@phdthesis{Gross2019,
  author    = {Groß, Heiko},
  title     = {Controlling Light-Matter Interaction between Localized Surface Plasmons and Quantum Emitters},
  doi       = {10.25972/OPUS-19209},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-192097},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Metal nanostructures have been known for a long time to exhibit optical resonances via localized surface plasmons. The high electric fields in close proximity to the metal surface have prospects to dramatically change the dynamics of electronic transitions, such as an enhanced spontaneous decay rate of a single emitter. However, there have been two major issues which impede advances in the experimental realization of enhanced light-matter interaction. (i) The fabrication of high-quality resonant structures requires state-of-the-art patterning techniques in combination with superior materials. (ii) The tiny extension of the optical near-field requires precise control of the single emitter with respect to the nanostructure. This work demonstrates a solution to these problems by combining scanning probe and optical confocal microscopy. Here, a novel type of scanning probe is introduced which features a tip composed of the edge of a single crystalline gold sheet. The patterning via focused ion beam milling makes it possible to introduce a plasmonic nanoresonator directly at the apex of the tip. Numerical simulations demonstrate that the optical properties of this kind of scanning probe are ideal to analyze light-matter interaction. Detailed experimental studies investigate the coupling mechanism between a localized plasmon and single colloidal quantum dots by dynamically changing coupling strength via their spatial separation. The results have shown that weak interaction affects the shape of the fluorescence spectrum as well as the polarization. For the best probes it has been found that it is possible to reach the strong coupling regime at the single emitter level at room temperature. The resulting analysis of the experimental data and the proposed theoretical models has revealed the differences between the established far-field coupling and near-field coupling. It has been found that the broad bandwidth of plasmonic resonances are able to establish coherent coupling to multiple transitions simultaneously giving rise to an enhanced effective coupling strength. It has also been found that the current model to numerically calculate the effective mode volume is inaccurate in case of mesoscopic emitters and strong coupling. Finally, light-matter interaction is investigated by the means of a quantum-dot-decorated microtubule which is traversing a localized nearfield by gliding on kinesin proteins. This biological transport mechanism allows the parallel probing of a meta-surface with nm-precision. The results that have been put forward throughout this work have shed new light on the understanding of plasmonic light-matter interaction and might trigger ideas on how to more efficiently combine the power of localized electric fields and novel excitonic materials.},
  subject      = {Plasmon},
  language  = {en}
}
@phdthesis{Razinskas2018,
  author    = {Razinskas, Gary},
  title     = {Functional plasmonic nanocircuitry},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166917},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {In this work, functional plasmonic nanocircuitry is examined as a key of revolutionizing state-of-the-art electronic and photonic circuitry in terms of integration density and transmission bandwidth. In this context, numerical simulations enable the design of dedicated devices, which allow fundamental control of photon flow at the nanometer scale via single or multiple plasmonic eigenmodes. The deterministic synthesis and in situ analysis of these eigenmodes is demonstrated and constitutes an indispensable requirement for the practical use of any device. By exploiting the existence of multiple eigenmodes and coherence - both not accessible in classical electronics - a nanoscale directional coupler for the ultrafast spatial and spatiotemporal coherent control of plasmon propagation is conceived. Future widespread application of plasmonic nanocircuitry in quantum technologies is boosted by the promising demonstrations of spin-optical and quantum plasmonic nanocircuitry.},
  subject      = {Nanooptik},
  language  = {en}
}
@phdthesis{Feichtner2017,
