Rozprawy doktorskie na temat „Plasmonic metal nanostructures”

This thesis is dedicated to the study of linear and nonlinear properties of closely spaced gold nanoparticle dimers, so-called nanoantennas, and hybrid nanoantenna devices consisting of metals and semiconductors. Coupled nanoparticles are of particular interest for nanophotonics because of their ability to focus light into subwavelength volumes and the associated large field enhancement in the gap. The samples used in this thesis are gold rectangles designed by electron-beam lithography, with both symmetric and asymmetric arms, as well as symmetric closely spaced 100 nm disk dimers which were fabricated by colloidal lithography in combination with angle-dependent evaporation. We investigate the linear interplay of modes in the two arms with Spatial Modulation Microscopy, an experimental technique which results in a measure directly proportional to the extinction cross-section. We find a variety of constructive and destructive interference between different order modes, which we can better understand by comprehensive simulations of antennas, varying the parameter space of gap size (coupling strength) and length-length ratio using advanced numerical methods such as the Fourier Domain Time Difference and the Boundary Element Method. We find that the presence of nonradiative modes is made visible by Electromagnetically Induced Transparency. In order to probe the nonlinear properties of the antennas and their interaction with Indium Tin Oxide substrates, a pump-probe setup is used to get an insight into ultrafast nonlinear response with picosecond resolution. These measurements (and corresponding fits using numerical simulations) lead us to identify a new energy transfer mechanism where fast electrons are injected from the nanoparticles into the semiconductor, resulting in a refractive index change due to heating of the surroundings. In follow-up experiments, we find this mechanism to be universal (and versatile) for other types of transparent conductive oxides. These results open new avenues towards application of nanoantennas for ultrafast switching.

Resumen en Español Las nanoestructuras metálicas están siendo objeto de gran atención dada su capacidad para generar resonancias plasmónicas, que son oscilaciones colectivas de electrones alojados en la banda de conducción en un metal excitado por efecto de un campo electromagnético. El creciente interés entorno a las nanoestructuras metálicas como fuentes de plasmones, ha resultado en el desarrollo de un nuevo campo, la plasmónica, definida como la ciencia y tecnología de la generación, control y manipulación de las excitaciones resultantes de las interaciones de la luz con la materia. Las nanoestructuras plasmónicas encuentran aplicaciones en diversos campos que cubren biología, física, química, ingeniería y medicina. Por ejemplo, son ampliamente usados en sensores, espectroscopía Raman aumentada por la superficie (SERS), celdas solares potenciadas con plasmones, fotodetectores, sistemas de transporte de medicamentos en el cuerpo y terapia de cáncer, así como nanoláseres, capas de invisibilidad y computación cuántica. Es bien sabido que las propiedades plasmónicas de las nanoestructuras metálicas se ven muy afectadas por diferentes parámetros, como el tamaño, la forma, la composición y las condiciones ambientales. Por tanto, entender y manipular las propiedades de los plasmones en la escala nanométrica es imprescindible para fabricar dispositivos con las características deseadas. En este manuscrito de tesis, presentamos un detallado estudio de caracterización de las propiedades plasmónicas de nanoestructuras huecas de AuAg, empleando técnicas espectroscópicas de pérdida de energía electrónica (en inglés, electron energy-loss spectroscopy, EELS). Se sabe que las nanoestructuras huecas muestran propiedades plasmónicas mejoradas si se comparan con las mismas estructuras macizas, debido al acoplamiento de las resonancias plasmónicas internas y externas. Este estudio incluye los primeros ejemplos de mapeo de plasmones resueltos espacialmente en nanoestructuras huecas de AuAg, tales come nanocajas y nanotubos, en 2 y 3D. Este manuscrito de tesis está divido en seis capítulos. El Capítulo 1 es la introducción, que incluye las bases teóricas de la resonancia de plasmones de superficie, revisiones de los diferentes parámetros que afectan a las propiedades plasmónicas de nanoestructuras metálicas, las áreas de aplicación de las nanoestructuras plasmónicas y las técnicas de caracterización usadas para determinar estas propiedades. En el Capítulo 2 se presentan los detalles metodológicos. Los resultados experimentales acompañados de simulaciones se presentan en los Capítulos 3, 4 y 5, donde realizamos caracterizaciones detalladas y estudios de modelaje de complejas nanoestructuras metálicas. Finalmente, el Capítulo 6 recoge las conclusiones generales de la tesis completa, así como los proyectos relacionados empezados o planeados a corto plazo.
Metallic nanostructures have received great attention due to their ability to generate surface plasmon resonances, which are the collective oscillations of conduction band electrons in a metal excited by an electromagnetic field. Ever-increasing interest in plasmonic metal nanostructures has emerged into the field of plasmonics, which can be defined as the science and technology of generation, control and manipulation of excitations resulted by the light-matter interactions. Plasmonic nanostructures have been used in many different applications spanning over the fields of biology, physics, chemistry, engineering and medicine. For instance, they are widely used in sensing, surface enhanced Raman spectroscopy (SERS), plasmon-enhanced solar cells, photodetectors, drug delivery and cancer therapy as well as nanolasers, invisibility cloaks and quantum computing. It is very-well known that plasmonic properties of metallic nanostructures are greatly affected by different parameters such as the size, shape, composition and local environment. Thus, understanding and manipulating the plasmonic properties at the nanoscale is essential to fabricate devices with the desired features. In this thesis manuscript, we present a detailed characterization study on the plasmonic properties hollow AuAg nanostructures by using electron energy-loss spectroscopy (EELS) technique. Hollow nanostructures are known to have enhanced plasmonic properties compared to their solid counterparts due to the coupling of inner and outer plasmon resonances. This study involves the first examples of spatially resolved plasmon mapping in hollow AuAg nanostructures such as nanoboxes and nanotubes, both in 2D and 3D. This thesis manuscript is divided into six chapters. Chapter 1 is the introduction, which includes the theoretical background of surface plasmon resonances, the reviews of different parameters that affect the plasmonic properties of metal nanostructures, the application areas of plasmonic nanostructures and characterization techniques used to determine the plasmonic properties. In Chapter 2, details of the methodology are presented. Experimental results and accompanying simulations are presented in Chapters 3, 4 and 5, where we perform a detailed characterization and modeling studies on complex metal nanostructures. Finally, Chapter 6 includes the general conclusions of the whole thesis and some future works that are already on-going or planned to be done in the near future.

In this thesis the interaction of ultrashort laser pulses with metal nanostructures is investigated via two different phenomena: coherent acoustic oscillations of nanoparticles and generation of THz pulses on metal surfaces. Both of these effects rely on the collective oscillations of free conduction electrons in metal surfaces, plasmons. The field of plasmonics gained a great interest in the last twenty years due to the unique properties of these surface modes. It is the effects of the resonant response of plasmonic structures to incident electromagnetic wave, in particular, in visible and infrared bands and the concentration of the electromagnetic field in small subwavelength regions with significant enhancement of the incident field that make plasmonics so attractive for various applications, such as biochemical sensing, enhanced fluorescence, surface-enhanced Raman scattering, and second harmonic generation, amongst others. Investigation of the coherent particle vibrations is performed using the pump-probe technique which allows measurement of the transient transmission signals. The expansion and subsequent contraction of the nanoparticle following the ultrashort laser pulse excitation lead to a shift of the plasmon band which can be traced by transient spectroscopy. We have investigated the effect of the particle thickness on the frequency of the fundamental vibrational mode. In addition, we measured the vibrational particle response during the particle shape deformation, both symmetrical and asymmetrical. Exploration of the THz generation phenomena on plasmonic structures was performed using THz time-domain spectroscopy, the method which allows tracing of the generated THz field in the time-domain. We were able for the first time to measure the THz pulses generated from arrays of metal nanoparticles. Our observations verify the role of the particle plasmon mode in the generation of THz pulses. In addition, by exploring the dependence of the THz emission on the femtosecond pulse intensity we showed a high nonlinearity in the THz generation mechanism. The experimental results were assessed in the context of a recently proposed model where the THz radiation is generated via the acceleration of the ejected electrons by ponderomotive forces. To reveal another proposed mechanism of the THz generation from plasmonic structures, namely optical rectification, we investigated the THz generation and electron emission from the arrays of nanoparticles and nanoholes. Our results suggest that both mechanisms may contribute to generation of THz pulses from the same sample under different illumination conditions. In addition to periodic arrays of nanoparticles and nanoholes, THz generation from random metal-dielectric films was investigated. The microstructuring of such films allowed selective THz frequency generation which was explained by a model of dipole THz emitters. In addition, the effects of low temperature and pressure on the THz generation efficiency were investigated.

