halide segregation obstructs bandgap tuning and is not fully understood. Here the authors study the effects of strain and iodide/bromide ratio on light-induced halide segregation in mixed-halide perovskites.

Mechanically controllable break junctions are one suitable approach to generate atomic point contacts and ultrasmall and controllable gaps between two metal contacts. For constant bias voltages, the tunneling current can be used as a ruler to evaluate the distance between the contacts in the sub-1-nm regime and with sub-Å precision. This ruler can be used to measure the distance between two plasmonic nanostructures located at the designated breaking point of the break junction. In this work, an experimental setup together with suitable nanofabricated break junctions is developed that enables us to perform simultaneous gap-dependent optical and electrical characterization of coupled plasmonic particles, more specifically bowtie antennas in the highly interesting gap range from few nanometers down to zero gap width. The plasmonic break junction experiment is performed in the focus of a confocal microscope. Confocal scanning images and current measurements are simultaneously recorded and exhibit an increased current when the laser is focused in the proximity of the junction. This setup offers a flexible platform for further correlated optoelectronic investigations of coupled antennas or junctions bridged by nanomaterials.

Nature Communications 13(1), 892 (2022). doi:10.1038/s41467-022-28486-3, We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffractionto reveal surface defects in supercrystals of self-assembled cesium lead halideperovskite nanocrystals and study their effect on the fluorescence properties. Upon comparisonwith density functional modeling, we show that a loss in structural coherence, anincreasing atomic misalignment between adjacent nanocrystals, and growing compressivestrain near the surface of the supercrystal are responsible for the observed fluorescenceblueshift and decreased fluorescence lifetimes. Such surface defect-related optical propertiesextend the frequently assumed analogy between atoms and nanocrystals as so-called quasiatoms.Our results emphasize the importance of minimizing strain during the self-assemblyof perovskite nanocrystals into supercrystals for lighting application such as superfluorescentemitters., Published by Nature Publishing Group UK, [London]

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid material under optical excitation. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation induced emission, can result in unprecedented emerging optical properties., 27 pages, 10 figures

We report an easy and broadly applicable method for the controlled self-assembly of atomically precise Au32(nBu3P)12Cl8 nanoclusters into micro-crystals. This enables the determination of emergent optoelectronic properties resulting from long-range order in such assemblies. Compared to the same nanoclusters in glassy, polycrystalline ensembles, we find a 100-fold increase in the electric conductivity and charge carrier mobility as well as additional optical transitions. We show that these effects are due to a vanishing energetic disorder and a drastically reduced activation energy to charge transport in the highly ordered assemblies. This first structure-transport correlation on self-assembled superstructures of atomically precise gold nanoclusters paves the way towards functional materials with novel collective optoelectronic properties., 30 pages, 11 Figures

The strong coupling of an IR-active molecular transition with an optical mode of the cavity results in vibrational polaritons, which opens a new way to control chemical reactivity via confined electromagnetic fields of the cavity. In this study, we design a voltage-tunable open microcavity and we show the formation of multiple vibrational polaritons in methyl salicylate. A Rabi splitting and polariton anticrossing behaviour is observed when the cavity mode hybridizes with the C=O stretching vibration of methyl salicylate. As this vibration contributes to the reaction coordinate of the photoinduced proton transfer process in methyl salicylate, we suggest the coupling might be used to modulate the photophysical properties of the molecule. Furthermore, the proposed theoretical model based on coupled harmonic oscillator reveals that the absorption of uncoupled molecules must also be considered to model the experimental spectra properly and that simultaneous coupling of multiple molecular vibrations to the same cavity mode has a significant influence on the Rabi splitting.

In this review, we focus on the experimental demonstration of enhanced emission from single plasmonic tunneling junctions consisting of coupled nano antennas or noble metal tips on metallic substrates in scanning tunneling microscopy. Electromagnetic coupling between resonant plasmonic oscillations of two closely spaced noble metal particles leads to a strongly enhanced optical near field in the gap between. Electron beam lithography or wet chemical synthesis enables accurate control of the shape, aspect ratio, and gap size of the structures, which determines the spectral shape, position, and width of the plasmonic resonances. Many emerging nano-photonic technologies depend on the careful control of such localized resonances, including optical nano antennas for high-sensitivity sensors, nanoscale control of active devices, and improved photovoltaic devices. The results discussed here show how optical enhancement inside the plasmonic cavity can be further increased by a stronger localization via tunneling. Inelastic electron tunneling emission from a plasmonic junction allows for new analytical applications. Furthermore, the reviewed concepts represent the basis for novel ultra-small, fast, optically, and electronically switchable devices and could find applications in high-speed signal processing and optical telecommunications.

Life as we know it cannot exist without photosynthesis, and even though the main photosynthetic mechanisms have been well investigated, some aspects are still unresolved. One example is the energy transfer to the reaction centers by accessory photosynthetic pigments after the absorption of photons. This process has an extremely high efficiency, which cannot be explained by a classical Foerster resonance energy transfer. However, a quantum mechanical process based on a coherent or wave-like energy transfer may provide an explanation for the high efficiency. In order to determine whether nature makes use of such a coherent process, we influence the potential coherence of photosynthetic pigments in vivo using an optical microresonator, which consists of two parallel silver mirrors separated only by the distance of a few wavelengths. The electromagnetic field inside such a microcavity is strongly confined, enabling coherent light-matter coupling. Here, we embedded living cyanobacteria of the species Synechococcus elongatus (strain PCC 7492) into the microresonator and exposed them to the confined electromagnetic field. The observation of vacuum Rabi splitting and anti-crossing observed in the transmission- and fluorescence spectra provides evidence of coherent coupling of the pigments with the resonator modes without harming the bacteria. Furthermore, we showed that not only some photosynthesis pigments are involved in this coupling, but all pigments in the excitation focus are coupled coherently. Our findings shed light on the function of quantum coherence in the evolution of photosynthetic organisms.

