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In this dissertation, I have verified that the striking purple color of the intermetallic compound AuAl₂, also known as purple gold, originates from surface plasmons (SPs). This contrasts to a previous assumption that this color is due to an interband absorption transition. The existence of SPs was demonstrated by launching them in thin AuAl2 films in the Kretschmann configuration, which enables us to measure the SP dispersion relation. I observed that the SP energy in thin films of purple gold is around 2.1 eV, comparable to previous work on the dielectric function of this material. Furthermore, SP sensing using AuAl₂ also shows the ability to measure the change in the refractive index of standard sucrose solution. AuAl₂ in nanoparticle form is also discussed in terms of plasmonic applications, where Mie scattering theory predicts that the particle bears nearly uniform absorption over the entire visible spectrum with an order magnitude higher than a lightabsorbing carbonaceous particle. The second topic of this dissertation focuses on plasmon enhanced fluorescence in gold nanoparticles (Au NPs). Here, I investigated the distance-dependent fluorescence emission of rhodamine green 110 fluorophores from Au NPs with tunable spacers. These spacers consist of polyelectrolyte multilayers (PEMs) consisting of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at pH 8.4. The distance between Au NPs and fluorophores was varied by changing the ambient pH from 3 to 10 and back, which causes the swelling and deswelling of PEM spacer. Maximum fluorescence intensity with 4.0-fold enhancement was observed with 7-layer coated Au NPs at ambient pH 10 referenced to pH 3. The last topic of this dissertation examines a novel approach to assemble nanoparticles, in particular, dimers of gold nanospheres (NSs). 16 nm and 60 nm diameter NSs were connected using photocleavable molecules as linkers. I showed that the orientation of the dimers can be controlled with the polarization of UV illumination that cleaves the linkers, making dipolar patches. This type of assembly provides a simple method with potential applications in multiple contexts, such as biomedicine and nanorobotics. / PHD / This dissertation covers three related topics. The first is an investigation of the optical properties of the unusually colored purple gold, which is a blend of gold and aluminum with the chemical formula is AuAl₂. This compound is interesting in that the origin of this color is different from most other metals. In the case of gold, for example, the metal gold is yellow color by absorbing the blue component from white light, leaving behind yellow color reflected light. The blue light is absorbed by electrons that change their state from a lower energy to a higher one. In purple gold, the color results from a different phenomenon known as “surface plasmons.” Surface plasmons are waves consisting of many electrons that move back and forth near an interface between a metal and an electrical insulator. The energy of surface plasmons in purple gold is low and corresponds to the purple color in this compound. Recently, published theoretical work supports the possibility of surface plasmons in purple gold. In this dissertation, I experimentally verify the presence of surface plasmons in purple gold. To launch surface plasmons, light was reflected off of a purple gold film deposited on the hypotenuse of a prism with varying angles of incidence. Surface plasmons can be observed by the sudden dimming of reflected light. From this, I was able to extract the surface plasmon dispersion relation, which is the relation between the inverse of the wavelength and the energy of the surface plasmons. In addition, I computed the light absorption properties of purple v gold when it is used in a nanoparticle form. The computational result showed that small purple gold nanoparticles absorb light very well, which may be useful in photothermal cancer therapy and solar steam generation. The second dissertation topic comprises a study of fluorescent molecules. These are compounds that reemit light with a different and redder color than the color of the light that illuminates them. In this experiment, green fluorescent molecules were placed near the surface of gold nanoparticles to observe how the brightness of the light emission is affected by the distance between the molecule and the metal. The underlying mechanism is based on localized surface plasmon resonances in gold nanoparticles. Localized surface plasmon resonances are waves consisting of many electrons that oscillate inside the particle, and they only occur when light at certain frequency illuminate the particle. On the resonance, the particle also exhibits the brighter light around the particle’s surface but the dimmer light away from the particle’s surface. The light enhancement from the particle can change the light emission of the fluorescent molecules. If the fluorescent molecules were placed in the range of localized surface plasmon resonances, the light emission is increased owing to the brighter light from the particle. However, if the fluorescent molecules were placed further away from the range of localized surface plasmon resonances, the light emission is decreased owing to the dimmer light from the particle. The distance between the surface of gold nanoparticle and the fluorescent molecules was varied by wrapping the gold