The field of Plasmonics deals with interaction processes between an electromagnetic radiation of appropriate wavelength and the conduction electrons of a metal. The induced collective oscillation of the electrons is called Plasmon Resonance. The Localized Surface Plasmon Resonance (LSPR) occur when the excitation involves surface electrons of nanostructures with dimensions less or comparable to the excitation wavelength. The excitation causes a strong enhancement of the local field around the metal nanostructure, which, combined with Raman Spectroscopy, could be very interesting for molecular sensing. The Raman technique is well known for providing a fingerprint spectrum of a given molecule, but has the great limitation of low sensibility. By adsorbing the analyte of interest on a plasmonic substrate in the region of enhanced local field, high detection sensitivity can be reached through Surface Enhanced Raman Spectroscopy (SERS). The first part of the present work is focused on the synthesis and characterization of gold and silver nanoparticles (Au and Ag NPs) and gold nanoshells (Au NSs) and their exploitation for the realization of SERS substrates, both in colloidal solutions and on solid supports. Different metal nanostructures give the possibility to exploit the LSPR in a wide spectral range, from the Vis to the near IR. Their optical and morphological characterization is carried out with conventional techniques, like TEM, AFM, UV-Vis absorption and Surface Enhanced Raman Spectroscopy, and with a new characterization technique, rarely used in this research field: the Photoacoustic Spectroscopy. It provides information about the absorption contribution to the total extinction of a plasmonic nanostructure. From a rigorous measurement of the SERS enhancement factor and from Photoacoustic Spectroscopy data at different excitation wavelengths, some considerations could be done concerning the relation of far field extinction and near field SERS properties. SERS EF profile measurements on liquid and solid SERS substrates demonstrated the presence of hot spots. The solid SERS substrates were chemically stable, homogeneous and reproducible and showed EF values of about 104-105. In colloidal solution, the EF values were about 103-106, depending on the metal nanostructure. Photoacoustic measurements performed on Au NSs in solution were in agreement with theoretical predictions found in literature. In the second part of the work, the plasmonic substrates, realized with Au NPs and Au NSs, were used for the realization of label free SERS sensors, to detect toxic aromatic chemical species and biological molecules. A sensor for toxic volatile compounds, based on Au NPs and Au NSs substrates coupled with a porous organic-inorganic hybrid sol-gel matrix, was realized. The matrix was specifically chosen for exhibiting a high-affinity interaction to aromatic hydrocarbons. The enhancement activity of the Au NPs and Au NSs substrates on the sol gel matrix alone was demonstrated. Some problems in the xylene detection process through SERS were probably due to the fast matrix regeneration under the laser radiation. Although, the enhanced SERS efficiency due to the detection design was demonstrated. Another application was based on the development of a novel label-receptor system, based on the cromophore 4-hydroxyazobenzene-2 carboxylic acid (HABA) and its specific antibody, to be used in bio-analytical applications. The interesting behaviour of the HABA dye relies in changing its tautomeric structure from an azo to a hydrazo form, thanks to the interaction with its antibody. This structural change can be exploited for SERS detection of the label-receptor interaction. Properly synthesized and characterized HABA derivatives were adsorbed onto SERS substrates, further incubated in the antibody solution. The HABA signals were well visible on both Au NSs and Au NPs substrates. No HABA change could be detected through SERS, because the antibodies extracted in vivo from two rabbits, do not cause the quantitative change of the HABA structure.
La Plasmonica si occupa dell’interazione di una radiazione elettromagnetica di opportuna lunghezza d’onda con gli elettroni di conduzione di un metallo. L’oscillazione collettiva degli elettroni, indotta da questa interazione, è chiamata appunto Risonanza Plasmonica. La risonanza plasmonica di superficie localizzata avviene quando gli elettroni coinvolti sono quelli di superficie di un metallo nanostrutturato con dimensioni minori o comparabili alla lunghezza d’onda di eccitazione. Da questa eccitazione deriva una forte amplificazione del campo elettromagnetico locale, localizzato nelle immediate vicinanze della nanostruttura metallica. Tale amplificazione, unita a una tecnica di rivelazione spettroscopica specifica, quale la spettroscopia Raman, può essere sfruttata per la realizzazione di sensori molecolari. La tecnica Raman è conosciuta come altamente specifica, perché in grado di fornire uno spettro caratteristico della singola molecola, identificandone univocamente la presenza e la costituzione. La sua maggiore limitazione, però, è la bassa sensibilità. Ponendo l’analita in prossimità di un substrato plasmonico, proprio nella regione di forte amplificazione del campo locale, la sensibilità di rivelazione viene fortemente aumentata, dando origine alla spettroscopia Raman amplificata da superfici (SERS). La prima parte del presente lavoro è focalizzata sulla sintesi e sulla caratterizzazione di nanoparticelle d’argento, d’oro e di nano gusci d’oro (chiamati nanoshell) e sul loro impiego per la realizzazione di substrati SERS, sia in soluzione colloidale che su substrato solido. L’utilizzo di differenti nanostrutture metalliche, dà la possibilità di sfruttare la risonanza plasmonica localizzata di superficie in un’ampia regione spettrale, che si estende dal visibile al vicino infrarosso. La caratterizzazione ottica e morfologica delle nanostrutture è stata effettuata con tecniche convenzionali, come la spettroscopia di assorbimento UV-visibile, il SERS, la microscopia elettronica a trasmissione e la microscopia a forza atomica. Ad esse è stata affiancata anche una tecnica raramente usata nell’ambito della plasmonica: la spettroscopia fotoacustica. Questa può fornire informazioni riguardanti il contributo di assorbimento, all’estinzione totale, di una nanostruttura plasmonica. Da una rigorosa misura dei fattori di amplificazione e delle proprietà di fotoacustica al variare della lunghezza d’onda, possono essere fatte alcune considerazioni riguardanti la possibile relazione tra l’estinzione (proprietà di campo lontano) e l’ amplificazione SERS (proprietà di campo vicino). Le misure dei profili di eccitazione SERS su substrati plasmonici in liquido e su supporto solido, hanno evidenziato la presenza di hot spots, ovvero di zone fortemente amplificate dall’interazione di due o più nanostrutture. I substrati SERS solidi sono risultati chimicamente stabili, omogenei e riproducibili; essi presentano valori di fattori di amplificazione attorno a 104-105. In soluzione colloidale, i fattori di amplificazione delle nanostrutture hanno raggiunto valori nell’intervallo 103-106, dipendentemente dal tipo di nanostruttura metallica investigata. Le misure di fotoacustica effettuate su soluzioni colloidali di nanoshell d’oro si sono rivelate in accordo con le predizioni teoriche di letteratura. Nella seconda parte del lavoro, i substrati plasmonici, realizzati principalmente con nanoparticelle e nanoshell d’oro, sono stati impiegati per la realizzazione di sensori SERS per la rivelazione di specie chimiche e biologiche. É stato realizzato un sensore di composti tossici aromatici volatili, accoppiando un substrato plasmonico con un film poroso di sol gel ibrido organico-inorganico. La componente organica della matrice sol gel è stata appositamente scelta per la sua alta affinità a composti aromatici, quali lo Xilene. È stata dimostrata l’amplificazione dei segnali della matrice da parte della componente plasmonica, ma si sono riscontrati alcuni problemi nella rivelazione delle molecole di analita attraverso il SERS. La difficoltà nella rivelazione è probabilmente dovuta al veloce deadsorbimento dello Xilene dalla matrice a causa del forte riscaldamento locale causato dalla radiazione laser. Nonostante questo, si è comunque dimostrata l’aumentata efficienza del sensore progettato, rispetto ai suoi componenti singoli. La seconda applicazione studiata ha riguardato la realizzazione di un sistema analita-accettore innovativo, che può essere utilizzato per diverse applicazioni bioanalitiche; esso è basato sull’interazione tra un cromoforo diazobenzenico (HABA) e il suo anticorpo specifico. Alla base dell’applicazione si trova una proprietà interessante del suddetto cromoforo, che è quella di cambiare la sua struttura molecolare, passando da una forma azo alla forma idrazo, dopo aver interagito con il suo anticorpo specifico. Questa variazione nella struttura molecolare può essere sfruttata per la rivelazione dell’avvenuta interazione analita-accettore, mediante SERS. Alcuni derivati di questo cromoforo sono stati sintetizzati e caratterizzati in modo da poter essere adsorbiti su un substrato SERS, che viene successivamente incubato in una soluzione di anticorpo. I segnali SERS della molecola di HABA sono risultati ben visibili sia sui substrati di nanoparticelle che di nanoshell d’oro. Purtroppo non è stato possibile rivelare la variazione strutturale del cromoforo, in quanto gli anticorpi, estratti in vivo da due coniglietti, inducono solo un parziale cambio di struttura, rendendo la rivelazione SERS alquanto difficile.