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Optical half-wave microresonators enable to control the optical mode density around a quantum system and thus to modify the temporal emission properties. If the coupling rate exceeds the damping rate, strong coupling between a microresonator and a quantum system can be achieved, leading to a coherent energy exchange and the creation of new hybrid modes. Here, we investigate strong coupling between two adjacent lambda/2 Fabry-P\'erot microresonators, where the resonance of one microresonator can be actively tuned across the resonance of the other microresonator. The transmission spectra of the coupled microresonators show a clear anticrossing behavior, which proves that the two cavity modes are strongly coupled. Additionally, we can vary the coupling rate by changing the resonator geometry and thereby investigate the basic principles of strong coupling with a well-defined model system. Finally, we will show that such a coupled system can theoretically be modelled by coupled damped harmonic oscillators.

Summary Integration of signalling on the cellular level is essential for the survival of organisms. Protein-protein interaction studies provide valuable insights in these signalling events. One of the best understood signalling pathways in plants is the brassinosteroid (BR) hormone signalling pathway, which is mediated by the receptor brassinosteroid insensitive 1 (BRI1) with its co-receptor bri 1- associated kinase (BAK1). Both BRI1 and BAK1 have been shown to interact with receptor like protein 44 (RLP44), which was implicated in cell wall integrity sensing by modulation of BL signalling. Here we provide evidence by quantitative in vivo three-fluorophore FRET-FLIM measurements, that RLP44, BRI1 and BAK1 form a trimeric complex in the plasma membrane of N. benthamiana leaf cells, with an estimated distance between them below 15 nm. The immune receptor flagellin sensing 2 (FLS2), which is also a receptor-like kinase like BRI1, is not integrated in a similar complex with RLP44 and BAK1. Our study supports that BRI1 and FLS2 are localized in distinct nanodomains in the PM. As the fluorescence lifetime of the donor is monitored, our method circumvents the extensive calculations necessitated by intensity-based FRET interaction assays and thus provides a feasible base for studying the sub-compartmentalization in the plasma membrane of living plant cells with a nanoscale resolution. Graphical abstract Principle of three fluorophore FRET-FLIM Förster resonance energy transfer (FRET) describes the energy transfer of a donor fluorophore (D) to an acceptor fluorophore (A1) if they are in close proximity, typically below 10 nm. Three fluorophore FRET-FLIM combines two FRET pairs in such a fashion, that the acceptor (A1) of the first pair, is at the same time donor to a second acceptor (A2). Thus, energy of D can either sequentially move from D via A1 to A2 and directly between D-A1 and D-A2. Previous studies have quantified the fluorescence intensities to monitor the distances between the three fluorophores. Here we establish the use of fluorescence lifetime imaging microscopy (FLIM), which measures the donor fluorescence lifetime (τ) to assess complex formation of three proteins of interest in the plasma membrane. Specifically, if both A1 and A2 drain energy from D, (or sequentially from D via A1 to A2), then the fluorescence lifetime decreases. Both the distance below 15 nm between D and A2 and the trimeric complex formation will result in an additional decrease of τ 3 compared to τ 2 and τ 2’ .

Hypericin can be found in nature in Hypericum perforatum (St. John's Wort) and has become subject of intense biochemical research. Studies report of antidepressive, antineoplastic, antitumor and antiviral activity of hypericin. Among the variety of potential applications hypericin can be used as photosensitizer in photodynamic therapy (PDT), where it is brought into cancer cells and produces singlet oxygen upon irradiation with a suitable light source. Therefore, the photophysical properties of hypericin are crucial for a successful application in a medical treatment. Here, we present the first single molecule optical spectroscopy study of hypericin. Its photostability is large enough to obtain single molecule fluorescence, surface enhanced Raman spectra (SERS), fluorescence lifetime, antibunching and blinking dynamics. Embedding hypericin in a PVA matrix changes the blinking dynamics, reduces the fluorescence lifetime and increases the photostability. Single molecule SERS spectra show both the neutral and deprotonated form of hypericin and exhibit sudden spectral changes, which can be associated with a reorientation of the single molecule with respect to the surface.

Optical half-wave microresonators enable to control the optical mode density around a quantum system and thus to modify the temporal emission properties. If the coupling rate exceeds the damping rate, strong coupling between a microresonator and a quantum system can be achieved, leading to a coherent energy exchange and the creation of new hybrid modes. Here, we investigate strong coupling between two adjacent lambda/2 Fabry-P��rot microresonators, where the resonance of one microresonator can be actively tuned across the resonance of the other microresonator. The transmission spectra of the coupled microresonators show a clear anticrossing behavior, which proves that the two cavity modes are strongly coupled. Additionally, we can vary the coupling rate by changing the resonator geometry and thereby investigate the basic principles of strong coupling with a well-defined model system. Finally, we will show that such a coupled system can theoretically be modelled by coupled damped harmonic oscillators.

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In this work we use femtosecond laser pulses of 800 nm wavelength to excite and characterize the multiphoton luminescence emission of single gold nanoparticles. For excitation with 100 fs laser pulses we observe a two and three photon emission dominated by radiative electron hole pair recombination, while the emission is caused by radiative plasmon decay for excitation with 500 fs pulses. For single gold nanorods with different aspect ratios, we study the interplay between the particle plasmon and electron hole pairs, which enables us to develop a quantitative model to fully describe the two and three photon luminescence emission of single gold nanoparticles.