particles with ultra-thin films of different plastic polymers before attaching fluorescent molecules to the surface of the films. These polymer films have the property that they swell and shrink when the acidity and basicity of the solution of gold particles changes, which allows me to vary the distance between the gold particles and fluorescent molecules. The results showed that the observed light gets dimmer when the solution is more acidic. On the other hand, the brighter light is noticed when the solution is more basic, and this observation is repeatable many times. Moreover, my work differs from other published works vi in that the particles with the polymer films are more robust and stable than the other particles. This allows more design flexibility and suggests applications in biomedical or environmental research where the particles can be used to locally measure properties, such as acidity in confined spacers such as living cells. It may be possible to use this technique for tumor cells in our body or toxic pollutants in the air or water. The last dissertation topic involves assembling nanoparticles to build them into larger structures. In this experiment, I fabricated particle dimers that consisted of two gold nanospheres of different sizes. They were attached together by using small molecules that are sensitive to ultraviolet (UV) light, where these molecules allow small gold nanospheres to be attached to large gold nanospheres only in those locations on the large nanospheres that have been illuminated with a sufficient amount of UV light. To achieve this alignment, UV light with a linear polarization (a specific electric field direction) was used to select the area on the large nanospheres where the UV light was particularly intense and therefore able to break the molecules, leaving positively charged surface patches on the spheres. This results in the electrostatic attraction between the positive patches on the large gold nanospheres and the negatively charged small gold nanospheres. With this method, I was able to make dimers of nanospheres in a preferred alignment by changing the polarization of UV light. The experimental results showed a good yield of dipolar patches, which allows multifunctional nanostructures with applications in nanomedicine, optical sensing, nanoelectronics, etc.

Artificial photosynthesis is used to harvest solar energy and store it in the form of chemical bonds. The system of interest in this study does this by splitting water into hydrogen and oxygen gas through a plasmon assisted process, collective oscillations from free electron gas. This is a renewable way to store energy that could be used as an alternative to fossil based fuel. In this study, a small part of this photo catalytic system is studied, namely the interaction between plasmonically active silver nanoparticles (Ag NPs) transferring photo-excited electrons via a linker molecule, 4-aminobenzoic acid (pABA). The pABA linker molecule transfers charge from the Ag surface to a semiconductor and a catalyst performing the water splitting. The pABA can bind in different ways onto the Ag-surface and the aim of this study is to examine which bond is strongest and which best enables charge transfer. To this purpose three systems where simulated quantum mechanically using a supercomputer. The total free energy of the systems was computed and compared. Out of the three studied binding sites, the hollow-site bond (pABA binding to three silver atoms) was found to have the lowest energy, meaningit's the strongest of the possible bonds. Additionally it was found that the band gap (the energy needed to transfer charge) for the pABA decreased when bound to the Ag-surface. The hollow-site bound pABA also had the smallest band gap, meaning it requires the least energy to transfer a charge and should therefore be the best bond fitted for the photo catalytic system. / Artificiell fotosyntes används för att absorbera solenergi och förvara den i formen av kemiska bindningar. Systemet som används i denna studie gör detta genom att splittra vatten till vätgas och syrgas genom en plasmon assisterad process. Detta är ett förnyelsebart sätt att förvara energi och kan användas som ett alternativ till fossila bränslen. I denna studie studeras en liten del utav detta fotokatalytiska system nämligen interaktionen där plasmonaktiva silvernanopartiklar (Ag NPs) överför foto-exciterade elektroner genom molekyllänken 4-aminobensoesyra (pABA). Molekyllänken pABA överför laddning från silverytan till en halvledare och en katalys som utför splittringen av vattnet. pABA kan binda på olika sätt tillen silveryta och denna studie syftar till att undersöka vilken utav bindningarna som är starkast och vilken som effektivast överför laddning. För att göra detta simulerades tre system kvantmekaniskt med hjälp av en superdator, ett system för varje sorts bindning. Den totala fria energin av systemen beräknades och jämfördes. Av de tre undersökta bindningarna hadehollow-site bindningen (pABA som binder till tre silveratomer) längst energi, vilket betyder att det är den starkaste av bindningarna. Utöver detta så visade det sig att bandgapet (energin som krävs för att överföra laddning) minskade för pABA när den var bunden till Ag-ytan. Hollow-site bundet pABA hade även minst bandgap, vilket betyder att den kräver minst energi för att överföra laddning och är därmed den mest effektiva bindningen för det fotokatalytiska systemet.