The purpose of the present thesis is the study of the interaction of plasmonic and pre-plasmonic nanostructures with an emitter in close proximity. The investigation was carried out following different approaches but always with the aim of inserting the experimental results in the frame- work of new or existing theoretical models in order to better understand the photophysical nature of the interaction. To this aim in the framework of this thesis different nanoarchitectures have been synthesised and coupled to Er-doped silica layers. The choice of Erbium as emitting source was driven by the great technological importance of this rare earth in photonics and optoelectronics, connected to the characteristic emission at 1540 nm that matches the window of minimum transmission loss for silica. For this reason the first step of the research activity was devoted to the optimization of the Erbium photoluminescent properties in silica. When an emitter is placed near an interface, its optical properties will be modified. To describe this variation different contributions have to be taken into account: the variation of the local density of state due to the reflection from the interface, the coupling of the emitted radiation with propagating surface plasmons on the metal-dielectric interface and the dissipation in the overlayer. All these aspects have been studied in detail for different overlayer materials demonstrating that the strong control of the excited state lifetime of the emitter can be obtained by tailoring the dielectric properties of the overlayer and the separation distance from the interface. Nanostructuring the overlayer offers further opportunities for changing the optical properties of a nearby emitter. Among different plasmonic nanostructures, nanohole arrays (NHAs) can represent the ideal candidate for this purpose due to their extraordinary optical transmission (EOT): at specific frequencies determined by the hole periodicity, the light transmitted through the NHA is orders of magnitude higher than the one predicted with the classical diffraction theory. When the EOT peak was tailored with the emission wavelength of the emitter strong plasmonic coupling was demonstrated, leading to lifetime shortening with almost no dissipation in the overlayer. The improvement of the optical performances of an emitter can be obtained not only acting on the decay from the excited state but also increasing the excitation efficiency. For this purpose, an interesting possibility that has been explored is the sensitization by of ultra-small molecular-like metal nanoclurters (NCs) produced by ion implantation. Noble metal NCs can indeed efficiently absorb light through broad-band interband transitions and transfer energy to a nearby emitter, acting as efficient nanoantennae for excitation of the emitter. Such interaction leads to the increase of the effective excitation cross-section by several orders of magnitude. Finally, all the obtained results allowed the development of predictive models that can be used in the design of novel devices for different photonic applications
Lo scopo del presente lavoro di tesi è l’analisi dell’interazione di nanostrutture plasmoniche e pre-plasmoniche con un emettitore. Lo studio è stato condotto seguendo diversi approcci, ma sempre con il fine di confrontare i risultati sperimentali con modelli teorici sia già noti che nuovi, in modo da comprendere appieno la natura foto-fisica dell’interazione. In questo senso nell’ambito della presente tesi diverse nano-architetture sono state sintetizzate ed accoppiate con film sottili di silice drogata con erbio. La scelta dell’erbio come emettitore è stata dettata dalla sua grande importanza tecnologica della terra rara nella fotonica e nell’optoelettronica, associata alla caratteristica emissione radiativa a 1540nm, che si trova nella finestra di minimo assorbimento ottico della silice. Per questa ragione il primo passo dell’attività di ricerca è stato volto all’ottimizzazione delle proprietà di fotoluminescenza dello ione erbio in silice. Quando un emettitore è posto in prossimità di un film sottile le sue proprietà ottiche vengono modificate. Per descrivere tale variazione è necessario tenere conto di contributi differenti: la variazione della densità locale degli stati dovuta alla riflessione all’interfaccia, l’accoppiamento della radiazione emessa con plasmoni di superficie propaganti sull’interfaccia metallo-dielettrico e infine la dissipazione nel film. Tutti questi aspetti sono stati studiati in dettaglio per film di diversi materiali, dimostrando che un ottimo controllo sul tempo di vita dello stato eccitato può essere ottenuto agendo sulle proprietà dielettriche del film e sulla distanza di separazione tra l’emettitore e l’interfaccia. La nanostrutturazione del film può offrire ulteriori opportunità nella modifica delle proprietà ottiche di un emettitore. Tra le diverse nanostrutture plasmoniche, i nanohole arrays (NHAs) possono essere visti come i candidati ideali per questo scopo grazie alla loro trasmissione ottica straordinaria (EOT): a determinate lunghezze d’onda definite dalla periodicità dei buchi e dalle proprietà dielettriche dei materiali coinvolti, la luce trasmessa attraverso il NHA è ordini di grandezza più grande rispetto a quella predetta dalla teoria classica della diffrazione. Quando il picco della EOT è risonante con la lunghezza d’onda di emissione dell’emettitore, è stato dimostrato un forte accoppiamento plasmonico che porta ad un marcato accorciamento del tempo di vita nella quasi assenza di dissipazione nella nanostruttura. Il miglioramento delle proprietà ottiche di un emettitore può essere ottenuto non solamente agendo sulla parte emissiva del processo, ma anche aumentando la probabilità di eccitazione. A questo scopo, una possibilità interessante è offerta dalla sensitizzazione da aggregati metallici ultra-piccoli ottenuti per impiantazione ionica. Cluster di metalli nobili composti da 10–20 atomi possono infatti assorbire efficientemente la radiazione di eccitazione attraverso transizioni interbanda e trasferire l’energia a un emettitore posto nelle vicinanze, agendo in questo modo da efficienti nanoantenne. Tale interazione può portare ad un aumento della sezione d’urto di eccitazione efficace di diversi ordini di grandezza. Infine, tutti questi risultati hanno permesso lo sviluppo di modelli predittivi che possono essere utilizzati nella progettazione di nuovi dispositivi per diverse applicazioni fotoniche

Research in nanoscience has gained noteworthy interest over the past three decades. As novel chemical and physical properties that are vastly different from extended solids are realized in nanosized materials, nanotechnology has become the center of attention for material in research community. Much to our amazement, investigations in the past two decades revealed that the nanocrystalline semiconductors are “THE PRIME CANDIDATES” to meet the growing energy demand, sensor development, cellular imaging and a number of other optoelectronic applications. Nonetheless, synthesis of nanostructures with control over physical parameters is not sufficient, yet assembling them into functional nanoarchitectures with unique and tunable physical properties is critical for device integration studies. Among bottom-up assembling methods, sol-gel method has received noteworthy interest to produce macroscopic nanostructures of metal and semiconductor NPs with no use of intervening ligands or supports. In 2005, condensation of pre-formed semiconductor NPs (CdSe, CdS, ZnS and PbS) into voluminous gels is reported via controlled destabilization of the surfactant ligands. The resultant chalcogenide aerogels are reported to exhibit extremely low density, high surface area and porosity, and quantum confined optical properties of the NP building blocks. More recently, this method has been extended for the assembly of metal NPs, where transparent and opaque nanostructures (aerogels) of Ag and Au/Ag NPs were produced. The aerogels produced by condensation of NPs are low dimensional (fractal) nanostructures and exhibit a physically connected network of colloidal NPs. Interactions between NPs in a gel structure are intermediate of those of the ligand stabilized NPs and core/shell hetero-nanostructures (e.g. Au@CdSe NPs) with the potential to couple chemically dissimilar systems. In this research study, NP condensation strategy has been utilized to efficiently integrate the plasmonic and excitonic properties of metal and semiconductor nanostructures to produce high-efficiency hybrids that exhibit unique tunable physical and photophysical properties. Two hybrid systems composed of spherical CdSe/Ag hollow NPs and rod shaped CdSe/Ag hollow NPs were investigated for the fabrication of metal-semiconductor hybrid aerogels. The first excitonic energy of spherical CdSe NPs is overlapped with the plasmonic energy of Ag hollow NPs at 515 - 530 nm. The second excitonic energy of rod shaped CdSe is overlapped with the plasmonic energy of Ag hollow NPs at 490 - 505 nm. The photophysical properties of both systems were thoroughly probed through UV-Visible absorption, photoluminescence (PL), and time-resolved (TR) PL spectroscopy. A novel hybrid emission emerged at 640 nm (for spherical CdSe/Ag hollow NPs) and 720 nm (for rod shaped CdSe/Ag hollow NPs) with ~0.2-1% Ag loading. TRPL studies revealed 685 ns and 689 ns PL decay times for hybrid emissions, which are vastly different from the band-edge and trap state emission of phase pure spherical and rod shaped CdSe aerogels respectively, supporting the generation of novel radiative decay pathways. Overall, synthesis of CdSe/Ag hybrid aerogels with novel/tunable photophysical properties will add to the toolbox of semiconductor aerogels with the potential application in future light harvesting technologies.