  author    = {Feichtner, Thorsten},
  title     = {Optimal Design of Focusing Nanoantennas for Light : Novel Approaches: From Evolution to Mode-Matching},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-140604},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {Optische Antennen arbeiten {\"a}hnlich wie Antennen f{\"u}r Radiowellen und wandeln elektromagnetische Strahlung in elektrische Wechselstr{\"o}me um. Ladungsdichteansammlungen an der Antennen-Oberfl{\"a}che f{\"u}hren zu starken und lokalisierten Nahfeldern. Da die meisten optischen Antennen eine Ausdehnung von wenigen hundert Nanometern besitzen, erm{\"o}glichen es ihre Nahfelder, Licht auf ein Volumen weit unterhalb des Beugungslimits zu fokussieren, mit Intensit{\"a}ten, die mehrere Gr{\"o}ßenordnungen {\"u}ber dem liegen, was man mit klassischer beugender und reflektierender Optik erreichen kann. Die Aufgabe, die Abstrahlung eines Quantenemitters zu maximieren, eines punktf{\"o}rmigen Objektes, welches einzelne Photonen absorbieren und emittieren kann, ist identisch mit der Aufgabe, die Feldintensit{\"a}t am Ort des Quantenemitters zu maximieren. Darum ist es erstrebenswert, den Fokus optischer Antennen zu optimieren Optimierte Radiofrequenz-Antennen, welche auf Gr{\"o}ßenordnungen von wenigen 100 Nanometern herunterskaliert werden, zeigen bereits eine gute Funktionalit{\"a}t. Jedoch liegen optische Frequenzen in der N{\"a}he der Plasmafrequenz von den Metallen, die f{\"u}r optische Antennen genutzt werden und die Masse der Elektronen kann nicht mehr vernachl{\"a}ssigt werden. Dadurch treten neue physikalische Ph{\"a}nomene auf. Es entstehen gekoppelte Zust{\"a}nde aus Licht und Ladungsdichte-Schwingungen, die sogenannten Plasmonen. Daraus folgen Effekte wie Volumenstr{\"o}me und k{\"u}rzere effektive Wellenl{\"a}ngen. Zus{\"a}tzlich f{\"u}hrt die endliche Leitf{\"a}higkeit zu thermischen Verluste. Das macht eine Antwort auf die Frage nach der optimalen Geometrie f{\"u}r fokussierende optische Antennen schwer. Jedoch stand vor dieser Arbeit der Beweis noch aus, dass es f{\"u}r optische Antennen bessere Alternativen gibt als herunterskalierte Radiofrequenz-Konzepte. In dieser Arbeit werden optische Antennen auf eine bestm{\"o}gliche Fokussierung optimiert. Daf{\"u}r wird ein Ansatz gew{\"a}hlt, welcher bei Radiofrequenz-Antennen f{\"u}r komplexe Anwendungsfelder (z.B. isotroper Breitbandempfang) schon oft Erfolg hatte: evolution{\"a}re Algorithmen. Die hier eingef{\"u}hrte erste Implementierung erlaubt eine große Freiheit in Bezug auf Partikelform und Anzahl, da sie quadratische Voxel auf einem planaren, quadratischen Gitter beliebig anordnet. Die Geometrien werden in einer bin{\"a}ren Matrix codiert, welche als Genom dient und somit Methoden wie Mutation und Paarung als Verbesserungsmechanismus erlaubt. So optimierte Antennen-Geometrien {\"u}bertreffen vergleichbare klassische Dipol-Geometrien um einen Faktor von Zwei. Dar{\"u}ber hinaus l{\"a}sst sich aus den optimierten Antennen ein neues Funktionsprinzip ableiten: ein magnetische Split-Ring-Resonanz kann mit Dipol-Antennen leitend zu neuartigen und effektiveren Split-Ring-Antennen verbunden werden, da sich ihre Str{\"o}me nahe des Fokus konstruktiv {\"u}berlagern. Im n{\"a}chsten Schritt wird der evolution{\"a}re Algorithmus so angepasst, so die Genome real herstellbare Geometrien beschreiben. Zus{\"a}tzlich wird er um eine Art ''Druckertreiber'' erweitert, welcher aus den Genomen direkt Anweisungen zur fokussierten Ionenstrahl-Bearbeitung von einkristallinen Goldflocken erstellt. Mit Hilfe von konfokaler Mikroskopie der Zwei-Photonen-Photolumineszenz wird gezeigt, dass Antennen unterschiedlicher Effizienz reproduzierbar aus dem evolution{\"a}ren Algorithmus heraus hergestellt werden k{\"o}nnen. Außerdem wird das Prinzip der Split-Ring-Antenne verbessert, indem zwei Ring-Resonanzen zu einer Dipol-Resonanz hinzugef{\"u}gt werden. Zu guter Letzt dient die beste Antenne des zweiten evolution{\"a}re Algorithmus als Inspiration f{\"u}r einen neuen Formalismus zur Beschreibung des Leistungs{\"u}bertrages zwischen einer optischen Antenne und einem Punkt-Dipol, welcher sich als "dreidimensionaler Moden{\"u}berlapp" beschreiben l{\"a}sst. Damit k{\"o}nnen erstmals intuitive Regeln f{\"u}r die Form einer optischen Antenne aufgestellt werden. Die G{\"u}ltigkeit der Theorie wird analytisch f{\"u}r den Fall eines Dipols nahe einer metallischen Nano-Kugel gezeigt. Das vollst{\"a}ndige