Pour la première fois, en associant un microscope à effet tunnel (STM) et un microscope optique inversé,nous avons imagé les plasmons de surface excités électriquement sur un film d’or avec la pointe d’un STM.Par microscopie de fuite radiative, en observant l’image de l’interface air/or et celle du plan de Fourierassocié, nous avons distingué les plasmons propagatifs des plasmons localisés sous la pointe. Les plasmonspropagatifs sont caractérisés par une distance de propagation et une direction d’émission en accord aveccelles de plasmons propagatifs créés par excitation laser sur des films d’or de mêmes épaisseurs. Les fuitesradiatives des plasmons localisés s’étalent jusqu’à l’angle maximum d’observation. Plasmons propagatifs etlocalisés ont une large bande spectrale dans le visible. Si la pointe est plasmonique (en argent), lesplasmons localisés ont une composante supplémentaire due au couplage associé. Pour différents types depointe, nous avons déterminé les intensités relatives des plasmons localisés et propagatifs. Nous trouvonsque chaque mode plasmon (propagatif ou localisé) peut être préférentiellement sélectionné en modifiant lematériau de la pointe et sa forme. Une pointe en argent produit une intensité élevée de plasmons localisés,tandis qu’une pointe fine de tungstène (rayon de l’apex inférieur à 100 nm) produit essentiellement desplasmons propagatifs. Nous avons étudié la cohérence spatiale des plasmons propagatifs excités par la pointe du STM. Avec un film d’or opaque (épaisseur 200 nm) percé de paires de nanotrous nous avons réalisé une expérienceanalogue à celle des fentes d’Young. Des franges d’interférences sont observées. La mesure de leurvisibilité en fonction de la distance des nanotrous donne une longueur de cohérence des plasmons de 4.7±0.5 μm. Cette valeur, très proche de la valeur 3.7± 1.2 μm déduite de la largeur de la distribution spectraledes plasmons, indique que l’élargissement spectral des plasmons propagatifs est homogène.Nous avons aussi étudié la diffusion des plasmons propagatifs excités par la pointe du STM par desnanoparticules d’or déposées sur un film d’épaisseur 50 nm. Nous observons une diffusion élastique et unediffusion radiative. Des franges d’interférences sont observées dans la région d’émission lumineuseinterdite du plan de Fourier, dont la période est inversement proportionnelle à la distancepointe-nanoparticule d’or avec un facteur de proportionnalité égal à la longueur d’onde moyenne desplasmons. Il y a donc interférence entre la radiation des plasmons localisés et la radiation provenant de ladiffusion des plasmons propagatifs sur les nanoparticules d’or. Ceci indique que les plasmons localisés etpropagatifs excités électriquement par la pointe du STM sont différentes composantes du plasmon uniqueproduit par effet tunnel inélastique avec la pointe du STM. Ces résultats originaux sur les plasmons créés sur film d’or par un effet tunnel inélastique localisé à l’échelle atomique (i) élargissent la compréhension du processus et (ii) offrent des perspectives intéressantes pour une association de la nanoélectronique et de la nanophotonique. / For the first time, using a equipment combining a scanning tunneling microscope (STM) and an invertedoptical microscope, we excite and directly image STM-excited broadband propagating surface plasmons ona thin gold film. The STM-excited propagating surface plasmons have been imaged both in real space andFourier space by leakage radiation microscopy. Broadband localized surface plasmons due to the tip-goldfilm coupled plasmon resonance have also been detected. Quantitatively, we compare the intensities ofSTM-excited propagating and localized surface plasmons obtained with different STM tips. We find that the intensity of each plasmon mode can be selectively varied by changing the STM tip shape or material composition. A silver tip produces a high intensity of localized surface plasmons whereas a sharp (radius < 100 nm) tungsten tip produces mainly propagating surface plasmons. We have investigated the coherence of STM-excited propagating surface plasmons by performingexperiments on a 200 nm thick (opaque) gold film punctured by pairs of nanoholes. This work is analogousto Young’s double-slit experiment, and shows that STM-excited propagating surface plasmons have acoherence length of 4.7±0.5 μm. This coherent length is very close to the value 3.7±1.2 μm expected fromthe spectrum, which indicates that the spectrum broadening of STM-excited surface plasmons ishomogeneous. We have also studied the in-plane and radiative scattering of STM-excited propagating surface plasmons bygold nanoparticles deposited on a 50 nm thick gold film. In the Fourier space images, interference fringesare observed in the forbidden light region. This interference occurs between STM-excited localized surfaceplasmons (radiating at large angles from the tip position) and the radiative scattering by the goldnanoparticle of STM-excited propagating surface plasmons. This indicates that STM-excited localized andpropagating surface plasmons are different components of the same single plasmon produced by inelasticelectron tunneling with the STM tip. These results not only broaden the understanding about the excitation process of STM excited surface plasmons but also offer interesting perspectives for the connection between nanoelectronics andnanophotonics.