The development of nanotechnology has provided a variety of noble metal nanostructures with unique optical properties that are useful for different application fields. Metal nanoparticles present strongly enhanced optical properties associated with localized surface plasmon resonance (LSPR): here, the effect on the optical properties of metal nanostructures is investigated by different techniques. The large AuNPs absorption cross section coupled with fast nonradiative decay rate and low radiative decay efficiency make them perfect converter of light into heat: the high temperatures reached can be used for photothermal terapy, light conversion in thermal and photovoltaic devices, but our interest has been focused on optical limiting application against cw laser. The study of the thermal conversion of incoming light could be useful for the protection of the human eye from accidental or intentional damage. A good protection device should be a “smart material” able to activate the protection at high energy with a large dynamic range and in a wide wavelength interval. The last property is especially required in the case of military use, for protection against laser pointing devices or blinding weapons of unpredictable emission wavelength. In this case, passive filters, commonly used for specific wavelengths, are useless because of their selectivity and lack of tuning properties. The irradiation of an optical limiting material with a focused cw laser beam induces energy absorption rapidly converted into a local heating and a temperature gradient corresponding to a refractivity index variation across the sample. In this way, even a flat sample acts as a focusing or defocusing lens and spreads the laser beam. We have studied different aspects of the phenomenon, as described below, to achieve the application in a solid state device with a broadband range of activity and a fast response time. In the first experimental part of this thesis different nanostructures have been synthesized, starting from gold nanoparticles, nanoshells and nanorods with different aspect ratio, in order to obtain plasmonic resonances in a wide range of the visible spectrum. Nanostructures has been then manipulated for the functionalization with a thiolated-fulleropyrrolidine (FULP-SH) to combine the thermal relaxation process with a faster one. A useful material for protection devices should preferably be in the solid state, so a thorough study has been centered on polycarbonate (PC) as matrix because of its good optical qualities. Film production and nanoparticles embedding require a specific study of the functionalization and transfer of nanostructures synthesized in aqueous solvent. We characterized the morphology and their linear optical properties with conventional techniques: transmission electron microscopy (TEM) gives information about the dimension of nanostructures to implement the synthesis, UV-Vis spectroscopy correlates structures with extinction properties and surface enhanced Raman spectroscopy (SERS) of the nanosystems defines the correct functionalization with organic molecules. In the second part of the project we studied and tried to improve the nonlinear optical response of these promising systems in order to obtain different characteristics. Using z-scan technique we define the nature of the defocusing mechanism, confirming the self-defocusing behavior and giving nonlinear efficiency parameters to compare different systems. Optical power limiting measurements give direct information on the protection ability of these systems. Thanks to the easy functionalization of nanostructures we figured out promising properties for a solid state protection device. First we have studied the optical limiting properties of gold nanoparticle solutions identifying a thermal response as the main mechanism. We have then compared these results with those obtained by coupling gold nanoparticles with a thiolated-fulleropyrrolidine. In this way we wanted to combine the thermal process with a faster one, to permit a stronger reduction of transmittance and a better limiting efficiency. Such a strategy has been proved to be effective for improving OL through a quite different mechanism that is activated in a much shorter time. Optical limiting measurements have been conducted on gold nanoparticles embedded in polycarbonate with good results that have been compared to the colloidal solutions. The study of a different matrix for optical limiting studies has been attempt: silk fibroin. This matrix has been selected because of the easier nanoparticles embedding. Furthermore it can be applied for instance in controlled release of drugs, thanks to the biocompatibility and gradual solubility of silk matrix. Preliminary studies discourage the use of this system for optical limiting but different application could be considered. The fibroin-nanoparticles solution can be easily transform to obtain a porous structure: the idea is to employ this matrix as a sensor for liquid samples with SERS characterization, taking advantage of the high porosity and the presence of plasmonic structures. In the last part we tried to compare thermal properties revealed by our systems through cw laser excitation to nonlinear optical properties classically expressed by pulsed laser excitation. Optical limiting related to photoacoustic measurements allow us to discriminate the contribution of the absorption and to choose the best system with higher linear transmittance and lower threshold for nonlinear behavior
Lo sviluppo delle nanotecnologie ha fornito una varietà di nanostrutture metalliche con proprietà ottiche uniche utili per diverse applicazioni. Le nanoparticelle metalliche presentano una forte amplificazione delle proprietà ottiche associate al plasmone di risonanza superficiali (LSPR): in questo lavoro abbiamo studiato le proprietà ottiche di nanoparticelle d’oro (AuNPs) con diverse tecniche. La grande cross section di assorbimento delle AuNPs accoppiata con la rapido decadimento non radiativo e la scarsa efficienza di decadimento rendono efficace la conversione di luce in calore: le alte temperature raggiunte possono essere utilizzate per terapia fototermica, conversione luminosa in dispositivi fotovoltaici, ma il nostro interesse si è focalizzato sull’applicazione nella limitazione ottica contro laser in continuo (cw). Lo studio della conversione termica della luce incidente può essere utilizzato per la realizzazione di dispositivi per la protezione dell’occhio contro danni accidentali o intenzionali. Un buon dispositivo di protezione dovrebbe essere un materiale intelligente in grado di attivarsi sopra una certa soglia di intensità, con un ampio intervallo di attività e a diverse lunghezze d’onda. Quest’ultima proprietà è di particolare interesse in ambito militare per la protezione contro dispositivi laser di puntamento o armi accecanti di lunghezze d’onda non note a priori. In questo caso sono i filtri passivi per specifiche lunghezze d’onda attualmente utilizzati risultano inefficaci data la loro alta selettività e scarsa versatilità. L’irraggiamento di un limitatore ottico con un raggio laser cw focalizzato induce un assorbimento dell’energia che viene rapidamente convertito in un riscaldamento locale e la formazione di un gradiente di temperatura che corrisponde ad una variazione di indice di rifrazione attraverso il campione. In questo modo anche un campione piatto agisce come una lente focalizzante o defocalizzante e diffonde la luce. Abbiamo studiato diversi aspetti del fenomeno, come descritto in seguito, per ottenere un dispositivo a stato solido con un ampio intervallo di attività e una risposta rapida. Nella prima parte sperimentale di questa tesi sono state sintetizzate diverse nanostrutture, a partire da nanoparticelle d’oro, nanoshells e nanorods con aspect ratio differenti, al fine di ottenere risonanze plasmoniche in un ampio intervallo dello spettro visibile. Le nanostrutture sono state in seguito funzionalizzate con molecole di fulleropirrolidina tiolata (FULP-SH) per combinare il processo di rilassamento termico con uno più rapido. Un limitatore ottico per un dispositivo di protezione deve essere preferibilmente solido, e quindi lo studio delle proprietà ottiche è stato effettuato anche in matrice, in particolare in polycarbonato (PC), scelto per le sue ottime qualità ottiche. La produzione dei film e l’inglobamento delle nanoparticelle ha richiesto degli studi sulla funzionalizzazione e la stabilizzazione delle nanostrutture sintetizzate in solvente acquoso. Abbiamo caratterizzato la morfologia e le proprietà ottiche lineari con tecniche convenzionali: microscopia a trasmissione elettronica (TEM), che fornisce informazioni sulle dimensioni e la forma delle nanostrutture al fine di implementarne la sintesi, spettroscopia UV-Visibile che correla le strutture con le proprietà di estinzione, e la spettroscopia Raman che ha verificato l’effettiva funzionalizzazione dei sistemi con le molecole organiche. Nella seconda parte del progetto abbiamo studiato le risposte ottiche non lineari di questi promettenti sistemi per poterne modulare le proprietà. Attraverso la tecnica Z-scan siamo stati in grado di definire la natura del meccanismo di defocalizzazione e di ottenere i parametri non lineari che ci hanno permesso di confrontare i nostri risultati con quelli attualmente presenti in letteratura. Misure di limitazione ottica hanno dato informazioni sull’efficacia di protezione dei nostri sistemi. Grazie alla semplicità di funzionalizzazione delle nanoparticelle abbiamo individuato delle nuove e promettenti proprietà per un dispositivo di protezione a stato solido. In primo luogo abbiamo studiato le proprietà di limitazione ottica di nanoparticelle in soluzione per identificare la tipologia di funzionamento. In seguito i risultati sono stati confrontati con quelli ottenuti con nanoparticelle funzionalizzate con FULP-SH. In questo modo abbiamo tentato di associare al processo di rilassamento termico un meccanismo più rapido, in modo da ridurre maggiormente la trasmittanza e migliorare l’efficienza di limitazione. Abbiamo quindi verificato l’efficacia della strategia utilizzata evidenziando un miglioramento della limitazione ottica in un tempo inferiore. Le misure di limitazione ottica eseguite su nanoparticelle in matrice di PC hanno dato ottimi risultati, paragonabili a quelli ottenuti in soluzione. Un primo di studio di matrici differenti si è concentrato sulla fibroina della seta, scelta per la semplicità di inglobamento delle nanoparticelle. Inoltre questo sistema AuNPs-fibroina potrebbe trovare sbocco anche in diverse applicazioni: grazie alla biocompatibilità della matrice ed alla sua solubilità graduale in acqua potrebbe essere usato per il rilascio controllato di farmaci. Studi preliminari scoraggerebbero l’utilizzo di questo sistema nella limitazione ottica ma possono essere comunque considerate altre applicazioni. Le nanoparticelle in fibroina possono infatti essere facilmente trasformate in strutture porose: un’idea potrebbe essere quella di utilizzarle come sensori per campioni in soluzione con caratterizzazione Raman amplificata (SERS), combinando l’alta porosità e la presenza di strutture plasmoniche. Nell’ultima parte abbiamo confrontato le proprietà termiche dei nostri sistemi attraverso studi di fotoacustica che ci hanno permesso di discriminare il contributo assorbitivo dall’estinzione totale e di scegliere il sistema migliore con alta trasmittanza lineare e basse soglie di attivazione nonlineari

This thesis focuses on the design, synthesis and development of dynamic multivalent nanostructures such as supramolecular dendrimers, liposomes and gold-functionalized nanostructures. These structures can be used for drug delivery and molecular sensing applications. This thesis is divided into three parts: In part one, a general introduction to self-assembly, dynamic systems, metalligand exchange, nanostructured dendritic scaffolds, liposomes and gold nanostructures is given. In part two, a microwave approach is presented as an efficient method for the regioselective deuteration of bipyridine scaffolds. Dynamic systems based on transition metal-bipyridine coordination complexes were investigated. The compositional self-adaptation and kinetics of these dynamic systems were successfully assessed by ESI-MS. Based on this amphiphilic dendrimers/metallodendrimers were also designed and synthesized via  a convergent strategy. Their ability to self-assemble into supramolecular assemblies and their controlled disassembly was effectively demonstrated. In part three, two types of drug delivery systems based on dynamic multivalent nanostructures of glycodendrimers/metalloglycodendrimers and drugpresenting liposomes were developed. The dynamic self-assembly of these architectures into supramolecular nanostructures with site-specific functionality through interacting carbohydrate or cholesterol moieties was assessed. The host-guest interaction/encapsulation and controlled release with external stimuli were studied using a fluorescent probe, as well as selected drug molecules. The antibacterial property of the drug delivery systems was also evaluated, demonstrating an enhanced bactericidal activity. A new, rapid and simple approach for the functionalization of plasmonic gold nanostructured surfaces was also developed. The optical performance and light-specific sensitivity of the fluorescent probe on the resulting nanostructures were also presented.

QC 20151119

Thesis (M. S.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008.
Committee Chair: El-Sayed, Mostafa A.; Committee Member: Lyon, L. Andrew; Committee Member: Sherrill, C. David; Committee Member: Wang, Zhong Lin; Committee Member: Whetten, Robert L.