Problem, Licht mittels einer optischen Antenne zu fokussieren, l{\"a}sst sich so auf die Erf{\"u}llung zweier Moden{\"u}berlapp-Bedingungen reduzieren -- mit dem Feld eines Punktdipols, sowie mit einer ebenen Welle. Damit lassen sich zwei Arten idealer Antennenmoden identifizieren, welche sich von der bekannten Dipol-Antennen-Mode grundlegend unterscheiden. Zum einen l{\"a}sst sich dadurch die Funktionalit{\"a}t der evolution{\"a}ren und Split-Ring-Antennen erkl{\"a}ren, zum lassen sich neuartige plasmonische Hohlraum-Antennen entwerfen, welche zu besserer Fokussierung von Licht f{\"u}hren. Dies wird numerisch im direkten Vergleich mit einer klassischen Dipolantennen-Geometrie gezeigt.},
  subject      = {Physik},
  language  = {en}
}
@phdthesis{Kern2014,
  author    = {Kern, Johannes},
  title     = {Optical and electrical excitation of nanoantennas with atomic-scale gaps},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-115492},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Nano-antennas are an emerging concept for the manipulation and control of optical fields at the sub-wavelength scale. In analogy to their radio- and micro-wave counterparts they provide an efficient link between propagating and localized fields. Antennas operating at optical frequencies are typically on the order of a few hundred nanometer in size and are fabricated from noble metals. Upon excitation with an external field the electron gas inside the antenna can respond resonantly, if the dimensions of the antenna are chosen appropriate. Consequently, the resonance wavelength depends on the antenna dimensions. The electron-density oscillation is a hybrid state of electron and photon and is called a localized plasmon resonance. The oscillating currents within the antenna constitute a source for enhanced optical near-fields, which are strongly localized at the metal surface. A particular interesting type of antennas are pairs of metal particles separated by a small insulating gap. For anti-symmetric gap modes charges of opposite sign reside across the gap. The dominating field-components are normal to the metal surface and due to the boundary conditions they are sizable only inside the gap. The attractive Coulomb interaction increases the surface-charge accumulation at the gap and enhanced optical fields occur within the insulating gap. The Coulomb interaction increases with decreasing gap size and extreme localization and strongest intensity enhancement is expected for small gap sizes. In this thesis optical antennas with extremely small gaps, just slightly larger than inter-atomic distances, are investigated by means of optical and electrical excitation. In the case of electrical excitation electron tunneling across the antenna gap is exploited. At the beginning of this thesis little was known about the optical properties of antennas with atomic scale gaps. Standard measurement techniques of field confinement and enhancement involving well-separated source, sample and detector are not applicable at atomic length-scales due to the interaction of the respective elements. Here, an elegant approach has been found. It is based on the fact that for closely-spaced metallic particles the energy splitting of a hybridized mode pair, consisting of symmetric and anti-symmetric mode, provides a direct measure for the Coulomb interaction over the gap. Gap antennas therefore possess an internal ruler which sensitively reports the size of the gap. Upon self-assembly side-by-side aligned nanorods with gap sizes ranging from 2 to 0.5nm could be obtained. These antennas exhibit various symmetric and anti-symmetric modes in the visible range. In order to reveal optical modes of all symmetries a novel scattering setup has been developed and is successfully applied. Careful analysis of the optical spectra and comparison to numerical simulations suggests that extreme field confinement and localization can occur in gaps down to 0.5 nm. This is possibly the limit of plasmonic enhancement since for smaller gaps electron tunneling as well as non-locality of the dielectric function affect plasmonic resonances. The strongly confined and intense optical fields provided by atomic-scale gaps are ideally suited for enhanced light-matter interaction. The interplay of intense optical-frequency fields and static electric fields or currents is of great interest for opto-electronic applications. In this thesis a concept has been