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In this work, we theoretically and experimentally study the influence of the optical environment on the radiative decay rate of luminescent nanoparticles forming a thin film. We use electric dipole sources in finite-difference time-domain simulations to analyze the effect of modifying the effective refractive index of transparent layers made of phosphor nanocrystals doped with rare earth cations, and propose a significant correction to previously reported analytical models for calculating the radiative decay rate. Our predictions are tested against an experimental realization of such films, in which we manage to vary the effective refractive index in a gradual and controllable manner. Our model accurately accounts for the measurements attained, allows us discriminating the radiative and non-radiative contributions to the time-resolved photoluminescence, and provides a way to rationally tune the spontaneous decay rate and hence the photoluminescence quantum yield of an ensemble of nanoparticles.

We have implemented the Bogoliubov-de Gennes equation in a screened Korringa-Kohn-Rostoker method for solving, self-consistently, the superconducting state for three-dimensional (3D) crystals including substitutional impurities. In this paper we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3D magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in in-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of a first-principles-based description which captures the quantitative details, making direct comparisons with experimental findings possible.

Photolumineszenz-Techniken spielen eine wichtige Rolle bei der Charakterisierung verschiedener funktionaler Fluorophore in den Lebens- und Materialwissenschaften, in der Biologie und bis hin zu den Quantentechnologien. Ein entscheidender und wichtiger Parameter für den Vergleich von Fluorophoren ist die Quanteneffizienz, die ein direktes Maß für die Umwandlungseffizienz von absorbierten Photonen in emittierte Photonen darstellt. Diese Größe charakterisiert die Nutzbarkeit von Emittern für Anwendungen in optischen Geräten, Einzelphotonenquellen und im biomedizinischen Bereich. Mehrere Techniken wie optische und photothermische Methoden werden verwendet, um die photolumineszente Quanteneffizienz zu messen, und so die Eignung von Fluorophoren für verschiedene Anwendungen zu bewerten. Quanteneffizienz-Messungen können jedoch eine Herausforderung für hochverdünnte Fluorophore sein, die in dünne Schichten eingebettet sind. Die in dieser Arbeit beschriebene Forschung überwindet die Herausforderungen der Quanteneffizienzmessung durch eine Modifikation der Wechselwirkung zwischen Licht und Umgebung., Photoluminescence techniques play an important role for characterization of various functional fluorophores in the life and material sciences from biology to quantum technologies. A crucial and key parameter for comparing the performance of fluorophores is the photoluminescence quantum efficiency or quantum yield, which presents a direct measure of conversion efficiency of absorbed photons into emitted photons. This quantity characterizes the performance of emitters for applications in optical devices as single photon sources and in the biomedical sector. Several techniques such as optical and photothermal methods are used to measure the photoluminescence quantum efficiency of emitters in various environments and aggregation states. Quantum efficiency measurements can be challenging for fluorophores in solid matrix, in scattering systems and for highly diluted fluorophores embedded to thin films. The research described in this thesis overcomes these challenges by performing quantum efficiency measurement via modifying the spontaneous emission as a fundamental process of light-matter interaction.

Photolumineszenz-Techniken spielen eine wichtige Rolle bei der Charakterisierung verschiedener funktionaler Fluorophore in den Lebens- und Materialwissenschaften, in der Biologie und bis hin zu den Quantentechnologien. Ein entscheidender und wichtiger Parameter für den Vergleich von Fluorophoren ist die Quanteneffizienz, die ein direktes Maß für die Umwandlungseffizienz von absorbierten Photonen in emittierte Photonen darstellt. Diese Größe charakterisiert die Nutzbarkeit von Emittern für Anwendungen in optischen Geräten, Einzelphotonenquellen und im biomedizinischen Bereich. Mehrere Techniken wie optische und photothermische Methoden werden verwendet, um die photolumineszente Quanteneffizienz zu messen, und so die Eignung von Fluorophoren für verschiedene Anwendungen zu bewerten. Quanteneffizienz-Messungen können jedoch eine Herausforderung für hochverdünnte Fluorophore sein, die in dünne Schichten eingebettet sind. Die in dieser Arbeit beschriebene Forschung überwindet die Herausforderungen der Quanteneffizienzmessung durch eine Modifikation der Wechselwirkung zwischen Licht und Umgebung., Photoluminescence techniques play an important role for characterization of various functional fluorophores in the life and material sciences from biology to quantum technologies. A crucial and key parameter for comparing the performance of fluorophores is the photoluminescence quantum efficiency or quantum yield, which presents a direct measure of conversion efficiency of absorbed photons into emitted photons. This quantity characterizes the performance of emitters for applications in optical devices as single photon sources and in the biomedical sector. Several techniques such as optical and photothermal methods are used to measure the photoluminescence quantum efficiency of emitters in various environments and aggregation states. Quantum efficiency measurements can be challenging for fluorophores in solid matrix, in scattering systems and for highly diluted fluorophores embedded to thin films. The research described in this thesis overcomes these challenges by performing quantum efficiency measurement via modifying the spontaneous emission as a fundamental process of light-matter interaction.