In this work, we investigated nano-metal plasmonic coupling between dissimilar metals. We measured the optical transmission of nano-Ag coupled to other nano-metals using glass and Si substrates respectively. The reflected colors shifted from yellow to violet were obtained through the plasmonic coupling with nearest-neighbor nano-metals such as aluminum, magnesium, and ytterbium nano-metals. They were deposited randomly next to the nano-Ag. The metal size is from 8 to 15 nanometers. The results show that the colors changing is essentially due to plasmonic coupling between nano-Ag and another the nano-metals e.g. nano-Al The coupling caused a red shift in plasmonic resonance frequency, thus, changing the reflection color. The resonance shift agrees well with the simulation result using COMSOL. The inter-particle distance and particle size dependency of the optical spectra correspond to surface plasmon resonance extinction peaks for isolated nano-Ag and coupled with those neighboring nano- metals. Due to plasmonic coupling between nanoparticles in small space can create new resonances; red shifts as the interparticle distance reduce. Wavelengths are tuned by the extent of the interparticles interactions which relate to the particles size, interparticles distance and the similarity of nano metals. Using different nano metals to fabricate thin films can change the plasmonic resonance frequency which makes the reflected colours become multihued. When we look into the effect of the nano-particle size, and the distance between nano-particles, we discovered that larger nano-particle size has larger distance between the particles, and since the plasmonic coupling is a function of Inverse Square of the distance between particles. Therefore, smaller nano-particles have the strongest plasmonic coupling. Al produced the smallest nano-particle therefore it has the shortest distance between nano-Al and nano-Ag. Since the size of the particles can be controlled during deposition, the color changing of nano-Ag can be well defined. Thus tunable color changing devices can be fabricated

Semiconductor quantum dots and metal nanoparticles feature very strong light-matter interactions, which has led to their use in many photonic applications such as photodetectors, biosensors, components for telecommunications etc.Under illumination both structures exhibit collective electron-photon resonances, described in the frameworks of quasiparticles as exciton-polaritons for semiconductors and surface plasmon-polaritons for metals.To date these two approaches to controlling light interactions have usually been treated separately, with just a few simple attempts to consider exciton-plasmon interactions in a system consisting of both semiconductor and metal nanostructures.In this work, the exciton-polaritons and surface \\plasmon-polaritons are first considered separately, and then combined using the Finite Difference Time Domain numerical method coupled with a master equation for the exciton-polariton population dynamics.To better understand the properties of excitons and plasmons, each quasiparticle is used to investigate two open questions - the source of the Stokes shift between the absorption and luminescence peaks in quantum dots, and the source of the photocurrent increase in quantum dot infrared photodetectors coated by a thin metal film with holes. The combined numerical method is then used to study a system consisting of multiple metal nanoparticles close to a quantum dot, a system which has been predicted to exhibit quantum dot-induced transparency, but is demonstrated to just have a weak dip in the absorption.

QC 20120417

Le domaine du photovoltaïque en couches minces s’attache à réduire le coût de l’énergie photovoltaïque, en réduisant considérablement la quantité de matières premières utilisées. Dans le cas du silicium cristallin en couches minces, la réduction de l’épaisseur de la cellule s’accompagne d’une baisse drastique de l’absorption, notamment pour les plus fortes longueurs d’onde. Nombreuses sont les techniques aujourd’hui mises en œuvre pour lutter contre cette baisse de performance, dont l’utilisation des effets plasmoniques induits par des nanostructures métalliques qui permettent un piégeage de la lumière accru dans la couche absorbante. Dans ces travaux, nous étudions l’influence de nanostructures d’argent organisées suivant un réseau périodique sur l’absorption d’une couche de silicium. Ces travaux s’articulent autour de deux axes majeurs. L’influence de ces effets plasmoniques sur l’absorption est d’abord mise en évidence à travers différentes simulations numériques réalisées par la méthode FDTD. Nous étudions ainsi les cas de réseaux périodiques finis et infinis de nanostructures d’argent situés sur la face arrière d’une couche mince de silicium. En variant les paramètres du réseau, nous montrons que l’absorption au sein du silicium peut être améliorée dans le proche infrarouge, sur une large plage de longueurs d’onde. Le second volet de la thèse concerne la réalisation des structures modélisées. Pour cela, deux voies de fabrication ont été explorées et développées. Pour chacune d’entre elles, trois briques élémentaires ont été identifiées : (i) définition du futur motif du réseau grâce à un masque, (ii) réalisation de pores dans le silicium et (iii) remplissage des pores par de l’argent pour former le réseau métallique. La première voie de fabrication développée fait appel à un masque d’alumine, réalisé par l’anodisation électrochimique d’une couche d’aluminium, pour définir les dimensions du réseau métallique. Une gravure chimique assistée par un métal est ensuite utilisée pour former les pores, qui seront alors comblés grâce à des dépôts d’argent par voie humide. La seconde voie de fabrication utilise un masque réalisé par lithographie holographique, une gravure des pores par RIE et un remplissage des pores par dépôt d’argent electroless. Les substrats plasmoniques fabriqués sont caractérisés optiquement, au moyen d’une sphère intégrante, par des mesures de transmission, réflexion et absorption. Pour tous les substrats plasmoniques caractérisés, les mesures optiques montrent une baisse de la réflexion et de la transmission et une hausse de l’absorption pour les plus grandes longueurs d’onde
Thin-film photovoltaics focus on lowering the cost reduction of photovoltaic energy through the significant reduction of raw materials used. In the case of thin-films crystalline silicon, the reduction of the thickness of the cell is linked to a drastic decrease of the absorption, particularly for the higher wavelengths. This decrease of the absorption can be fought through the use of several different light trapping methods, and the use of plasmonic effects induced by metallic nanostructures is one of them. In this work, we study the influence of a periodic array of silver nanostructures on the absorption of a silicon layer. This work is decomposed into two main axes. First, the influence of the plasmonic effects on the silicon absorption is highlighted through different numerical simulations performed by the FDTD method. Both finite and infinite arrays of silver nanostructures, located at the rear side of a thin silicon layer, are studied. By varying the parameters of the array, we show that the silicon absorption can be improved in the near infrared spectral region, over a wide range of wavelengths. The second part of the thesis is dedicated to the fabrication of such modeled structures. Two different approaches have been explored and developed inside the lab. For each of these two strategies, three major building blocks have been identified: (i) definition of the future array pattern through a mask, (ii) etching of the pattern in the silicon layer and (iii) filling of the pores with silver in order to form the metallic array of nanostructures. In the first fabrication method, an anodic alumina mask, produced by the electrochemical anodization of an aluminium layer, is used in order to define the dimensions of the metallic array. A metal assisted chemical etching is then performed to produce the pores inside the silicon, which will then be filled with silver through a wet chemical process. The second fabrication method developed involves the use of holographic lithography to produce the mask, the pores in silicon are formed by reactive ion etching and they are filled during an electroless silver deposition step. The fabricated plasmonic substrates are optically characterized using an integrating sphere, and transmission, reflection and absorption are measured. All the characterized plasmonic substrates shown a decrease of their reflection and transmission and an absorption enhancement at the largest wavelengths

Diese Arbeit ist der theoretische Untersuchung nichtlinearer optischer Prozesse in metall-dielektrischen Medien gewidmet, wobei Möglichkeiten zur Ausnutzung der erhöhten nichtlinearen Koeffizienten und der Feldüberhöhung durch metallische Nanoteilchen untersucht wurden. Die wichtigsten Ergebnisse beziehen sich auf eine Untersuchung der zeitabhängigen sättigbaren Absorption in Gläsern, die mit metallischen Nanoteilchen dotiert sind, ihrer physikalischen Ursache sowie verschiedener Anwendungen in der nichtlinearen Optik. Zur Untersuchung der Zeitabhängigkeit der nichtlinearen Rückwirkung wird unter Verwendung des semi-klassischen Zwei-Temperatur-Modells eine zeitabhängige Gleichung für die nichtlineare dielektrische Funktion der Metalle hergeleitet. Die Ergebnisse zeigen, dass solche Gläser, sich als sehr effiziente sättigbare Absorber im Spektralbereich vom sichtbaren bis nahen IR eignen. Für kurzwellige Laser im blau/violetten Spektralbereich wird die Dynamik der Modenkopplung in Festkörper- und Halbleiter-Scheibenlaser untersucht. Weiterhin wird ein neuer Mechanismus für die Realisierung von langsamem Licht vorgeschlagen und im Detail untersucht, der in solchen dotierten Gläsern in einem Pump-Probe Regime infolge der sättigbaren Absorption in der Nähe der Plasmonresonanz realisierbar ist. Weiterhin untersuchten wir die Möglichkeit einer Femtosekunden Plasmon Impulserzeugung durch Modenkopplung eines Oberflächen Plasmonlasers mit einem Bragg Resonator, der aus einer dünnen Schicht aus Silber sowie einem sättigbaren Absorbers und einem Verstärker besteht. Im letzten Teil der Arbeit werden Ergebnisse zur Erzeugung hoher Harmonischer in Edelgasen in der Nähe einer metallischen fraktalen rauen Oberfläche untersucht. Die Berechnungen zeigen eine Reduzierung der geforderten Intensität um drei Größenordnungen und eine um zwei Größenordnungen erhöhte Effizienz gegenüber der bisher experimentell realisierten HHG in der Nähe von metallischen "bow-tie"Nanostrukturen.
This work reports results of a theoretical study of nonlinear optical processes in metal-dielectric nanocomposites used for the increase of the nonlinear coefficients and for plasmonic field enhancement. The main results include the study of the transient saturable nonlinearity in dielectric composites doped with metal nanoparticles, its physical mechanism as well its applications in nonlinear optics. For the study of the transient response, a time-depending equation for the dielectric function of the nanocomposite using the semi-classical two-temperature model is derived. By using this approach, we study the transient nonlinear characteristics of these materials in comparison with preceding experimental measurements. The results show that these materials behave as efficient saturable absorbers for passive mode-locking of lasers in the spectral range from the visible to near IR. We present results for the modelocked dynamics in short-wavelength solid-state and semiconductor disk lasers; in this spectral range other efficient saturable absorbers do not exist. We suggest a new mechanism for the realization of slow light phenomenon by using glasses doped with metal nanoparticles in a pump-probe regime near the plasmonic resonance. Furthermore, we study femtosecond plasmon generation by mode-locked surface plasmon polariton lasers with Bragg reflectors and metal-gain-absorber layered structures. In the final part of the thesis, we present results for high-order harmonic generation near a metallic fractal rough surface. The results show a possible reduction of the pump intensities by three orders of magnitudes and two orders of magnitudes higher efficiency compared with preceding experimental results by using bow-tie nanostructures.