developed, which allows for the electrical connection of optical antennas. By means of numerical simulations the concept was first verified for antennas with gap sizes on the order of 25 nm. It could be shown, that by attaching the leads at positions of a field minimum the resonant properties are nearly undisturbed. The resonance wavelengths shift only by a small amount with respect to isolated antennas and the numerically calculated near-field intensity enhancement is about 1000, which is just slightly lower than for an unconnected antenna. The antennas are fabricated from single-crystalline gold and exhibit superior optical and electrical properties. In particular, the conductivity is a factor of 4 larger with respect to multi-crystalline material, the resistance of the gap is as large as 1 TOhm and electric fields of at least 10^8 V/m can be continuously applied without damage. Optical scattering spectra reveal well-pronounced and tunable antenna resonances, which demonstrates the concept of electrically-connected antennas also experimentally. By combining atomic-scale gaps and electrically-connected optical antennas a novel sub-wavelength photon source has been realized. To this end an antenna featuring an atomic scale gap is electrically driven by quantum tunneling across the antenna gap. The optical frequency components of this fluctuating current are efficiently converted to photons by the antenna. Consequently, light generation and control are integrated into a planar single-material nano-structure. Tunneling junctions are realized by positioning gold nanoparticles into the antenna gap, using an atomic force microscope. The presence of a stable tunneling junction between antenna and particle is demonstrated by measuring its distinct current-voltage characteristic. A DC voltage is applied to the junction and photons are generated by inelastically tunneling electrons via the enhanced local density of photonic states provided by the antenna resonance. The polarization of the emitted light is found to be along the antenna axis and the directivity is given by the dipolar antenna mode. By comparing electroluminescence and scattering spectra of different antennas, it has been shown that the spectrum of the generated light is determined by the geometry of the antenna. Moreover, the light generation process is enhanced by two orders of magnitude with respect to a non-resonant structure. The controlled fabrication of the presented single-crystalline structures has not only pushed the frontiers of nano-technology, but the extreme confinement and enhancement of optical fields as well as the light generation by tunneling electrons lays a groundwork for a variety of fundamental studies and applications. Field localization down to the (sub-)nanometer scale is a prerequisite for optical spectroscopy with near-atomic resolution. Indeed, recently first pioneering experiments have achieved molecular resolution exploiting plasmon-enhanced Raman scattering. The small modal volume of antennas with atomic-scale gaps can lead to light-matter interaction in the strong coupling regime. Quantum electro-dynamical effects such as Rabi splitting or oscillations are likely when a single emitter is placed into resonant structures with atomic-scale gaps. The concept of electrically-connected optical antennas is expected to be widely applied within the emerging field of electro-plasmonics. The sub-wavelength photon source developed during this thesis will likely gain attention for future plasmonic nanocircuits. It is envisioned that in such a circuit the optical signal provided by the source is processed at ultrafast speed and nanometer-scales on the chip and is finally converted back into an electronic signal. An integrated optical transistor could be realized by means of photon-assisted tunneling. Moreover, it would be interesting to investigate, if it is possible to imprint the fermionic nature of electrons onto photons in order to realize an electrically-driven source of single photons. Non-classical light sources with the potential for on-chip integration could be built from electrically-connected antennas and are of great interest for quantum communication. To this end single emitters could be placed in the antenna gap or single electron tunneling could be achieved by means of a single-channel quantum point contact or the Coulomb-blockade effect.},
  subject      = {Nanooptik},
  language  = {en}
}