The spectral properties and dynamics of the fluorescence emission of photosystem II core complexes are investigated by single-molecule spectroscopy at 1.6 K. The emission spectra are dominated by sharp zero-phonon lines (ZPLs). The sharp ZPLs are the result of weak to intermediate exciton-vibrational coupling and slow spectral diffusion. For several data sets, it is possible to surpass the effect of spectral diffusion by applying a shifting algorithm. The increased signal-to-noise ratio enables us to determine the exciton-vibrational coupling strength (Huang-Rhys factor) with high precision. The Huang-Rhys factors vary between 0.03 and 0.8. The values of the Huang-Rhys factors show no obvious correlation between coupling strength and wavelength position. From this result, we conclude that electrostatic rather than exchange or dispersive interactions are the main contributors to the exciton-vibrational coupling in this system.

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Contains fulltext : 136521.pdf (Publisher’s version ) (Open Access) During photosynthesis, energy conversion at the two photosystems is controlled by highly complex and dynamic adaptation processes triggered by external factors such as light quality, intensity, and duration, or internal cues such as carbon availability. These dynamics have remained largely concealed so far, because current analytical techniques are based on the investigation of isolated chloroplasts lacking full adaptation ability and are performed at non-physiologically low temperatures. Here, we use non-invasive in planta spectro-microscopic approaches to investigate living chloroplasts in their native environment at ambient temperatures. This is a valuable approach to study the complex function of these systems, because an intrinsic property-the fluorescence emission-is exploited and no additional external perturbations are introduced. Our analysis demonstrates a dynamic adjustment of not only the photosystemI/photosystemII (PSI/PSII) intensity ratio in the chloroplasts but also of the capacity of the LHCs for energy transfer in response to environmental and internal cues.

The enhancement of fluorescence and Raman scattering by plasmonic nanostructures is studied theoretically with special focus on the effects of the observed molecule's properties and the realistic geometry of the plasmonic nanostructure. Numerical experiments show that the enhancement factor may vary by many orders of magnitude depending on a fluorophore's transition rates or intrinsic quantum yield. For different molecules, boosting fluorescence enhancement means optimizing different factors, leading to a different ideal geometric and spectral configuration. This framework, coupled with powerful new simulation tools, will facilitate the design and characterization of fluorescence-enhancing plasmonic nanostructures as well as yield experimental access to the intrinsic properties of the molecules under study.

Second-harmonic generation (SHG) is investigated from three kinds of lithographically fabricated plasmonic systems: Al monomers, Au monomers and Au-Al heterodimers with nanogaps of 20 nm. Spectrally integrated SHG intensities and the linear optical responses are recorded and compared. The results show that for the monomer nanoantennas, the SHG signal depends sensitively on the linear excitation of the plasmon resonance by the fundamental wavelength. For Au-Al heterodimer nanoantennas, apart from fundamental resonant excitation, nonlinear optical factors such as SH driving fields and phase interferences need to be taken into account, which play significant roles at the excitation and scattering stages of SHG radiation. It is interesting to note that a possible energy transfer process could take place between the two constituting nanoparticles (NPs) in the Au-Al heterodimers. Excited at the linear plasmon resonance, the Au NP transfers the absorbed energy from the fundamental field to the nearby Al NP, which efficiently scatters SHG to the far-field, giving rise to an enhanced SHG intensity. The mechanisms reported here provide new approaches to boost the far-field SHG radiation by taking full advantage of strongly coupled plasmonic oscillations and the synergism from materials of different compositions.

Diese Arbeit behandelt einen Teilaspekt der Photosynthese. Sie liefert einen Erklärungsansatz dafür, dass photosynthetische Organismen nicht nur die newtonsche Mechanik und Thermodynamik, sondern auch nicht-triviale Quanteneffekte nutzen und davon profitieren. Ein neuartiger quantenoptischen Ansatz soll dies ermöglichen. In einem eigens für diese Arbeit optimierten Fabry-Pérot-ähnlichen Mikroresonator und einem dafür gebauten zeitaufgelösten konfokalen Mikroskop werden lebende Cyanobakterien der Art Synechococcus elongatus unter physiologischen Bedingungen untersucht. Parameter wie die Fluoreszenzlebensdauer des Photosystems, die quantenoptische Kopplung von individuellen Cyanobakterien mit einem Mikroresonator und die optisch ausgelesene photosynthetische Aktivität liefern starke Hinweise auf die Existenz nicht-trivialer Quanteneffekte im Photosystem. Ein zweiter Schwerpunkt dieser Arbeit ist die Aufnahme und Analyse von Fluoreszenzlebensdauern von Farbstoffen in Tumorsphäroiden sowie von Quanten Dots in einer speziellen chemischen Umgebung.

This thesis addresses the interdisciplinary application of spectromicroscopic techniques and the subsequent design of a comprehensive microscopic platform. In classical microscopy, the detected light intensity of a sample is the main source of information. Spectroscopy, i.e., the analysis of light-matter interactions, additionally analyses the physical properties of light and is thus an ideal complement to microscopy. It provides additional information that enables a much more comprehensive characterization of samples. Especially in the life sciences, the combination of imaging with spatially resolved spectroscopy (spectromicroscopy) is key to modern microscopy methods and serves as an important tool to gain ever deeper insights into the complexity of biological processes. Accordingly, the demands on a modern research microscope have steadily increased. By means of interdisciplinary research projects within the scope of this work, the successful application of several spectromicroscopy methods are demonstrated, including the detection of protein-protein interactions with FLIM-FRET (fluorescence lifetime imaging microscopy with Förster resonance energy transfer), contrast enhancement for autofluorescent samples with FIDSAM (fluorescence intensity decay shape analysis), the photophysical characterization of biological interaction markers with single-molecule spectroscopy, super-resolution imaging with single-molecule localization microscopy and single particle tracking of individual plasma membrane proteins in living cells. Using the knowledge gained from these projects, a microscope platform was designed and built to provide a unique combination of advanced light microscopy techniques for interdisciplinary research. This work offers the conceptual design as well as a detailed description of the microscope.