We present a fully relativistic first-principles-based theoretical approach for the calculation of the spectral properties of magnetic impurities on the surface of a superconducting substrate, providing a material specific framework for the investigation of the Yu-Shiba-Rusinov (YSR) states. By using a suitable orbital decomposition of the local densities of states we discuss in great detail the formation of the YSR states for an Mn adatom and for two kinds of Mn dimers placed on the Nb(110) surface and compare our results to recent experimental findings. In the case of the adatom we find that the spin-orbit coupling slightly shifts some of the YSR peaks and also the local spin polarization on the Nb atoms has marginal effects on their positions. Moreover, by scaling the exchange field on the Mn site we could explain the lack of the dx2−y2-like YSR state in the spectrum. While our results for a close packed ferromagnetic dimer are in satisfactory agreement with the experimentally observed splitting of the YSR states, in the case of an antiferromagnetic dimer we find that the spin-orbit coupling is not sufficiently large to explain the splitting of the YSR states seen in the experiment. Changing the relative orientation of the magnetic moments in this dimer induces splitting of the YSR states and also shifts their energy, leading even to the formation of a zero bias peak in the case of the deepest YSR state.

We propose a method to probe the local density of states (LDOS) of atomic systems that provides both spatial and energy resolution. The method combines atomic and tunneling techniques to supply a simple, yet quantitative and operational, definition of the LDOS for both interacting and non-interacting systems: It is the rate at which particles can be siphoned from the system of interest by a narrow energy band of non-interacting states contacted locally to the many-body system of interest. Ultracold atoms in optical lattices are a natural platform for implementing this broad concept to visualize the energy and spatial dependence of the atom density in interacting, inhomogeneous lattices. This includes models of strongly correlated condensed matter systems, as well as ones with non-trivial topologies.

Many applications in (quantum) nanophotonics rely on controlling light-matter interaction through strong, nanoscale modification of the local density of states (LDOS). All-optical techniques probing emission dynamics in active media are commonly used to measure the LDOS and benchmark experimental performance against theoretical predictions. However, metal coatings needed to obtain strong LDOS modifications in, for instance, nanocavities, are incompatible with all-optical characterization. So far, no reliable method exists to validate theoretical predictions. Here, we use sub-nanosecond pulses of focused electrons to penetrate the metal and excite a buried active medium at precisely-defined locations inside sub-wavelength resonant nanocavities. We reveal the spatial layout of the spontaneous-emission decay dynamics inside the cavities with deep-subwavelength detail, directly mapping the LDOS. We show that emission enhancement converts to inhibition despite an increased number of modes, emphasizing the critical role of optimal emitter location. Our approach yields fundamental insight in dynamics at deep-subwavelength scales for a wide range of nano-optical systems., ImPhys/Quantitative Imaging, ImPhys/Charged Particle Optics

We propose a method to probe the local density of states (LDOS) of atomic systems that provides both spatial and energy resolution. The method combines atomic and tunneling techniques to supply a simple, yet quantitative and operational, definition of the LDOS for both interacting and non-interacting systems: It is the rate at which particles can be siphoned from the system of interest by a narrow energy band of non-interacting states contacted locally to the many-body system of interest. Ultracold atoms in optical lattices are a natural platform for implementing this broad concept to visualize the energy and spatial dependence of the atom density in interacting, inhomogeneous lattices. This includes models of strongly correlated condensed matter systems, as well as ones with non-trivial topologies.

Many applications in (quantum) nanophotonics rely on controlling light-matter interaction through strong, nanoscale modification of the local density of states (LDOS). All-optical techniques probing emission dynamics in active media are commonly used to measure the LDOS and benchmark experimental performance against theoretical predictions. However, metal coatings needed to obtain strong LDOS modifications in, for instance, nanocavities, are incompatible with all-optical characterization. So far, no reliable method exists to validate theoretical predictions. Here, we use sub-nanosecond pulses of focused electrons to penetrate the metal and excite a buried active medium at precisely-defined locations inside sub-wavelength resonant nanocavities. We reveal the spatial layout of the spontaneous-emission decay dynamics inside the cavities with deep-subwavelength detail, directly mapping the LDOS. We show that emission enhancement converts to inhibition despite an increased number of modes, emphasizing the critical role of optimal emitter location. Our approach yields fundamental insight in dynamics at deep-subwavelength scales for a wide range of nano-optical systems., ImPhys/Quantitative Imaging, ImPhys/Charged Particle Optics