The modification of the band edge or emission energy of semiconductor quantum well light emitters due to image charge induced phenomenon is an emerging field of study. This effect observed in quantum well light emitters is critical for all metal-optics based light emitters including plasmonics, or nanometallic electrode based light emitters. This dissertation presents, for the first time, a systematic study of the image charge effect on semiconductor–metal systems. the necessity of introducing the image charge interactions is demonstrated by experiments and mathematical methods for semiconductor-metal image charge interactions are introduced and developed.

Thesis advisor: Michael J. Naughton
The nanoscale morphology of metals can enable special functionality in plasmonic and electrochemical devices, with applications in energy conversion and storage, sensors, and computers. In particular, sharp edges on metal nano and microstructures are understood to affect the density of electrons on the metal surface. The associated concentration of electric field can concentrate surface plasmon polaritons (SPPs) and enable waveguiding of the SPPs, as we show in this thesis for sharp ridges along aluminum nanowires. Also important is the presence of facets on the metal structures, which determines the orbitals that electrons occupy on the metal surface. Changes in both the electron density and orbitals can affect the binding of molecules to the metal, which can improve reaction kinetics in catalysis. We demonstrate this on gold dendrite and plate electrocatalysts for CO2 electrolysis. Regarding metal nanostructure fabrication, electrochemical deposition and corrosion have demonstrated promising control over the morphology, including the topography, crystallinity, grain boundaries, and crystal faceting. This is important, because existing methods for metal nanostructure fabrication can only produce a circumscribed assortment of morphologies. In contrast, semiconductors and insulators have many new deposition techniques that produce a wide range of controlled morphologies. Of further appeal, electrochemical techniques are solution-based and typically operate at room temperature and pressure, allowing facile scale-up to industrial production. Here we demonstrate and discuss the mechanisms of two new techniques, which produce the aluminum nanowires and gold dendrites and plates discussed above
Thesis (PhD) — Boston College, 2018
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

The study of interactions between plasmonic nanomaterials and dielectrics is a thriving field of research, which in recent years proved that such nanostructures can be applied in a wide range of applications, from sensing to catalysts. These are all based on the nanoscale surface interactions happening between the nanomaterials and their surrounding environment. In this thesis, the possible interaction between plasmonic nanoparticles and the V doping states in the Anatase (TiO_2) bandgap, rather than in their undoped counterpart, is studied. The aim is to better understand the dynamics of these phenomena, and obtaining insights on the V states position in the TiO_2 bandgap. The work done encompasses all the steps needed to achieve the experimental results: from the preparation and characterisation of the samples, to the simulations of the phenomena involved, until the actual measurements of their optical properties and the discussion of the results. The findings achieved are not decisive in explaining the dynamics involved, but preliminary interpretations could be formulated. Moreover, the specific investigations displayed in this thesis have never been done before in literature, and the work performed might be used in the future as a starting point for more thorough and deep studies of these phenomena.

Surface plasmons have been attracted extensive interests in recent decades due to the novel properties in nanometer sized dimensions. My work focused on the novel optical properties of metal nanostructures based on surface plasmons using theoretical simulation methods. In the first part, we investigated metal nanofilms and nanorods and demonstrated that extremely low scattering efficiency, high absorption efficiency and propagation with long distance could be obtained by different metal nanostructures. With a perforated silver film, we demonstrated that an extremely low scattering cross section with an efficiency of less than 1% can be achieved at tunable wavelengths with tunable widths. The resonance wavelength, width, and intensity are influenced by the shape, size and arrangement pattern of the holes, as well as the distance separating the holes along the polarization direction. The extremely low scattering could be used to obtain high absorption efficiency of a two-layer silver nanofilm. Using the discrete dipole approximation method, we achieved enhanced absorption efficiencies, which are close to 100%, at tunable wavelengths in a two-layer silver thin film. The film is composed of a 100 nm thick perforated layer facing the incident light and a 100 nm thick solid layer. Resonance wavelengths are determined by the distances between perforated holes in the first layer as well as the separation between two layers. The resonance wavelengths shift to red with increasing separation distance between two layers or the periodic distance of the hole arrays. Geometries of conical frustum shaped holes in the first layer are critical for the improved absorption efficiencies. When the hole bottom diameter equals the periodic distance and the upper diameter is about one-third of the bottom diameter, close to unit absorption efficiency can be obtained. We examined the electromagnetic wave propagation along a hollow silver nanorod with subwavelength dimensions. The calculations show that light may propagate along the hollow nanorod with growing intensities. The influences of the shape, dimension, and length of the rod on the resonance wavelength and the enhanced local electric field, |E|2, along the rod were investigated. In the second part, a generalized electrodynamics model is proposed to describe the enhancement and quenching of fluorescence signal of a dye molecule placed near a metal nanoparticle (NP). Both the size of the Au NPs and quantum yield of the dye molecule are crucial in determining the emission intensity of the molecule. Changing the size of the metal NP will alter the ratio of the scattering and absorption efficiencies of the metal NP and consequently result in different enhancement or quenching effect to the dye molecule. A dye molecule with a reduced quantum yield indicates that the non-radiative channel is dominant in the decay of the excited dye molecules and the amplification of the radiative decay rate will be easier. In general, the emission intensity will be quenched when the size of metal NP is small and the quantum yield of dye molecule is about unity. A significant enhancement factor will be obtained when the quantum yield of the molecule is small and the particle size is large. When the quantum yield of the dye molecule is less than 10-5, the model is simplified to the surface enhanced Raman scattering equation.
Ph.D.
Doctorate
Chemistry
Sciences
Chemistry

The theme of my thesis is to investigate the diffraction behavior of subwavelength holes in metal films, and to understand the surface plasmons' (SPs) role in aperture far-field diffraction. We have built a home-made goniometer setup with high-level quality. A series of single hole continuously ranging from k*r>>1 to k*r<

Les premières études dans ce domaine ont examiné l'interaction entre les structures métalliques et les molécules photosensibles et ont prouvé la possibilité de déclencher une photo-polymérisation à l'échelle nanométrique, par le biais des plasmons de surface de ces nanoparticules. Il a été également montré que la nanophotopolymérisation constitue une technique puissante pour l'imagerie du champ proche des nanostructures, évitant ainsi la perturbation de la physique de l'échantillon en apportant une sonde à proximité. Au cours de cette thèse, nous avons été beaucoup plus quantitatifs que nos prédécesseurs dans ce domaine. En irradiant les nanoparticules de métal à leur résonance, nous avons moulé le profil dipolaire du champ électromagnétique par un polymère photo-actif, avec une résolution inédite de 5 nm. Ensuite et par une caractérisation précise des moules polymères, des valeurs précises du facteur d'exaltation et de la profondeur du champ proche de colloïdes d'argent ont été extraites. En outre, nous avons montré notre capacité à avoir une signature spectrale de la résonance plasmon d'une nanoparticule métallique unique directement en champ proche. De plus, nous présentons des cartes de résolution nanométrique de la distribution spatiale de la densité surfacique de charge créée par la discontinuité du champ électrique au niveau d’une interface métal non-résonant/diélectrique. Enfin, ce travail a prouvé que l'approche de nanophotopolymérisation constitue, d’un point de vue fondamental, une opportunité pour étudier la nanophotochimie
While past research has considered the interaction between metal nanoparticles and photo-sensitive molecules, especially the possibility of initiating nanoscale photopolymerization based on the localized surface plasmons of such particles, this PhD dissertation describes the in-depth characterization and optimization of such interactions that result in nanoscale photopolymerization. The present work demonstrates our ability to use the nanophotopolymerization process to quantitatively map with unprecedented resolution, better than 5 nm, both, the near-field of metallic nanoparticles associated with their localized surface plasmons, and the local electric fields resulting from surface charges density at metal/dielectric interfaces. We will emphasize that a precise characterization of the nanoscale molecular mold of the confined electromagnetic field of metal colloids enabled us to quantify the near-field depth and its enhancement factor. Moreover, a near-field spectrum corresponding to the response of localized surface plasmons of a single metal nanoparticle will be assessed. Additionally, we present nanoscale resolution maps of the spatial distribution of the surface charge density created by the electric field dis-continuity at a non-resonant metal/dielectric interface. Furthermore, this work will prove that the nanoscale photopolymerization approach does not only map the near-field of metal nanoparticles, yet it constitutes, from a more fundamental point of view, a unique opportunity to investigate nanophotochemistry

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Bachelors
Sciences
Physics

This thesis investigates the nonlinear optical properties of ZnO and ZnO-Au composite nanostructures. For applications such as photodynamic therapy, it is desirable to use nanoparticles to generate localised UV emission while illuminating them with visible or infrared light. This is possible using nonlinear optical processes such as two photon absorption. Nonlinear optical processes however, are extremely weak, so this work investigates the potential of increasing the efficiency of two photon absorption in ZnO nanoparticles by coupling them to metal nanoparticles. Using new experimental methods, the two photon absorption and resulting UV emission from the nanoparticles are measured.