Mikroresonatoren, die für Licht im optischen Bereich entworfen sind, stellen nützliche Werkzeuge dar, um in die Licht-Materie-Wechselwirkung einzugreifen und sind deshalb oft verwendete Instrumente, um die spektralen und zeitlichen Eigenschaften nanoskopischer Objekte zu überwachen und zu beeinflussen. In dieser Arbeit wird die Lumineszenz von organischen Farbstoffmolekülen, energietransfergekoppelten Modellsystemen, photosynthetischen Komplexen und Metallnanopartikeln untersucht und durch den direkten Einfluss der photonischen und phononischen Umgebung kontrolliert. Die experimentellen Beobachtungen wurden durch konfokale Mikroskopie, in Kombination mit Spektroskopie und zeitkorrelliertem Einzelphotonenzählen, erzielt. Der photonische Einfluss wurde durch einen stimmbaren Fabry-Pérot Mikroresonator erreicht und der phononische Einfluss durch einen Helium-Bad-Kryostaten der das Experimentieren bei Temperaturen bis zu 1.6 K ermöglicht.

Combining confocal microscopy with higher order laser modes, so-called radially and azimuthally polarized Doughnut modes, allows to uncover subdiffractional information about individual gold nanoparticles. Either the elastically scattered light or the photoluminescence emission of spatially isolated particles was detected. Thus, differently shaped particles render characteristic patterns. For gold nanorods and triangles, the 2D particle orientation is directly visualized and can be precisely determined. In-situ atomic force microscopy (AFM) was used to independently measure the particles’ orientations. Especially when the scattered light is detected, the sensitive dependence of the signal amplitude and the signal-to-noise ratio allows to probe dielectric interfaces. Moreover, combining experimental scattering images with simulated data, the 3D orientation of gold nanorods embedded in a polymer film can be detected. Finally, apertureless scanning near-field optical microscopy (SNOM) shows enhanced photoluminescence emission at the tips of gold nanorods. Durch die Kombination von konfokaler Mikroskopie mit Lasermoden höherer Ordnung, so genannten radial und azimutal polarisierten Doughnut-Moden, können Informationen auf Nanometerskala über einzelne Goldnanopartikel erhalten werden, die mit beugungsbegrenzter Mikroskopie nicht ohne Weiteres zugänglich sind. Dabei kann sowohl das elastisch gestreute Licht als auch die Photolumineszenzemission der Partikel detektiert werden. Verschiedene Partikelformen lassen sich auf Grund ihrer charakteristischen Muster unterscheiden. Im Fall von Goldnanostäbchen und -dreiecken wird darüber hinaus die zweidimensionale Partikelorientierung direkt abgebildet und kann präzise bestimmt werden. Die Orientierung einzelner Partikel wurde parallel mit in-situ Atomkraftmikroskopie bestätigt. Besonders bei der Detektion des elastisch gestreuten Lichts lassen die Signalintensität und das Signal-Rausch-Verhältnis Rückschlüsse auf die dielektrische Probengrenzfläche zu. Werden Goldnanostäbchen in einen Polymerfilm eingebettet, kann ihre dreidimensionale Orientierung durch Vergleich mit theoretischen Simulationen der Streumuster abgeschätzt werden. Mittels aperturloser optischer Nahfeldmikroskopie (SNOM) wird die verstärkte Photolumineszenzemission an den Enden von Goldnanostäbchen bestimmt.

This work describes the development and the implementation of several new functions in an existing near-field optical microscope based on a parabolic mirror for excellent tip illumination and optical signal collection. The improved microscope is an ideal tool for performing imaging and spectroscopy on opaque samples, in particular molecular films deposited on flat substrates with a spatial resolution on the order of several nm. The new functions were hyperspectroscopic imaging, fluorescence life time imaging (FLIM) and scanning bias voltage tip enhanced Raman spectroscopy (SBV-TERS) based on a self built Scanning Tunneling Microscope (STM)-scan head. Utilizing these improvements, several new measurement methods have been developed, were the bias voltage is used to apply a strong electric field in the gap mode to manipulate and directly monitor the sample on a molecular scale. The results out of this thesis demonstrate how optical enhancement inside the gap mode can be further increased by a stronger localization via tunneling through a molecule. The bias voltage activated stimulated emission from an illuminated single molecule tunneling junction can improve the fundamental understanding of quantum plasmonics and lead to new analytical applications. Furthermore, this concept represents the basis for novel ultra-small, fast, optically and electronically switchable devices and could find applications in high-speed signal processing and optical telecommunications. The thesis is composed on the development of the setup and the following experiments exploring different questions on organic semi conductors. This work consists three chapters, describing the evolution of the microscope and the methods used for investigating different molecular systems which are relevant for organic electronics.

The scope of this work is the interaction of plasmonic nano structures of cylindrical symmetry with radially and azimuthally polarized laser modes. Five different nanostructures, namely gold nanoposts, gold nano disks, gold nanocones, gold nanorings and plasmonic oligomers, and their interaction with radially and azimuthally polarized lasermodes are investigated by studying the spectral properties of the one photon luminescence of gold. Das Ziel dieser Arbeit ist die Untersuchung der Wechselwirkung von zylindersymmetrischen plasmonischen Nanostrukturen mit radial und azimutal polarisierten Laser Moden. Die Wechselwirkung von fünf verschiedene Nanostrukturen, nämlich Gold Nanopfosten, Gold Nanoscheiben, Gold Nanokegel, Gold Nanoringe und plasmonischer Oligomere, mit radial und azimutal polarisierten Laser Moden werden durch spektroskopische Studien der Einphotonen-Goldlumineszenz untersucht.