Metamaterials are composites that are engineered purposefully for realizing electromagnetic characteristics that do not occur naturally in mineral or organic form. These characteristics are realized by regular arrangements of meta atoms, the building blocks of metamaterials, that mimic the atoms in an element. The electric and magnetic responses of these building blocks are engineered in such a way that the effective permittivity and permeability of metamaterials can be tuned. Similarly, the anisotropy of constitutive electromagnetic parameters can also be controlled efficiently. This dissertation focuses on the enhancement of the interaction between light and matter using bulk anisotropic metamaterials and magnetic meta atoms, in order to enhance, control and/or isolate electric and magnetic nature of emitters. This is achieved first through utilization of hyperbolic metamaterials (HMs) which are a subcategory of uniaxially anistotropic materials exhibiting opposite signs of permittivity or permeability along and orthogonal the axis of anisotropy. HMs host a wide spatial spectrum of propagating waves, i.e., high gradient field features can be transferred in HMs owing to the propagating waves in ideally indefinitely large spatial spectrum. Optical HMs are mainly constructed by periodic alternating layers of plasmonic metals and dielectrics. The power emitted by point sources (and arrays of point sources) in the vicinity of HMs is highly boosted compared to regular dielectric media. Moreover, most of the power is absorbed by the HM. Similar characteristics can be realized using graphene layers instead of metals in the infrared regime, where the chemical potential can be an effective means of controlling emission enhancement. Thereafter, a reactively loaded transmission line grid is presented as an example of a two-dimensional HMs, where the canalization of large spectral waves leads to transferring high resolution features. HMs can also be molded into resonators that

Metamaterials are composites that are engineered purposefully for realizing electromagnetic characteristics that do not occur naturally in mineral or organic form. These characteristics are realized by regular arrangements of meta atoms, the building blocks of metamaterials, that mimic the atoms in an element. The electric and magnetic responses of these building blocks are engineered in such a way that the effective permittivity and permeability of metamaterials can be tuned. Similarly, the anisotropy of constitutive electromagnetic parameters can also be controlled efficiently. This dissertation focuses on the enhancement of the interaction between light and matter using bulk anisotropic metamaterials and magnetic meta atoms, in order to enhance, control and/or isolate electric and magnetic nature of emitters. This is achieved first through utilization of hyperbolic metamaterials (HMs) which are a subcategory of uniaxially anistotropic materials exhibiting opposite signs of permittivity or permeability along and orthogonal the axis of anisotropy. HMs host a wide spatial spectrum of propagating waves, i.e., high gradient field features can be transferred in HMs owing to the propagating waves in ideally indefinitely large spatial spectrum. Optical HMs are mainly constructed by periodic alternating layers of plasmonic metals and dielectrics. The power emitted by point sources (and arrays of point sources) in the vicinity of HMs is highly boosted compared to regular dielectric media. Moreover, most of the power is absorbed by the HM. Similar characteristics can be realized using graphene layers instead of metals in the infrared regime, where the chemical potential can be an effective means of controlling emission enhancement. Thereafter, a reactively loaded transmission line grid is presented as an example of a two-dimensional HMs, where the canalization of large spectral waves leads to transferring high resolution features. HMs can also be molded into resonators that

This thesis is dedicated to analytical and numerical studiesof a few models of strongly correlated systems and thedevelopment of techniques for such studies. The common name"strongly correlated" refers to systems in which theinteractions (correlations) between the constituent particleslead to nontrivial properties, such as various types ofmagnetism, superconductivity, and exotic phases. The thesisbegins with a three-part introductory review and ends withresearch results, presented in the form of scientificarticles. In the first part of the thesis the application of meanfield theory to models of magnetic multilayer systems isdiscussed. Magnetic multilayers are widely used as a medium formagnetooptical recording. The technologically importantproperties of such multilayers are high magnetic anisotropy anda compensation temperature (at which the magnetizations of thedifferent layers cancel each other and the net magnetic momentvanishes) close to room temperature. It is thus beneficial tohave a simple theoretical model, allowing to predict the netmagnetization and compensation temperature of a multilayer,depending on the composition and thickness of the componentlayers. Such a model, featuring long-range interactions betweenthemagnetic layers, has been developed and studied, using meanfield theory. A satisfactory agreement with experiment wasobtained. The second part of the thesis is devoted to possibleenhancements of an experimental technique, known as scanningtunneling microscopy (STM). In this technique a sharp (ideally- atomically sharp) metallic tip, whose movement may beprecisely controlled, is brought within a few Å(1Å =10-10m) of the studied surface and a bias voltage isapplied between them, so that the quantum-mechanical phe-nomenon of tunneling starts to take place. The extremesensitivity of the tunneling current (which typically decaysexponentially with increasing tip-surface distance) allows toobtain the topological image of the surface with atomicresolution, NR 20140805

Brillouin light scattering (BLS) has been successfully applied to investigate the elastic properties of hard films were studied. The systems investigated include diamond-like-carbon, supported GaN/AlN structures and hard nano-crystalline zirconium-boride films. Highlights of the project include report of the first evidence for the existence of the high frequency longitudinal guided mode (LGM) in a supported hard films (diamond-like-carbon) and emerging evidence for the role of acoustic barriers offered by AlN in localizing elastic waves in GaN films. Preliminary work on amorphous ZrB3 films reveals its transformation to hard nano-crystallites of the boride layer with dramatic enhancement of the elastic properties upon high temperature anneal.