The objective of this study is to examine core-shell type plasmonic metamaterials aimed at the development of materials with unique electromagnetic properties. The building blocks of metamaterials under study consist of gold as a metal component, and silica and precipitated calcium carbonate (PCC) as the dielectric media. The results of this study demonstrate important applications of the core-shells including scattering suppression, airborne obscurants made of fractal gold shells, photomodification of the fractal structure providing windows of transparency, and plasmonics core-shell with a gain shell as an active device. Plasmonic resonances of the metallic shells depend on their nanostructure and geometry of the core, which can be optimized for the broadband extinction. Significant extinction from the visible to mid-infrared makes fractal shells very attractive as bandpass filters and aerosolized obscurants. In contrast to the planar fractal films, where the absorption and reflection equally contribute to the extinction, the shells' extinction is caused mainly by the absorption. This work shows that the Mie scattering resonance of a silica core with 780 nm diameter at 560 nm is suppressed by 75% and only partially substituted by the absorption in the shell so that the total transmission is noticeably increased. Effective medium theory supports our experiments and indicates that light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate. Broadband extinction in fractal shells allows as well for a laser photoburning of holes in the extinction spectra and consequently windows of transparency in a controlled manner. Au fractal nanostructures grown on PCC flakes provide the highest mass normalized extinction, up to 3 m^2/g, which has been demonstrated in the broad spectral range. In the nanoplasmonic field active devices consist of a Au nanoparticle that acts as a cavity and the dye molecules attached to it via thin silica shell as the active medium. Such kind of devices is considered as a nano-laser or nano-amplifier. The fabricated nanolasers were studied for their photoluminescence kinetic properties. It is shown that the cooperative effects due to the coupling of dye molecules via Au nanoparticle plasmons result in bi-exponential emission decay characteristics in accord with theory predictions. These bi-exponential decays involve a fast superradiant decay, which is followed by a slow subradiant decay. To summarize, this work shows new attractive properties of core-shell nanoparticles. Fractal Au shells on silica cores prove to be a good scattering suppressor and a band pass filter in a broadband spectral range. They can also be used as an obscurant when PCC is used as the core material. Finally, gold nanoparticles coated with silica with dye results in bi-exponential decays.

Les cellules solaires en couches minces permettent de produire de l'énergie à bas-coût et sans émission de gaz à effet de serre. Dans le but de réaliser des dispositifs toujours plus performants, nous étudions l'impact de l'intégration de nanostructures métalliques (NSs) au sein de cellules solaires organiques (CSO). Ces NSs peuvent alors générer des effets diffusifs et des résonances issues de plasmons de surface. A l'aide d'un modèle numérique FDTD, nous démontrons que l'ingénierie plasmonique peut servir à augmenter l'absorption dans le matériau photoactif tout en limitant l'énergie perdue sous forme de chaleur dans les NSs. L'influence de paramètres opto-géométriques de structures associant matériaux organiques et effets plasmoniques est étudiée (diamètre, position des particules dans la couche et période du réseau de particules sphériques). Expérimentalement, des NSs d'argent ont été réalisées par évaporation sous vide puis intégrées dans des couches organiques. Nous avons mesuré une exaltation de l'absorption optique dans la gamme spectrale utile à la photo-conversion. Trois architectures différentes de CSO plasmonique ont été fabriquées et caractérisées par MEB, TEM et ToF-SIMS, puis modélisées, permettant d'identifier des verrous technologiques et de proposer des pistes d'amélioration. Nous avons aussi intégré des NSs au sein d'un empilement transparent et conducteur de type oxyde/métal/oxyde, dans le but de remplacer l'électrode classique en oxyde d'indium et d'étain d'une CSO. Le rôle de chaque couche de l'empilement sur le comportement optique de l'électrode est discuté. Les épaisseurs des couches d'une électrode de type ZnO/Ag/ZnO ont été optimisées.

Les hologrammes de sécurité composés de structures sub-diffractives se développent à la croisée des besoins de protection des documents sensibles et de l'avènement des technologies modernes de reprographie servant la falsification grand public. L’objectif de ce travail est de concevoir des dispositifs de sécurité dont les effets visuels, basés sur la génération de couleurs structurales, soient facilement authentifiables, difficiles à contrefaire et compatibles avec la production de films holographiques. Nous étudions la réponse plasmonique de réseaux métalliques dissymétriques uni et bi-dimensionnels, fabriqués par lithographie interférentielle et transposés à grande échelle sur film polymère par des procédés de réplication roll-to-roll. L’analyse approfondie des mécanismes résonants observés dans les réseaux hybrides métal-diélectrique ouvre des perspectives nouvelles de conception et d’amélioration de la qualité des couleurs perçues. Les résultats montrent que l'utilisation de processus d'optimisation modernes, avant la fabrication, constitue une approche efficace permettant d'adapter et d'optimiser la réponse résonante des réseaux de diffraction. Nous démontrons que l’approche multi-objectifs surpasse les stratégies mono-objectif ouvrant la possibilité d'augmenter la complexité des structures employées pour la reproduction des couleurs. Enfin, nous montrons que le recours à des outils d’intelligence artificielle constitue une alternative efficace aux méthodes électromagnétiques traditionnelles
Security holograms based on sub-wavelength gratings (SWGs) are increasingly used not only to protect sensitive documents, but also to combat against the reprographic technologies used in counterfeiting.The aim of the present work is to design optical security devices to produce visual and chromatic effects, based on the generation of structural colors, easily recognizable but difficult to counterfeit and compatible with high-tech foil production. To this end, we study the optical response of one and two-dimensional asymmetric SWGs fabricated by laser interferometric lithography and scaled up to larger scales on polymer film using roll-to-roll replication processes. The in-depth physical analysis of the resonance mechanisms generated by metallic and hybrid metal-dielectric SWGs allows to understand and tailor their chromatic response. We also demonstrate that hybrid SWGs open new design perspectives and enhance the quality of the perceived colors. The research evidence presented in this contribution clearly shows that the use of modern optimization tools, prior to fabrication, provides an efficient way to tailor and to optimize the resonant response of diffraction gratings. We demonstrate that the multi-objective approach outperforms single-objective strategies and opens the possibility of increasing the complexity of the diffractive structures used for color reproduction. We emphasize that Artificial Intelligence tools constitute an efficient alternative to the traditional time-consuming electromagnetic methods

Orientadores: Daniela Zanchet, Jose Antonio Brum
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Nanopartículas de metais nobres têm atraído uma renovada atenção nos últimos anos devido às novas aplicações científicas e tecnológicas explorando suas propriedades ópticas únicas. No regime nanométrico, é bem conhecido que a resposta óptica de metais, associada aos plásmons de superfície, depende fortemente do tamanho e também da forma. De fato, grande parte das aplicações ópticas de nanopartículas de ouro e prata baseia-se na exploração dos efeitos de forma. Porém, apesar dos esforços realizados, os processos que levam à formação de morfologias anisotrópicas ainda não são bem compreendidos e a formulação de um mecanismo geral ainda é um desafio. Nesse trabalho, foram abordados os mecanismos de formação e crescimento de nanoprismas triangulares de prata produzidos por métodos de síntese coloidal. Uma combinação de diferentes técnicas experimentais foi utilizada para estudar diversos aspectos da síntese fotoquímica, dentre eles, a evolução morfológica, a cinética da reação e a estrutura cristalina das nanopartículas. As sólidas evidências experimentais obtidas associadas a outras observações da literatura foram utilizadas na formulação de um modelo fenomenológico para explicar a formação e crescimento dos nanoprismas de prata em métodos fotoquímicos. Esse modelo baseia-se na influência dos defeitos cristalográficos, que induzem a formação dos nanoprismas nos momentos iniciais da síntese, e na excitação de plásmons de superfície, que ocorre em estágios avançados, sendo responsável pela definição do tamanho final dos nanoprismas. Adicionalmente, cálculos teóricos indicaram que aspectos energéticos podem ter um papel ativo nesse sistema, favorecendo o crescimento dos nanoprismas em relação às nanopartículas esféricas durante os estágios iniciais da síntese. Finalmente, os nanoprismas triangulares de prata produzidos foram aplicados ao estudo de efeitos de intensificação do espalhamento Raman de moléculas. Medidas espectroscópicas de moléculas depositadas na superfície de nanoprismas com diferentes tamanhos foram realizadas e a comparação quantitativa dos resultados indicou a presença de um tamanho ótimo, que é determinado por processos de perda de energia dos plásmons de superfície
Abstract: Noble metal nanoparticles have attracted a recent renewed interest due to the new scientific and technological applications exploiting their unique optical properties. At nanometric scale, it is well known that the optical response of metals, related to the excitation of surface plasmons, strongly depends not only on the size of the particles but also on their shape. Several methodologies to produce silver and gold nanoparticles with different shapes are available in the literature. However, notwithstanding the efforts that have been made, the process that lead to the formation of anisotropic morphologies has not been fully understood yet and a general mechanism is still a challenge. In this work, we address the formation and growth mechanisms of silver triangular nanoprisms produced by photochemical methods. A set of characterization tools was used to study different aspects of the photochemical synthesis, namely, the morphological evolution, the reaction kinetics and the crystalline structure of the nanoprisms. The solid experimental evidences obtained here were used to build a phenomenological model that explains the formation and growth of silver triangular nanoplates in photochemical methods. This model was based on the influence of crystallographic defects, which induce the formation of the nanoprismas in the initial stages of the synthesis, and on the excitation of surface plasmons, which occurs in advanced stages and it is responsible for the definition of the final size of the nanoprismas. Additionally, theoretical calculations indicate that energetics might play an important role in this system, favoring the growth of nanoprismas relative to spheres. Finally, the silver triangular nanoprisms were used to study enhancement effects in the Raman scattering of molecules. We performed spectroscopic measurements for nanoplates with different sizes and the quantitative comparison of the curves indicated the existence of an optimum size that is dictated by surface plasmon energy losses
Doutorado
Física da Matéria Condensada
Doutor em Ciências