Diese Arbeit beinhaltet experimentelle und theoretische Untersuchungen der optischen Frequenzverdopplung (second-harmonic generation, kurz SHG) an metallischen Nanopartikeln. Frequenzverdopplung bedeutet, daß ein bei der Frequenz omega angeregtes Nanopartikel Strahlung der Frequenz 2*omega emittiert. Dieser Effekt tritt nicht nur in Materialien mit nichtzentrosymmetrischer Kristallstruktur, sondern auch an der Oberfläche von Metallen auf. Deshalb läßt er sich gut mit plasmonischen Feldüberhöhungen an metallischen Nanoantennen verbinden. Die Frequenzverdopplung wird an verschiedenen Nanostrukturen wie dreieckförmigen, stäbchenförmigen und vor allem kegelförmigen Nanopartikeln experimentell untersucht, welche aufgrund ihrer scharfen Spitzen starke SHG-Signale emittieren. Besonders die Kegel sind interessant: Bei Anregung mit einem fokussierten, radial polarisierten Strahl dominiert je nach Kegelgröße und Umgebungsmedium ein SHG-Signal entweder von der Spitze oder von der Bodenkante des Kegels. Diese an den Kegeln gemessenen Resultate werden durch theoretische Untersuchungen untermauert. In diesen Rechnungen werden die plasmonischen Feldüberhöhungen und die sich daraus ergebende Frequenzverdopplung für einen Kegel mit verschiedenen Parametern modelliert. An einem einzelnen Kegel gewonnene Resultate werden auch mit den Fällen eines kugelförmigen und eines stäbchenförmigen Partikels verglichen. Ein weiterer Gegenstand der theoretischen Untersuchungen ist die Superposition der zweiten Harmonischen von mehreren emittierenden Nanopartikeln zu einem Feldmaximum. Dabei wird eine kreisförmige Anordnung von 8 Nanostäbchen bzw. Nanokegeln von einer radial polarisierten Mode angeregt. Die Superposition der emittierten zweiten Harmonischen ergibt ein Feldmaximum innerhalb der Anordnung der Emitter. Durch eine Verkippung des anregenden Strahls kann dieser Fokus im Raum bewegt werden. Letztere Untersuchung ist insbesondere interessant im Hinblick auf lokalisierte photochemische Reaktionen, die durch das frequenzverdoppelte Licht von Nanopartikeln ausgelöst werden sollen. Mit chemischen Substanzen, die bei omega transparent, bei 2*omega aber photoreaktiv sind, wäre im Nahfeld dieser Nanoantennen eine starke Lokalisierung der Reaktion auf Bereiche kleiner als 100~nm möglich. Anhand von Photolacken und Polymermatrizen mit diesen Eigenschaften wird experimentell untersucht, ob frequenzverdoppeltes Licht überhaupt solche Reaktionen auslösen kann oder ob die photochemische Reaktionen überwiegend durch direkte Zwei-Photonen-Absorption des anregenden Lichts ausgelöst werden. Die Ergebnisse zeigen allerdings, daß die Zwei-Photonen-Absorption dominant ist. Durch die Zwei-Photonen-Absorption im Nahfeld von Partikeln ist aber dennoch eine vergleichbare Lokalisierung der Reaktion möglich.:1. Einführung 1.1 Frequenzverdopplung an Nanopartikeln 1.2 Photochemisches Schreiben auf kleinen Längenskalen 2. Theoretische Grundlagen 2.1 Nichtlineare optische Effffekte zweiter Ordnung 2.2 Frequenzverdopplung in Metallen 2.3 Frequenzverdopplung bei metallischen Nanopartikeln 2.4 Überlagerung der Strahlung mehrerer frequenzverdoppelter Dipole 2.5 Core-Shell-Nanopartikel mit nichtzentrosymmetrischem Kern 3. Experimenteller Aufbau 3.1 Beleuchtung der Proben und Detektionspfad 3.2 Objektiv und Probenhalter 3.3 Realisierung der radial polarisierten Mode 4. Messungen der zweiten Harmonischen an Nanostrukturen 4.1 Einzelne kugel- und stäbchenförmige Goldnanopartikel 4.2 Nanodreiecke (Fischer-Pattern) 4.3 Nanokegel 4.4 Nanostäbchen-Teppiche 4.5 Zusammenfassung 5. Nichtlinear-optisches photochemisches Schreiben auf kleinen Längenskalen 5.1 Photochemische Reaktionen auf der Sub-100nm-Skala 5.2 Erste Versuche an Photolacken 5.3 Photochemisches Schreiben auf azobenzolhaltigen PMMA-Copolymerschichten 5.4 Photochemisches Schreiben auf azosulfonathaltigen PMMA-Copolymerschichten 5.5 Ausblick 6. Zusammenfassung und Ausblick Anhang A. Darstellung der radialen Mode und des z-polarisierten Fokus B. Mehode der multiplen Multipole (MMP) C. Präparation der Proben Literaturverzeichnis Abbildungsverzeichnis Verzeichnis der Tabellen Verwendete Abkürzungen Liste der Veröffffentlichungen Danksagung Erklärung This work includes experimental and theoretical investigations of second-harmonic generation (SHG) at metallic nanoparticles. SHG means that a nanoparticle that is excited at the frequency omega emits radiation at the frequency 2*omega. SHG does not only occur in materials with noncentrosymmetric structure, but also on metal surfaces. Hence, SHG can be combined well with plasmonic field enhancement at metallic nanoantennae. SHG is investigated experimentally at different nanostructures such as triangle-like, rod-like and especially cone-like nanoparticles. With their sharp tips these structures show a much stronger SHG signal than spherical nanoparticles. Especially the cones are interesting: Excited with a focused radially polarized beam, for different cone sizes and in different surrounding media either the signal from the tip or the signal from the bottom edge dominates. The measurement results from the cones are underpinned by theoretical investigations. In these calculations the plasmonic field enhancements and the resulting SHG are modeled for a cone with different parameters. The single-cone results are also compared with the cases of a spherical or rod-shaped particle. A further subject of the theoretical investigations is the superposition of the SHG radiation from a number of emitting nanoparticles to a field maximum. For that, a circular arrangement of 8 nanorods or nanocones is excited by a radially polarized beam. The superposition of the second-harmonic radiation fields yields a field maximum in the space between the emitters. A tilt of the exciting beam can move this focus in space. The latter item is of special interest concerning localised photochemical reactions induced by the second-harmonic light from nanoparticles. In the near field of these nanoantennae, a strong localisation of the reaction on regions smaller than 100 nm would be possible by using chemical substances being transparent at omega, but photoreactive at 2*omega. With photoresists and polymer matrices, experiments are carried out to investigate whether SHG light can trigger such reactions at all, or if these photochemical reactions are triggered predominantly by direct two-photon absorption of the exciting light. The results show that the two-photon absorption is the dominant process. Yet, through two-photon absorption in the near field of particles, the localisation of the reaction is still similar.:1. Einführung 1.1 Frequenzverdopplung an Nanopartikeln 1.2 Photochemisches Schreiben auf kleinen Längenskalen 2. Theoretische Grundlagen 2.1 Nichtlineare optische Effffekte zweiter Ordnung 2.2 Frequenzverdopplung in Metallen 2.3 Frequenzverdopplung bei metallischen Nanopartikeln 2.4 Überlagerung der Strahlung mehrerer frequenzverdoppelter Dipole 2.5 Core-Shell-Nanopartikel mit nichtzentrosymmetrischem Kern 3. Experimenteller Aufbau 3.1 Beleuchtung der Proben und Detektionspfad 3.2 Objektiv und Probenhalter 3.3 Realisierung der radial polarisierten Mode 4. Messungen der zweiten Harmonischen an Nanostrukturen 4.1 Einzelne kugel- und stäbchenförmige Goldnanopartikel 4.2 Nanodreiecke (Fischer-Pattern) 4.3 Nanokegel 4.4 Nanostäbchen-Teppiche 4.5 Zusammenfassung 5. Nichtlinear-optisches photochemisches Schreiben auf kleinen Längenskalen 5.1 Photochemische Reaktionen auf der Sub-100nm-Skala 5.2 Erste Versuche an Photolacken 5.3 Photochemisches Schreiben auf azobenzolhaltigen PMMA-Copolymerschichten 5.4 Photochemisches Schreiben auf azosulfonathaltigen PMMA-Copolymerschichten 5.5 Ausblick 6. Zusammenfassung und Ausblick Anhang A. Darstellung der radialen Mode und des z-polarisierten Fokus B. Mehode der multiplen Multipole (MMP) C. Präparation der Proben Literaturverzeichnis Abbildungsverzeichnis Verzeichnis der Tabellen Verwendete Abkürzungen Liste der Veröffffentlichungen Danksagung Erklärung