We have developed a general theory of the plasmonic enhancement of many-body phenomena resulting in a closed expression for the surface plasmon-dressed Coulomb interaction. It is shown that this interaction has a resonant nature. We have also demonstrated that renormalized interaction is a long-ranged interaction whose intensity is considerably increased compared to bare Coulomb interaction over the entire region near the plasmonic nanostructure. We illustrate this theory by re-deriving the mirror charge potential near a metal sphere as well as the quasistatic potential behind the so-called perfect lens at the surface plasmon (SP) frequency. The dressed interaction for an important example of a metal–dielectric nanoshell is also explicitly calculated and analyzed. The renormalization and plasmonic enhancement of the Coulomb interaction is a universal effect, which affects a wide range of many-body phenomena in the vicinity of metal nanostructures: chemical reactions, scattering between charge carriers, exciton formation, Auger recombination, carrier multiplication, etc. We have described the nanoplasmonic-enhanced Förster resonant energy transfer (FRET) between quantum dots near a metal nanoshell. It is shown that this process is very efficient near high-aspect-ratio nanoshells. We have also obtained a general expression for the force exerted by an electromagnetic field on an extended polarizable object. This expression is applicable to a wide range of situations important for nanotechnology. Most importantly, this result is of fundamental importance for processes involving interaction of nanoplasmonic fields with metal electrons. Using the obtained expression for the force, we have described a giant surface-plasmoninduced drag-effect rectification (SPIDER), which exists under conditions of the extreme nanoplasmonic confinement. Under realistic conditions in nanowires, this giant SPIDER generates rectified THz potential differences up to 10 V and extremely strong electric fields up to 10^5-10^6 V/cm. It can serve as a powerful nanoscale source of THz radiation. The giant SPIDER opens up a new field of ultraintense THz nanooptics with wide potential applications in nanotechnology and nanoscience, including microelectronics, nanoplasmonics, and biomedicine. Additionally, the SPIDER is an ultrafast effect whose bandwidth for nanometric wires is 20 THz, which allows for detection of femtosecond pulses on the nanoscale.

Surface polaritons, i.e., collective oscillations of the surface charges, strongly influence the optical response at the micro- and nanoscale and have to be accounted for in modern nanotechnology. Within this thesis, certain basic phenomena involving surface polaritons are investigated by means of the semianalytical multiple-multipole (MMP) method. The results are compared to experiments. In the first part, the surface plasmon resonance (SPR) of metal nanoparticles is analyzed. This resonant collective oscillation of the free electrons in a metallic nanoparticle leads to an enhancement and confinement of the local electric field at optical frequencies. The local electric field can be further increased by tailoring the shape of the particle or by using near-field-interacting dimers or trimers of gold nanospheres. The hot spots found under such conditions increase the sensitivity of surface-enhanced Raman scattering by several orders of magnitude and simultaneously reduce the probed volume, thereby providing single-molecule sensitivity. The sub-wavelength-confined strong electromagnetic field associated with a SPR provides the basis for scattering-type near-field optical microscopy or tip-enhanced Raman spectroscopy, where the metal particle serves as a probe that is scanned laterally in the vicinity of a substrate. The presence of the latter causes a characteristic shift of the SPR towards lower frequencies. This effect originates in the near-field interaction of the surface charges on the objects. Furthermore, the excitation of higher-order modes becomes possible in case of an excitation by a strongly inhomogeneous wave, such as an evanescent wave. These modes may significantly contribute to the near field but have only very little influence on the far-field signature. Instead of using resonant probes, one may place a nonresonant probe in the vicinity of a substrate having a high density of