Sähkökentän vahvistumisen simulointi plasmonisissa nanomateriaaleissa

Localized surface plasmon resonances (LSPRs) are collective excitations of nearly free electrons in plasmonic nanomaterials, such as metal nanoparticles, and they are characterized by charge density oscillation over the whole system. LSPRs lead to strong optical response and enhanced electric near field around the structure, which has been utilized in numerous applications. This thesis focuses on plasmonic nanomaterials with features of a few nanometers size. The accurate description of the optical properties of such structures necessitates quantum mechanics. The main part of the thesis comprises on implementing computational tools for analyzing optically excited plasmon resonances and the associated electric near field enhancement in these materials. In the thesis, time-dependent density functional theory (TDDFT) and its implementation in GPAW package are used. The methods developed in this thesis are implemented both in real time and in frequency domain and direct comparison between these approaches is presented. In the literature part of this thesis, the underlying quantum theory of the optical properties of finite nanostructures is reviewed. Additionally, previously published quantum studies on the plasmon resonances are presented. The used methodology is also reviewed, including TDDFT and the projector augmented wave (PAW) method which is employed in GPAW to retain the atomic treatment of material. The implemented methods are used to study plasmon resonances in a molecular metal atom chain, metal clusters in close contact and a graphene nanoribbon. The obtained results are compared with similar studies from literature. The results show additional details due to small size and the exact atomic background of the studied systems. The effect of different PAW-corrections is also studied. The methods implemented in the thesis are open source, which enables their extensive use also in further studies.Lokalisoituneet pintaplasmoniresonanssit (LSPR:t) ovat vapaiden elektronien kollektiivisia viritystiloja plasmonisissa nanomateriaaleissa, kuten nanometrien kokoisissa metallipartikkeleissa. LSPR voidaan havainnollistaa koko rakenteen yli olevana varaustiheyden värähtelynä. LSPR aiheuttaa voimakkaan optisen vasteen ja vahvistuneen sähkökentän rakenteen ympärille, mitä on hyödynnetty monissa sovelluksissa. Tämä diplomityö käsittelee plasmonisia nanomateriaaleja, joissa on muutaman nanometrin kokoisia rakenteita. Tällaisten rakenteiden optisten ominaisuuksien tarkka mallintaminen vaatii kvanttimekaniikkaa. Diplomityön pääsisältönä on optisesti viritettyjen plasmoniresonanssien ja niiden aiheuttaman sähkökentän vahvistumisen mallintamiseen soveltuvien laskennallisten menetelmien toteuttaminen. Tähän on käytetty ajasta riippuvaa tiheysfunktionaaliteoriaa (TDDFT), jolle on toteutus GPAW-nimisessä ohjelmassa. Diplomityössä kehitetyt menetelmät on toteutettu sekä aika- että taajuusavaruuksissa ja näiden lähestymistapojen tuottamia tuloksia on vertailtu. Diplomityön kirjallisuusosuudessa kuvataan kvanttimekaaninen teoria pienten rakenteiden optisten ominaisuuksien määrittämiseen ja esitellään aiemmin julkaistuja kvanttimekaniikkaan pohjautuvia tutkimuksia plasmoniresonansseista. Työssä kerrataan myös käytetyt menetelmät, kuten TDDFT-teoria ja PAW-menetelmä, jota käytetään GPAW:ssa materiaalien atomistiseen käsittelyyn. Toteutetuilla menetelmillä on tutkittu plasmoniresonansseja molekyylien kokoluokkaa olevassa metalliatomiketjussa, lähekkäisissä metalliklustereissa ja grafeenihiutaleessa. Saatuja tuloksia on verrattu vastaaviin kirjallisuudessa esitettyihin tuloksiin. Tuloksissa havaitaan yksityiskohtia, jotka aiheutuvat tutkittujen rakenteiden pienestä koosta ja tarkasta atomirakenteesta. Diplomityössä toteutetut menetelmät ovat avointa lähdekoodia, mikä mahdollistaa niiden laajamittaisen käyttämisen myös myöhemmissä tutkimuksiss