In dieser Arbeit untersuchen wir die kontrollierte Änderung der Strahlungsübergangsrate und des Fluoreszenzspektrums eines einzelnen Farbstoffmoleküls und SiO2-Nanopartikels (NP) durch Einbetten in eine durchstimmbare plane Mikrokavität mit Subwellenlängenabstand. Wir haben ein theoretisches Modell entwickelt und finden hervorragende Übereinstimmungen zwischen theoretischen Voraussagen und experimentellen Ergebnissen. Während die Fluoreszenz einzelner Farbstoffmoleküle in Glas-Luft-Grenzfläche (d.h. im freien Raum) sehr gut bekannt ist, sind die Einzelheiten der optischen Eigenschaften individueller SiO2-NP immer noch unklar. Daher widmet sich ein Teil dieser Arbeit der Untersuchung ihrer Fotolumineszenz im freien Raum. Einleitend präsentieren wir die Konstruktion der durchstimmbaren Mikrokavität, die für die Messungen verwendet wurde. In this thesis we study controlled modification of the radiative transition rate and fluorescence spectrum of a single dye molecule and SiO2 nanoparticle (NP) by embedding it within a tunable planar microcavity with subwavelength spacing. We develop a theoretical model and find excellent agreement between theoretical prediction and experimental results. Whereas fluorescence of single dye molecules in glass-air confinement (i.e., in free space) is fairly well known, the details of optical properties of individual SiO2 NPs are still unclear. Therefore, a part of this thesis is dedicated to investigation of their photoluminescence in free space. In introduction of the thesis we present a tunable microcavity construction, which has been used for the measurements.