electromagnetic surface states. This also produces a resonance of the light scattering by the system. Especially polar crystals, such as the investigated silicon carbide, feature such a high density of surface phonon polariton states in the mid-infrared spectral region, which can be excited due to the near-field interaction with a polarized particle. Thereby, a resonance is created leading to a strong increase of the electric field at the interface. In the second part of the thesis, special emphasis is put on surface plasmon polaritons (SPPs). Such propagating surface waves can be excited directly by plane waves only at patterned interfaces. This process is studied for the case of a groove. The groove breaks the translational invariance, so that the SPPs can be launched locally at the edges of the groove. Additionally, the mode(s) inside the groove are excited. These modes can basically be understood as metal-insulator-metal cavity modes. Their dispersion strongly depends on the groove width. The cavity behavior caused by the finite depth provides another degree of freedom for optimizing the SPP excitation by plane waves. Thin metallic films deposited on glass offer two different SPP waveguide modes, each of which can be addressed preferentially by a proper choice of the width of the groove. The reflection, transmission, scattering, and the conversion of the modes at discontinuities such as edges, steps, barriers, and grooves can be controlled by appropriately designing the geometry at the nanoscale. Furthermore, the excitation of SPPs at single and multiple slits in thin-film metal waveguides on glass and their propagation and scattering is shown by scanning near-field optical experiments. Such waveguide structures offer a means for transporting light in a confined way. Especially triangularly shaped waveguides can be used to guide light in sub-wavelength spaces. / Die Wechselwirkung von elektromagnetischer Strahlung mit subwellenlängenkleinen Teilchen bzw. Oberflächenstrukturen ermöglicht nicht nur eine Miniaturisierung optischer Geräte, sondern erlaubt sehr interessante Anwendungen, beispielsweise in der Sensorik und Nahfeldoptik. In der vorliegenden Arbeit werden die zu Grunde liegenden Effekte im Rahmen der klassischen Elektrodynamik mit Hilfe der semianalytischen Methode der multiplen Multipole (MMP) analysiert, und die Ergebnisse werden mit Experimenten verglichen. Im ersten Teil werden Oberflächenplasmonenresonanzen (engl. surface plasmon resonance - SPR) einzelner und wechselwirkender Metallteilchen untersucht. Die dabei auftretende resonante kollektive Schwingung der freien Elektronen des Partikels bewirkt eine deutliche Erhöhung und Lokalisierung des elektromagnetischen Feldes in seiner Umgebung. Die spektrale Position und die Stärke der SPR eines Nanoteilchens, die von dessen geometrischer Form, Permittivität und Umgebung abhängen, können nur im Grenzfall sehr kleiner Teilchen elektrostatisch beschrieben werden, wohingegen der verwendete semianalytische MMP-Ansatz weitaus flexibler ist und insbesondere auch auf größere Partikel, Teilchen mit komplizierterer Form bzw. Ensembles von Partikeln anwendbar ist. Die betrachteten einzelnen kleinen (&amp;lt; Wellenlänge) Goldkügelchen und Silberellipsoide besitzen eine stark ausgeprägte SPR im sichtbaren optischen Bereich. Diese ist auf eine dipolartige Polarisierung des Teilchens zurückzuführen. Höhere Moden der Polarisation können entweder als Folge von Retardierungseffekten an größeren (mit der Wellenlänge vergleichbaren) Teilchen oder bei der Verwendung inhomogener (z.B. evaneszenter) Wellen angeregt werden. Partikel, die sich in der Nähe eines Substrates befinden, unterliegen der Nahfeldwechselwirkung zwischen den (lichtinduzierten) Oberflächenladungen auf der Oberfläche des Teilchens und des Substrats. Dies führt zu einer Verschiebung der SPR zu niedrigeren Frequenzen und einer Erhöhung des lokalen elektrischen Feldes. Letzteres bildet die Grundlage z.B. der spitzenverstärkten Raman-Spektroskopie und der optischen Nahfeldmikroskopie mit Streulichtdetektion. Dasselbe Prinzip bewirkt ein stark überhöhtes elektrisches Feld zwischen miteinander wechselwirkenden Nanopartikeln, welches z.B. die Sensitivität der oberflächenverstärkten Raman-Mikroskopie um mehrere Größenordnungen steigern kann. Im Gegensatz zur SPR einzelner Nanopartikel kann die Resonanz der