Localization in photonic crystals

A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of doctor of philosophy (Physics) School of Physics. University of the Witwatersrand, Johannesburg. 24 March 2017.This thesis is an accumulation of the work and that was carried out and published as two articles and two book chapters. Throughout the thesis, we develop and present theoretical as well as numerical model to extend the existing techniques to study the optical properties of photonic crystals, plasmonic photonic crystals and photonic quasicrystals. We start with a background review, where we cover the theoretical aspects of light–matter interaction. That is followed by a review of the physics of photonic crystals. In that chapter, we discuss the different properties of photonic crystals, plasmonic photonic crystals as well as the topic of localization. We then delve into the numerical aspects of the subject. We provide a review on the frequency domain method and the finite–differences–time–domain methods which they are both used in the work to perform different types of simulations. The frequency domain method is, then, extended to enable the numerical analysis of the optical properties in plasmonic photonic crystals. We use first order perturbation theory to study the effect of surface plasmon polaritons on the photonic band structure of plasmonic photonic crystals. We developed a simple numerical tool that extends the standard frequency domain methods to compute the photonic band structure of plasmonic photonic crystals. We then employ the two stage cut and project scheme to generate a dodecagonal two–dimensional quasiperiodic structure. The finite-differences-time–domain method is applied to simulate the propagation of electromagnetic modes in the system. We compute the transmission coefficients as well as the inverse participation ratio for a quasicrystal consisting of dielectric cylindrical rods. The analysis has shown that crystal has critical states. Furthermore, we apply the frequency domain method to quantify the localized modes in the vicinity of defects in a two–dimensional photonic crystal. We compute the intensity of those modes in the surroundings of the defects sites to identify their nature. Finally, we use the finite–differences–time–domain method to provide a second example of a quasicrystalline structure, where the states are localized.GR201