In this study we investigate photo-physical and photo-chemical properties of single molecules and semiconductor quantum dots by imaging their excitation transition dipole moment (TDM) using confocal microscopy combined with azimuthally and radially polarized laser beams. We show that using this technique the three-dimensional (3D) orientation, dimensionality and dynamical effects of excitation TDM of single quantum emitters can be determined. The work can be divided into the three main parts: 1. The determination of 3D TDM orientation and study of the tautomerism process in single dye molecules. 2. Investigation of direct/indirect band gap semiconductor single quantum dots and single silicon dioxide nanoparticles. 3. The study of host-guest compounds: the orientation of the guest molecules or quantum dots in the channels of host compounds. Each of the chapters presents independent topic of the research. In chapter 1, we introduce and motivate the research described in this work and define the scope of this thesis. Chapter 2 gives an overview of the experimental methods and equipment used in this study. Each chapter starts with an introduction of the object of the research and experimental part which refers to the specific instrumentation briefly presented in chapter 2.1. In chapters 3, we investigate the changing of the TDM orientation during the excited-state tautomerism process in single metal-free phthalocyanine and porphyrin molecules. In the case of phthalocyanine molecules the fluorescence excitation patterns show that the angle between the transition dipole moments of the two trans forms is near 90o. Most of porphyrin molecules exhibit the absence of the tautomerism process, while for 7% of the total number of investigated molecules we observe excitation patterns of two different trans forms of the same single molecule. Experimental results are in good agreement with theoretical prediction based on quantum chemical calculations. In chapter 4, we investigate the excitation TDM of the single CdSe/ZnS quantum dots (QD). We study different types of commercially available water/toluene soluble QDs. We show that single CdSe/ZnS QDs possess 3D excitation TDM in contrast to dye molecules which possess linear excitation TDM. We demonstrate that using the cylindrical vector beams we can determine the dimensionality of the excitation TDM. Investigated QDs show characteristic dynamical effects featuring single QDs such as fluorescence intermittency (blinking and bleaching). In chapter 5, we study single SiO2 nanoparticles (NPs) which possess defect luminescence. The obtained results clearly demonstrate the presence of the one-dimensional TDM of SiO2 NPs. Moreover, image series of the same sample area revealed the possibility for some silica NPs to change their TDM orientation from one image to another. In chapter 6 and 7, we investigate single Si/SiO2 core-shell nanocrystals (Si NCs) produced by laser pyrolysis. In chapter 6, we study samples with single Si NCs embedded in polymer matrix which were prepared by method of spin coating. In chapter 7, we study the Si NCs which were directly deposited on the surface of the glass cover slide. We show that samples with Si NCs prepared by method of spin coating possess defect photoluminescence, while some of directly deposited Si NCs possess emission due to the quantum confined electron-hole recombination. We demonstrate that Si NCs with defect photoluminescence possess linear excitation TDM, while Si NCs with exciton photoluminescence - 3D excitation TDM. In chapters 8 and 9, we demonstrate that the orientation of the single quantum emitters can be determined not only in polymer layer spin coated on the glass cover slide surface but, for example, inside the nanochannels of host materials. We investigate two different types of host-guest compounds. In chapter 8, we show the ability of new type of stopcock molecules close the channels of the organic crystals. Using an azimuthally polarized laser beam we determine the single stopcock molecules orientation in the nanochannel entrances of the crystals. In chapter 9, we study mesoporous silica beads with included CdSe/ZnS QDs in the channels. The beads with different concentration of the QDs included in the channels were investigated. Using an azimuthally polarized laser beam for the sample excitation we determine the orientation of single QDs in the channels of mesoporous silica beads. In dieser Arbeit untersuchen wir fotophysikalische und fotochemische Eigenschaften einzelner Moleküle und Halbleiterquantenpunkte durch Abbildung ihrer Anregungs- Übergangsdipolmomente (transition dipole moments TDMs) mit konfokaler Mikroskopie in Kombination mit azimutal und radial polarisierten Laserstrahlen. Wir zeigen, dass mit dieser Technik die dreidimensionale (3D) Orientierung, Dimensionalität und dynamische Effekte des Anregungs-TDM einzelner Quantenemitter bestimmt werden können. Die Arbeit kann in drei Hauptabschnitte unterteilt werden: 1. Die Bestimmung der 3D TDM-Orientierung und Untersuchung der Tautomerieprozesse in einzelnen Farbstoffmolekülen. 2. Die Untersuchung direkter/indirekter Bandlücken in Halbleiterquantenpunkten und einzelnen Siliziumdioxidnanopartikeln. 3. Die Analyse von Wirt-Gast-Systemen: Orientierung der Gastmoleküle oder Quantenpunkte in den Kanälen der Wirtsteilchen. In Kapitel 1 führen wir in die Forschung dieser Arbeit ein und definieren den Rahmen. Kapitel 2 gibt einen Überblick über die experimentellen Methoden und die Ausstattung, die in dieser Studie verwendet werden. Die folgenden Kapitel stellen unabhängige Forschungsgebiete dar. Jedes Kapitel beginnt mit einer Einleitung über das entsprechende Forschungsobjekt und den experimentellen Teil, der sich auf die in Kapitel 2.1 kurz beschriebene Messtechnik bezieht. In Kapitel 3 untersuchen wir die Änderung der TDM-Orientierung während der Tautomerie im angeregten Zustand in einzelnen metallfreien Phthalocyanin- und Porphyrinmolekülen. Im Falle der Phthalocyaninmoleküle zeigen die Fluoreszenzanregungsmuster, dass der Winkel zwischen den Übergangsdipolmomenten der beiden trans-Formen etwa 90° beträgt. Die meisten Porphyrinmoleküle weisen keine Tautomerie auf, während wir für 7% aller untersuchten Moleküle Anregungsmuster von zwei unterschiedlichen trans-Formen desselben einzelnen Moleküls beobachten. Die Versuchsergebnisse stimmen gut mit den theoretischen Erwartungen, basierend auf quantenchemischen Berechnungen, überein. In Kapitel 4 untersuchen wir das Anregungs-TDM der einzelnen CdSe/ZnS Quantenpunkte (quantum dots QDs). Wir erforschen verschiedene Arten kommerziell erhältlicher, in Wasser/Toluol löslicher QDs. Wir zeigen, dass einzelne CdSe/ZnS QDs ein 3D Anregungs-TDM besitzen, im Gegensatz zu Fluoreszenzmolekülen, die ein lineares Anregungs-TDM besitzen. Wir zeigen auf, dass wir unter Verwendung der zylindrischen Vektorstrahlen die Dimensionalität des Anregungs-TDM bestimmen können. Untersuchte QDs weisen charakteristische dynamische Effekte einzelner QDs auf wie Fluoreszenzperiodizität (Blinken und Bleichen). In Kapitel 5 analysieren wir einzelne SiO2 Nanopartikel (NPs), die Defektlumineszenz aufweisen. Die Ergebnisse zeigen eindeutig die Anwesenheit des eindimensionalen TDM von SiO2 NPs. Darüberhinaus decken Bilderserien derselben Probenbereiche die Wahrscheinlichkeit auf für einige Silica NPs die Orientierung ihres TDM von einer Aufnahme zur nächsten zu ändern. In den Kapiteln 6 und 7 erforschen wir einzelne, mit Laserpyrolyse hergestellten, Si/SiO2 Kern-Hülle-Nanokristalle (Si NKs). In Kapitel 6 untersuchen wir Proben mit einzelnen Si NKs eingebettet in einer Polymermatrixt, die mit dem spin-coating- Verfahren hergestellt wurden. In Kapitel 7 untersuchen wir Si NKs, die direkt auf die Oberfläche der Deckgläschen gebracht wurden. Wir zeigen, dass Proben mit Si NKs, die mit Hilfe des spin-coating-Verfahrens hergestellt wurden, Defektlumineszenz aufweisen, während einige der direkt aufgebrachten Si NKs Emission aufgrund quantenbegrenzter Elektron-Loch-Rekombination besitzen. Wir zeigen, dass Si NKs mit Defektlumineszenz ein lineares Anregungs-TDM besitzen, jedoch Si NKs mit Excitonenfotolumineszenz 3D Anregungs-TDM. In den Kapiteln 8 und 9 legen wir dar, dass die Orientierung einzelner Quantenemitter nicht nur in einer Polymerschicht, aufgeschleudert auf die Deckgläschenoberfläche, bestimmt werden kann, sondern z.B. innerhalb der Nanokanäle von Wirtsmaterialien. Wir untersuchen zwei unterschiedliche Arten von Wirt- Gast- Systemen. In Kapitel 8 zeigen wir die Eigenschaft einer neuen Art von Stopcockmolekülen die Kanäle der organischen Kristalle zu schließen. Mit einem azimutal polarisierten Laserstrahl bestimmen wir die Orientierung der einzelnen Stopcockmoleküle in den Eingängen der Nanokanäle der Kristalle. In Kapitel 9 untersuchen wir mesoporöse Silica Kügelchen mit eingeschlossenen CdSe/ZnS QDs in den Kanälen. Die Kügelchen mit verschiedenen Konzentration der in den Kanälen eingeschlossenen QDs werden analysiert. Mit einem azimutal polarisierten Laserstrahl zur Anregung bestimmen wir die Orientierung einzelner QDs in den Kanälen mesoporöser Silicakügelchen.