Lichtstreuung im Fall eines Partikels in der Nähe eines Substrats aus der durch die Nahfeldwechselwirkung induzierten Anregung elektromagnetischer Oberflächenzustände entstehen. Diese wirken ihrerseits auf das Nanopartikel zurück, wobei eine resonante Lichtstreuung beobachtbar ist. Dieser, am Beispiel einer metallischen Nahfeldsonde über einem Siliziumcarbid-Substrat analysierte, Effekt ermöglicht bei einer ganzen Klasse von polaren Kristallen interessante Anwendungen in der Mikroskopie und Sensorik basierend auf der hohen Dichte von Oberflächenphononpolaritonen dieser Kristalle im mittleren infraroten Spektralbereich und deren nahfeldinduzierten Anregung. Im zweiten Teil der Arbeit werden kollektive Anregungen von Elektronen an Metalloberflächen untersucht. Die dabei auftretenden plasmonischen Oberflächenwellen (engl. surface plasmon polaritons - SPPs) weisen einen exponentiellen Abfall der Intensität senkrecht zur Grenzfläche auf. Diese starke Lokalisierung der Energie an der Oberfläche bildet die Grundlage vieler Anwendungen, z.B. im Bereich der hochempfindlichen Detektion (bio)chemischer Verbindungen oder für eine zweidimensionale Optik (engl. plasmonics). Das Aufheben der Translationsinvarianz längs der Oberfläche ermöglicht die direkte Anregung von SPPs durch ebene Wellen. Die Abhängigkeit dieser Kopplung von der Geometrie wird am Beispiel eines Nanograbens untersucht. Dabei werden neben den SPPs ebenfalls eine oder mehrere Moden im Graben angeregt. Folglich ermöglicht die geeignete Wahl der Grabengeometrie die Optimierung der Umwandlung von ebenen Wellen in SPPs. Im - in der Praxis weit verbreiteten - Fall asymmetrisch eingebetteter metallischer Dünnschichtwellenleiter existieren zwei Moden. In Abhängigkeit von der Grabenbreite kann die eine oder die andere Mode bevorzugt angeregt werden. Die Analyse der Wechselwirkung von SPPs mit Oberflächenstrukturen, z.B. Kanten, Stufen, Barrieren und Gräben, zeigt die Möglichkeit der Steuerung der Reflexions-, Transmissions- und Abstrahleigenschaften durch die gezielte Wahl der Geometrie der &amp;quot;Oberflächendefekte&amp;quot; auf der Nanoskala und deckt die zu Grunde liegenden Mechanismen und die daraus resultierenden Anforderungen bei der Herstellung neuer plasmonischer Komponenten auf. Exemplarisch wird das Prinzip der SPP-Anregung an einzelnen und mehreren Gräben in dünnen metallischen Filmen sowie der subwellenlängen Feldlokalisierung an sich verjüngenden metallischen Dünnschichtwellenleitern unter Verwendung der optischen Nahfeldmikroskopie experimentell gezeigt.

Optoplasmonic microtube cavities, the combination of dielectric microcavities and noble metal layers, allow for the interactions between photonic modes and surface plasmons, leading to several novel phenomena and promising applications. In this thesis, the hybrid modes with different plasmon-types of evanescent field in the optoplasmonic microtube cavities are discussed. The basic physical mechanism for the generation of plasmon-type field is comprehensively investigated based on an effective potential approach. In particular, when the cavity wall becomes ultra-thin, the plasmon-type field can be greatly enhanced, and the hybrid modes are identified as strong photon-plasmon hybrid modes which are experimentally demonstrated in the metal-coated rolled-up microtube cavities. By designing a metal nanocap onto microtube cavities, angle-dependent tuning of hybrid photon-plasmon modes are realized, in which TE and TM polarized modes exhibit inverse tuning trends due to the polarization match/mismatch. And a novel sensing scheme is proposed relying on the intensity ratio change of TE and TM modes instead of conventionally used mode shift. In addition, localized surface plasmon resonances coupled to resonant light is explored by designing a vertical metal nanogap on microtube cavities. Selective coupling of high-order axial modes is demonstrated depending on spatial-location of the metal nanogap. A modified quasi-potential well model based on perturbation theory is developed to explain the selective coupling mechanism. These researches systematically explore the design of optoplasmonic microtube cavities and the mechanism of photon-plasmon coupling therein, which provide a novel platform for the study of both fundamental and applied physics such as the enhanced light-matter interactions and label-free sensing.