Tailoring Plasmon-Enhanced Light-Matter Interaction

Plasmons are the collective oscillation of free electrons in materials. They concentrate light into nanoscale volumes and trigger optical processes in nearby materials. My thesis is devoted to the understanding of optical processes that are mediated by localized surface plasmons. The fundamental excitation of plasmonic modes and the enhancement of optical absorption and Raman scattering in nanoscale materials are studied using experimental and theoretical approaches. I introduce a novel type of plasmonic excitation in layered films of metallic nanoparticles. Because of field retardation, incident light induces antiparallel dipoles in adjacent layers of metallic nanoparticles exciting a dark interlayer plasmon. It benefits from reduced radiative damping and efficient light absorption as I demonstrate with simulations and experiments. The self-assembled nanoparticle films pave the way for large-area coatings with tunable plasmon resonances. An application is the decay of plasmons into hot charge carriers that trigger photocatalytic reactions in molecules. I propose dark interlayer plasmons as ideal excitation channels for hot electrons because of their small radiative damping. Using plasmonic nanostructures for photodetection and sensing requires an understanding of the interaction with adjacent materials. I introduce microscopic theories for the enhancement of optical absorption and Raman scattering by localized surface plasmons. The plasmonic near field of nanoparticle arrays induced non-vertical optical transitions in graphene in dependence of the periodicity of the plasmonic lattice. For plasmon-enhanced Raman scattering I developed a general theoretical framework using perturbation theory. It provides analytic expressions for the enhanced Raman cross section. In a molecular dipole coupled to a plasmonic nanoparticle the enhancement is strongly affected by interference between different scattering channels. Plasmon-enhanced Raman scattering is an ideal tool to study the properties of materials interfaced with plasmonic nanostructures. I analyzed nanoscale strain and doping in graphene on top of a gold nanostructure. I developed a method for separating the contributions from strain and doping in the Raman spectrum of graphene, which is applicable to graphene on arbitrary substrates and in arbitrary strain configurations.Ziel dieser Arbeit ist es, ein besseres Verständnis von optischen Prozessen zu erlangen, die durch lokalisierte Oberflächenplasmonen gesteuert werden. Dafür habe ich grundlegende Anregungsmechanismen von Plasmonmoden, sowie die Verstärkung von optischer Absorption und Ramanstreuung, mit experimentellen und theoretischen Methoden untersucht. Ich stelle eine neuartige plasmonische Anregung in geschichteten Filmen von metallischen Nanopartikeln vor. Dieses dunkle Plasmon besteht aus antiparallelen plasmonischen Dipolen in den Nanopartikeln benachbarter Lagen und kann aufgrund von Feldretardierung direkt mit Licht angeregt werden. Ich zeige mit Experimenten und Simulationen, dass diese Anregung eine reduzierte Strahlungsdämpfung aufweist und zu einer ausgeprägten, durchstimmbaren Lichtabsorption im nahinfraroten Spektralbereich führt. Da die Nanopartikelfilme mittels Selbstorganisation von Nanopartikeln hergestellt werden können, eignen sie sich für die großflächige Beschichtung von Oberflächen. Aufgrund der unterdrückten radiativen Dämpfung sind dunkle Plasmonen in Nanopartikelfilmen ein idealer Anregungskanal für heiße Elektronen, mit Anwendungen in der Fotokatalyse. Mit mikroskopischen Theorien habe ich die Interaktion von plasmonischen Nanostrukturen mit angrenzenden Nanomaterialien untersucht. Ich zeige, dass das plasmonische Nahfeld eines Gitters von Goldnanopartikeln nicht-vertikale optische Übergänge in Graphen anregt. Die Auswahlregeln für diese Übergänge hängen von der Periodizität der plasmonischen Nanostruktur ab. Die mikroskopische Theorie führt zu einem besseren Verständnis der Photostromentstehung in Graphen-basierten optoelektronischen Detektoren. Als Zweites stelle ich ein allgemeines Konzept zur Beschreibung von plasmon-verstärkter Ramanstreuung mit Störungstheorie vor. Die analytischen Ausdrücke aus dieser Theorie eignen sich, um die Abhängigkeit der plasmonischen Verstärkung von der Anregungsenergie zu untersuchen. Mittels einer Implementierung für ein Molekül nahe eines plasmonischen Nanopartikels zeige ich, dass die Verstärkung stark von der Interferenz verschiedener Streuprozesse beeinflusst wird. Plasmon-verstärkte Ramanstreuung ist ideal, um zu untersuchen, wie Materialeigenschaften von einer angrenzenden plasmonischen Nanostruktur beeinflusst werden. Das zeige ich für Materialverspannungen und Dotierung in Graphen durch eine Gold-Nanostruktur. Ich habe dafür eine allgemeine Methodik entwickelt, mit der die Beiträge von Verspannung und Dotierung zum Ramanspektrum von Graphen voneinander getrennt und quantifiziert werden können. Diese eignet sich zur Auswertung von unbekannten Verspannungs-Konfigurationen in Graphen auf verschiedensten Substraten

Plasmons in nanoparticles: atomistic Ab Initio theory for large systems

205 p.El trabajo realizado en esta tesis doctoral se centra en la implementación de nuevos algoritmos y de suaplicación en diferentes tipos de nanoestructuras. El programa científico en el que se han llevado a cabolas extensiones es una implementación eficiente de la teoría funcional de densidad dependiente deltiempo, conocida como MBPT-LCAO.Las principales extensiones realizadas son las siguientes: implementación de la espectroscopía de pérdidade energía de electrones en el espacio real, mejora del procedimiento iterativo para permitir cálculos degran tamaño sin precedentes, cálculo del campo eléctrico inducido e implementación de la espectroscopíade dispersión Raman.Estas implementaciones se han utilizado en agregados y agregados dímeros de sodio y plata, así como ennanotubos de carbono y nitruro de boro. Se han calculado tanto el espectro de absorción como los camposeléctricos inducidos para todos estos sistemas. De esta forma, este trabajo nos ha permitido entendermejor la respuesta de tales nanoestructuras bajo la influencia de una perturbación externa