Im Rahmen dieser Arbeit wurde die Strukturierbarkeit ultradünner, funktioneller Polymerfilme anhand von Diazosulfonat-Terpolymer- und Aminoterpolymer-Schichten untersucht. Beide Polymersysteme enthalten eine photoaktive Gruppe in der Seitenkette, die sich durch gezielte UV-Bestrahlung verändern läßt. In den Diazosulfonat-Terpolymeren wird durch die Belichtung die Funktionalität zerstört, während bei den Aminoterpolymeren die Funktionalität durch die Belichtung erst freigelegt wird. Dafür wurden Strukturierungsmethoden für verschiedene Längenskalen erarbeitet und auf ihre Eignung geprüft. Der Nachweis der erfolgreichen Strukturierung wurde durch an die Längenskala angepaßte Methoden geführt und damit die erzeugten Strukturen sichtbar gemacht. Die Veränderungen im optischen Absorptionsverhalten konnten an makroskopischen Probenbereichen nachgewiesen werden. Sowohl der verwendete Aufbau für die Strukturierung (Belichtung) als auch die Detektion mit dem 2-Stahl-Spektrometer erwies sich als geeignet. Es konnte deutlich der Abbau der UV-Absorptionsbande der Diazosulfonat-Terpolymerfilme gezeigt und quantitativ untersucht werden. Dafür wurden Lichtdosen von etwa 0,35 ... 39 nJ/µm² eingebracht und deren Auswirkungen auf die Absorptionsänderung des Polymers direkt festgestellt. Diese Messungen zeigen, daß die eingebrachte Energie und nicht die Leistung (sofern diese unterhalb 2,5 mW liegt) entscheidend für die Modifikation der optischen Eigenschaften dieser Polymere ist. Anhand der Meßergebnisse konnte eine Abschätzung der Quantenausbeute durchgeführt werden, die für die Diazosulfonat-Terpolymerfilme einen Wert von (12 ± 6) % ergab. Auf der Mikrometer-Skala wurden unterschiedliche Ansätze verfolgt, um die optische Strukturierung nachzuweisen. Der Nachweis optischer Modifikationen der Diazosulfonat-Terpolymerfilme wurde nach Belichtung mit hohen Lichtdosen geführt, da er sich nur in diesem Energiebereich mit der erforderlichen Empfindlichkeit realisieren ließ. Für die Aminoterpolymerfilme wurden Strukturen durch Fluoreszenzmarkierung nachgewiesen, welche sich als sehr sensitiv herausstellte. Im Anschluß an die Belichtung konnten topographische Modifikationen mit dem AFM gemessen werden. Mit dem SNOM konnten diese Modifikationen bereits während der Belichtung direkt analysiert werden. Die getesteten Methoden der Raman-Spektroskopie und der Metallisierung mit anschließender Röntgen-Photoelektronenspektroskopie zeigten weder die benötigte Sensitivität noch Selektivität. Die untersuchten Polymersysteme können in Form ultradünner Filme auf unterschiedliche Substrate aufgebracht werden. In diesen Polymerfilmen wurden Strukturen von der Millimeter-Skala bis Nanometer-Skala erzeugt. Anhand von an die Größenskala angepaßten direkten und indirekten Nachweismethoden konnten Veränderungen der optischen, mechanischen und chemischen Eigenschaften der Polymere analysiert werden.