Surface polaritons, i.e., collective oscillations of the surface charges, strongly influence the optical response at the micro- and nanoscale and have to be accounted for in modern nanotechnology. Within this thesis, certain basic phenomena involving surface polaritons are investigated by means of the semianalytical multiple-multipole (MMP) method. The results are compared to experiments. In the first part, the surface plasmon resonance (SPR) of metal nanoparticles is analyzed. This resonant collective oscillation of the free electrons in a metallic nanoparticle leads to an enhancement and confinement of the local electric field at optical frequencies. The local electric field can be further increased by tailoring the shape of the particle or by using near-field-interacting dimers or trimers of gold nanospheres. The hot spots found under such conditions increase the sensitivity of surface-enhanced Raman scattering by several orders of magnitude and simultaneously reduce the probed volume, thereby providing single-molecule sensitivity. The sub-wavelength-confined strong electromagnetic field associated with a SPR provides the basis for scattering-type near-field optical microscopy or tip-enhanced Raman spectroscopy, where the metal particle serves as a probe that is scanned laterally in the vicinity of a substrate. The presence of the latter causes a characteristic shift of the SPR towards lower frequencies. This effect originates in the near-field interaction of the surface charges on the objects. Furthermore, the excitation of higher-order modes becomes possible in case of an excitation by a strongly inhomogeneous wave, such as an evanescent wave. These modes may significantly contribute to the near field but have only very little influence on the far-field signature. Instead of using resonant probes, one may place a nonresonant probe in the vicinity of a substrate having a high density of electromagnetic surface states. This also produces a resonance of the light scattering by the system. Especially polar crystals, such as the investigated silicon carbide, feature such a high density of surface phonon polariton states in the mid-infrared spectral region, which can be excited due to the near-field interaction with a polarized particle. Thereby, a resonance is created leading to a strong increase of the electric field at the interface. In the second part of the thesis, special emphasis is put on surface plasmon polaritons (SPPs). Such propagating surface waves can be excited directly by plane waves only at patterned interfaces. This process is studied for the case of a groove. The groove breaks the translational invariance, so that the SPPs can be launched locally at the edges of the groove. Additionally, the mode(s) inside the groove are excited. These modes can basically be understood as metal-insulator-metal cavity modes. Their dispersion strongly depends on the groove width. The cavity behavior caused by the finite depth provides another degree of freedom for optimizing the SPP excitation by plane waves. Thin metallic films deposited on glass offer two different SPP waveguide modes, each of which can be addressed preferentially by a proper choice of the width of the groove. The reflection, transmission, scattering, and the conversion of the modes at discontinuities such as edges, steps, barriers, and grooves can be controlled by appropriately designing the geometry at the nanoscale. Furthermore, the excitation of SPPs at single and multiple slits in thin-film metal waveguides on glass and their propagation and scattering is shown by scanning near-field optical experiments. Such waveguide structures offer a means for transporting light in a confined way. Especially triangularly shaped waveguides can be used to guide light in sub-wavelength spaces. / Die Wechselwirkung von elektromagnetischer Strahlung mit subwellenlängenkleinen Teilchen bzw. Oberflächenstrukturen ermöglicht nicht nur eine Miniaturisierung optischer Geräte, sondern erlaubt sehr interessante Anwendungen, beispielsweise in der Sensorik und Nahfeldoptik. In der vorliegenden Arbeit werden die zu Grunde liegenden Effekte im Rahmen der klassischen Elektrodynamik mit Hilfe der semianalytischen Methode der multiplen Multipole (MMP) analysiert, und die Ergebnisse werden mit Experimenten verglichen. Im ersten Teil werden Oberflächenplasmonenresonanzen (engl. surface plasmon resonance - SPR) einzelner und wechselwirkender Metallteilchen untersucht. Die dabei auftretende resonante kollektive Schwingung der freien Elektronen des Partikels bewirkt eine deutliche Erhöhung und Lokalisierung des elektromagnetischen Feldes in seiner Umgebung. Die spektrale Position und die Stärke der SPR eines Nanoteilchens, die von dessen geometrischer Form, Permittivität und Umgebung abhängen, können nur im Grenzfall sehr kleiner Teilchen elektrostatisch beschrieben werden, wohingegen der verwendete semianalytische MMP-Ansatz weitaus flexibler ist und insbesondere auch auf größere Partikel, Teilchen mit komplizierterer Form bzw. Ensembles von Partikeln anwendbar ist. Die betrachteten einzelnen kleinen (&amp;lt; Wellenlänge) Goldkügelchen und Silberellipsoide besitzen eine stark ausgeprägte SPR im sichtbaren optischen Bereich. Diese ist auf eine dipolartige Polarisierung des Teilchens zurückzuführen. Höhere Moden der Polarisation können entweder als Folge von Retardierungseffekten