Localized surface plasmon resonance for biosensing lab-on-a-chip applications

In recent times, metallic nanoparticle plasmonics coupled with applications towards biosensing has gathered momentum to the point where commercial R&D are investing large resources in developing the so-called localized surface plasmon resonance (LSPR) biosensors. Conceptually, the main motivation for the research presented within this thesis is achievement of fully-operational LSPR biosensor interfaced with the state-of-the-art microfluidics, allowing for very precise control of sample manipulation and stable read-out. LSPR sensors are specifficaly engineered by electron beam lithography nanofabrication technique, where nanoparticle interactions are optimized to exhibit increased sensitivity and higher signal-to-noise ratio. However, the overall performance of LSPR lab-on-a-chip device depends critically on the biorecognition layer preparation in combination with surface passivation. As an introduction, the principles of plasmonic biosensing are identified encompassing both Surface Plasmon Resonance (SPR) and Localized SPR. Being successfully implemented into commercial product, the governing physics of SPR is compared to LSPR in chapter 1, together with advantages and disadvantages of both. Chapter 2 describes methods necessary for LSPR biosensor development, beginning with nano-fabrication methods, the modelling tool (COMSOL Multiphisics), while the basics of micro-fabrication in microfluidics conclude this chapter, where passive and active microfluidics networks are discerned. Particularly attractive optical properties are exhibited by closely-coupled nanoparticles (dimers), with the dielectric gap of below tens of nm, which were theoretically predicted to be very suitable as LSPR biosensing substrates. Chapter 3 is subjected to optical characterization (dependence on the size of the dielectric gap) of nanofabricated dimer arrays. The acquired data demonstrate the advantages of the nanofabrication methods presented in chapter 2 and the technique for fast and reliable determination of nanoparticle characteristic parameters. The initial biosensing-like experiments presented in chapter 4 (no integration with microfluidics) proved for the first time, the theoretical predictions of higher sensitivity, yielding additionally the specific response as function of analyte size and dielectric gap between nanoparticles. The overall response of different dimer arrays (various gaps) provides information about adopted conformation of analyte protein once immobilized. Broad resonances of dimers feature higher noise when employing them for the real-time LSPR biosensing. As a way to circumvent such problem, the feasibility of employing far-field interaction within the nanoparticle array to spectrally narrow resonance is investigated in chapter 5 by optimizing the array periodicity and introducing thin waveguiding layers. Finally, the concluding chapter 6 is dedicated to a full assembly of a Lab-on-a-chip (LOC) LSPR biosensor, starting with interfacing plasmonic substrates with compatible active microfluidic networks, allowing the precise sample delivery and multiplexing. The prototype device consisting of 8 individual sensors is presented with typical modes of operation. The bulk refractive index determination of various samples demonstrates the working principle of such device. Finally, various strategies of biorecognition layer formation are discussed within the on-going research

Theoretical approach to atomic-scale nanoplasmonics as probed by light and swift electrons

223 P.This thesis tackles the theoretical description of atomic-scale features in plasmonic nanostructures asprobed by light and swift electrons. Plasmonic nanostuctures are known to localize and enhanceelectromagnetic fields in their proximity, and thus serve as building blocks to perform improved andenhanced molecular spectroscopy on them. We focus on the analysis of the effect of atomic-scale featuresin the overall response of plasmonic nanoparticles and nanocavities. We apply ab initio atomisticquantum time-dependent density functional theory (TDDFT) to unveil the near-field distribution aroundmetallic antennas, and describe "classically" various atomic-scale features such as continuous protrusionson the surfaces of the metal using a Boundary Element Method (BEM), providing an extra localization ofthe field. Moreover, we propose an analytical model to address the signal increase observed in surfaceenhancedRaman scattering (SERS) spectra related to local variations of the electron density associated toatomic-scale defects. Last, we identify the excitation of confined bulk plasmons (CBP) within theTDDFT calculations for the electron energy loss (EEL) probability of atomistic clusters, and provide asemi-analytical expression within a Hydrodynamic Model (HDM) to address such excitation