an größeren (mit der Wellenlänge vergleichbaren) Teilchen oder bei der Verwendung inhomogener (z.B. evaneszenter) Wellen angeregt werden. Partikel, die sich in der Nähe eines Substrates befinden, unterliegen der Nahfeldwechselwirkung zwischen den (lichtinduzierten) Oberflächenladungen auf der Oberfläche des Teilchens und des Substrats. Dies führt zu einer Verschiebung der SPR zu niedrigeren Frequenzen und einer Erhöhung des lokalen elektrischen Feldes. Letzteres bildet die Grundlage z.B. der spitzenverstärkten Raman-Spektroskopie und der optischen Nahfeldmikroskopie mit Streulichtdetektion. Dasselbe Prinzip bewirkt ein stark überhöhtes elektrisches Feld zwischen miteinander wechselwirkenden Nanopartikeln, welches z.B. die Sensitivität der oberflächenverstärkten Raman-Mikroskopie um mehrere Größenordnungen steigern kann. Im Gegensatz zur SPR einzelner Nanopartikel kann die Resonanz der Lichtstreuung im Fall eines Partikels in der Nähe eines Substrats aus der durch die Nahfeldwechselwirkung induzierten Anregung elektromagnetischer Oberflächenzustände entstehen. Diese wirken ihrerseits auf das Nanopartikel zurück, wobei eine resonante Lichtstreuung beobachtbar ist. Dieser, am Beispiel einer metallischen Nahfeldsonde über einem Siliziumcarbid-Substrat analysierte, Effekt ermöglicht bei einer ganzen Klasse von polaren Kristallen interessante Anwendungen in der Mikroskopie und Sensorik basierend auf der hohen Dichte von Oberflächenphononpolaritonen dieser Kristalle im mittleren infraroten Spektralbereich und deren nahfeldinduzierten Anregung. Im zweiten Teil der Arbeit werden kollektive Anregungen von Elektronen an Metalloberflächen untersucht. Die dabei auftretenden plasmonischen Oberflächenwellen (engl. surface plasmon polaritons - SPPs) weisen einen exponentiellen Abfall der Intensität senkrecht zur Grenzfläche auf. Diese starke Lokalisierung der Energie an der Oberfläche bildet die Grundlage vieler Anwendungen, z.B. im Bereich der hochempfindlichen Detektion (bio)chemischer Verbindungen oder für eine zweidimensionale Optik (engl. plasmonics). Das Aufheben der Translationsinvarianz längs der Oberfläche ermöglicht die direkte Anregung von SPPs durch ebene Wellen. Die Abhängigkeit dieser Kopplung von der Geometrie wird am Beispiel eines Nanograbens untersucht. Dabei werden neben den SPPs ebenfalls eine oder mehrere Moden im Graben angeregt. Folglich ermöglicht die geeignete Wahl der Grabengeometrie die Optimierung der Umwandlung von ebenen Wellen in SPPs. Im - in der Praxis weit verbreiteten - Fall asymmetrisch eingebetteter metallischer Dünnschichtwellenleiter existieren zwei Moden. In Abhängigkeit von der Grabenbreite kann die eine oder die andere Mode bevorzugt angeregt werden. Die Analyse der Wechselwirkung von SPPs mit Oberflächenstrukturen, z.B. Kanten, Stufen, Barrieren und Gräben, zeigt die Möglichkeit der Steuerung der Reflexions-, Transmissions- und Abstrahleigenschaften durch die gezielte Wahl der Geometrie der &amp;quot;Oberflächendefekte&amp;quot; auf der Nanoskala und deckt die zu Grunde liegenden Mechanismen und die daraus resultierenden Anforderungen bei der Herstellung neuer plasmonischer Komponenten auf. Exemplarisch wird das Prinzip der SPP-Anregung an einzelnen und mehreren Gräben in dünnen metallischen Filmen sowie der subwellenlängen Feldlokalisierung an sich verjüngenden metallischen Dünnschichtwellenleitern unter Verwendung der optischen Nahfeldmikroskopie experimentell gezeigt.

Optical trapping of metal nanoparticles investigates phenomena at the interface of plasmonics and optical micromanipulation. This thesis combines ideas of optical properties of metals originating from solid state physics with force mechanism resulting from optical trapping. We explore the influence of the particle plasmon resonance of gold and silver nanospheres on their trapping properties. We aspire to predict the force mechanisms of resonant metal particles with sizes in the Mie regime, beyond the Rayleigh limit. Optical trapping of metal nanoparticles is still considered difficult, yet it provides an excellent tool to investigate their plasmonic properties away from any interface and offers opportunities to investigate interaction processes between light and nanoparticles. Due to their intrinsic plasmon resonance, metal nanoparticles show intriguing optical responses upon interaction with laser light. These differ greatly from the well-known bulk properties of the same material. A given metal nanoparticle may either be attracted or repelled by laser light, only depending on the wavelength of the latter. The optical forces acting on the particle depend directly on its polarisability and scattering cross section. These parameters vary drastically around the plasmon resonance and thus not only change the magnitude but also the direction and entire nature of the acting forces. We distinguish between red-detuned and blue-detuned trapping, that is using a trapping wavelength shorter or longer than the plasmon resonance of the particle. So far optical trapping of metal nanoparticles has focussed on a wavelength regime far from the particle’s resonance in the infrared. We experiment with laser wavelengths close to the plasmon resonance and expand the knowledge of metal nanoparticle trapping available to date. Existing theoretical models are put to the test when we compare these with our real experimental situations.