La génération de seconde harmonique magnétique (mSHG pour magnetic Second Harmonic Generation) est un phénomène physique très sensible apparaissant grâce aux brisures de symétrie aux niveaux des surfaces et interfaces des structures métalliques magnétiques. Elle constitue donc un outil puissant pour explorer ce type d'interfaces et des nanostructures. Dans ce travail, nous nous intéressons aux couplages et interactions entre la mSHG et les ondes électromagnétiques pouvant se propager en surface des matériaux. Un intérêt spécifique est porté sur l’ excitation de (i) plasmon polaritons de surfaces (SPP) dans des films métalliques en structures multicouches, (ii) d'anomalies de diffraction (dîtes de Wood) dans des nanostructures métalliques périodiques. Pour étudier l'influence de l'excitation linéaire et non-linéaire des SPP sur la mSHG, l'intensité du signal réfléchi par génération de seconde harmonique (SH) et le contraste magnétique lié à ce signal ont été mesurés par la technique de l'effet Kerr magnéto-optique transverse (MOKE) en fonction de l'angle d'incidence. Via l'utilisation de sources lasers femtosecondes émettant dans le proche infrarouge, domaine spectral où les variations de la dispersion des SPP et du coefficient d'amortissement sont significatives, nous avons pu distinguer les différentes contributions linéaires et non-linéaires aux processus d'excitation. Ce travail de thèse a ainsi permis de montrer que l’accord de phase entre la mSHG et les ondes électromagnétiques de surface peuvent contribuer très efficacement à l'augmentation des signaux SH et de contraste magnétique associé
Owing to surface and interface sensitivity, the magnetic Second Harmonic Generation (mSHG) represents a useful tool to probe magnetic interfaces and nanostructures. This work investigates the coupling and interaction of the mSHG with electromagnetic waves propagating along the surface. Two types of surface waves have been studied: (i) surface plasmon polaritons (SPP) at surfaces of metallic thin films and multilayers, and (ii) the diffraction anomaly at the surface of periodically arranged metallic nanostructures. To study influence of linear and nonlinear excitation of surface waves on the mSHG, the reflected second harmonic (SH) intensity and the magnetic SH contrast in the transverse magneto-optical geometry were measured as a function of the angle of incidence. The use of different femtosecond light sources in the near-infrared optical range, where the SPP dispersion and damping exhibit significant variations, made it possible to disentangle linear and nonlinear contributions to the excitation of surface waves. In this thesis, it is proven that phase-matching of the mSHG and surface electromagnetic waves can lead to the enhancement of both the SH yield and the nonlinear magneto-optical signal. These results are important for controlling of the nonlinear magneto-optical response and could impact the development of magnetic storage devices, label-free biosensors and nonlinear magneto-optical switches

通過與具有表面等離子體激元特性的金屬納米晶複合，半導體納米材料的捕光能力可以得到很大地提高。銀、金納米晶因其獨特的表面等離子體特性，已被廣泛應用于半導體複合物的製備。其中通過沉積或者粘合方式得到的複合物存在一定弊端，比如：金屬納米晶暴露在實驗環境中，導致其團聚、變形、脫落、或者長大，使原有的獨特表面等離子特性改變或消失。核/殼結構納米材料可以有效地避免以上問題，因而表現出優越的光活性。爲了進一步拓寬金屬/半導體核/殼結構在光能捕獲方面的應用，我们需要深入理解製備過程中表面等離子體激元特性及材料結構的變化、設計合成新的納米材料。在這篇畢業論文中，我研究了在製備Ag/Ag2S核/殼結構過程中的表面等離子體特性及材料結構的變化，制備了Au/TiO₂核/殼結構，并對他們的應用及表面等離子體共振激元特性進行了研究。
理解硫化過程有助於更好的控制其表面等離子體特性和結構組成。因此，我分別從實驗和數值模擬兩方面研究了銀納米立方塊在硫化過程中表面等離子體特性及其相應Ag/Ag₂S 核/殼的組成及結構的變化。硫化反應分別在溶液及單顆粒環境下進行。同時，我們應用數值模擬計算揭示硫化過程中表面等離子體特性及模式變化。實驗和數值計算均表明硫化反應首先發生在銀納米立方塊的棱角和頂點。隨著反應的進行，銀立方塊被逐步鈍化為球狀銀納米顆粒。與此同時，納米立方結構的尺寸也隨之小幅增加。
二氧化鈦是一種重要的被應用於光能捕獲的半導體納米材料。因其低毒性、生物兼容性、化學及熱穩定性、耐光腐蝕性以及資源豐富等特點，TiO₂ 已經被廣泛研究。但是TiO₂僅在紫外光區具有光化學活性，這大大限制了其在光能捕獲方面的應用。儘管Au/TiO₂核/殼結構複合物可以提高TiO₂在可見區的光催化活性，但是對於該核/殼結構的合成鮮為報導，而且已報導的工作也是限制在以金納米球作為核層。與金納米球相比，金納米棒具有更引人關注的表面等離子體特性，例如金納米棒具有更高的電場增強，而且金棒的縱向共振波長可以從可見區調控到近紅外區。因此金納米棒/二氧化鈦核/殼結構可以更有效的提高二氧化鈦的光捕獲能力。在此論文中，我發展了一種合成Au/TiO₂核/殼結構的方法，并研究其在光能捕獲方面的應用。在該方法中，我選擇三價鈦作為鈦源，可控合成了Au/TiO₂ 核/殼結構。通過對核的尺寸及殼層厚度的調節，實現了對核/殼結構的共振波長的調控。另外這種方法也適用于其他單組份或者雙組份的鉑、鈀、金納米晶。爲了驗證在光能捕獲方面的應用潛能，Au/TiO₂核/殼結構納米材料被作為散射層而應用於染料敏化太陽能電池中，結果發現這種電池具有較高光電轉化效率。另外，我們還研究了表面等離子體共振激元增強下的活性氧化物的生成。再者，具有較高介電常數的二氧化鈦殼層可以與金納米晶核耦合產生法諾共振效應。結果表明金納米棒的橫向、縱向共振峰均能和殼層材料發生共振耦合而產生對應的法諾效應。納米棒的縱向共振峰的可調性實現了對應的法諾共振峰的可調性。同時，包覆二氧化鈦殼層后，金納米棒的橫向共振模式被大幅放大。
本論文的研究有利於人們了解金屬/半導體納米結構的設計及應用。硫化過程中表面等離子體共振激元特性及結構變化的研究，對具有特定組分及共振特性的複合物的設計合成具有指導意義。對貴金屬/半導體核殼結構製備、共振特性及應用的研究也擴展了其在光能捕獲方面的應用。
Over the past several years, integration of metal nanocrystals that can support localized surface plasmon has been demonstrated as one of the most promising methods to the improvement of the light-harvesting efficiency of semiconductors. Ag and Au nanocrystals have been extensively hybridized with semiconductors by either deposition or anchoring. However, metal nanocrystals tend to aggregate, reshape, detach, or grow into large nanocrystals, leading to a loss of the unique properties seen in the original nanocrystals. Fortunately, core/shell nanostructures, circumventing the aforementioned problems, have been demonstrated to exhibit superior photoactivities.To further improve the light-harvesting applications of (plasmonic metalcore)/(semiconductor shell) nanostructures, it is vital to understand the plasmonic and structural evolutions during the preparation processes, design novel hybridnanostructures, and improve their light-harvesting performances. In this thesis, I therefore studied the plasmonic and structural evolutions during the formation of (Ag core)/(Ag₂S shell) nanostructures. Moreover, I also prepared (noble metal core)/(TiO₂shell) nanostructures and investigated their plasmonic properties and photon-harvesting applications.
Clear understanding of the sulfidation process can enable fine control of the plasmonic properties as well as the structural composition of Ag/Ag₂S nanomaterials.Therefore, I investigated the plasmonic and structural variations during the sulfidation process of Ag nanocubes both experimentally and numerically. The sulfidation reactions were carried out at both the ensemble and single-particle levels.Electrodynamic simulations were also employed to study the variations of theplasmonic properties and plasmon modes. Both experiment and simulation results revealed that sulfidation initiates at the vertices of Ag nanocubes. Ag nanocubes arethen gradually truncated and each nanocube becomes a nanosphere eventually. The cubic shape is maintained throughout the sulfidation process, with the edge lengthii being increased gradually.
TiO₂ is one of the most important semiconductors that are employed inlight-harvesting applications. It has been extensively studied for a variety of applications by virtue of its low toxicity, biological compatibility, chemical and thermal stability, resistance to photocorrosion, and relative abundance. However, the photocatalytic activity of TiO₂ is limited to the UV region because of its wide bandgap, which limits its applications in light harvesting. Although (Au core)/(TiO₂ shell)nanostructures can improve the photocatalytic activities of TiO₂ in visible light, it hasonly been demonstrated in a few experiments and has been limited with Au nanospheres. Compared with Au nanospheres, Au nanorods offer more attractive plasmonic features, including stronger electric field enhancements and synthetically tunable longitudinal plasmon wavelengths over the visible to near-infrared region. The coating of Au nanorod therefore can largely improve light harvesting capabilityof TiO₂. In this thesis, I developed a facile and versatile method for the preparation of(Au nanocrystal core)/(TiO₂ shell) nanostructures by using a Ti(III) compound as thetitania precursor. By employing Au nanorods with different sizes and varying the shellthickness, the plasmonic bands of the core/shell nanostructures can be tailored. TiO₂can also be grown on other monometallic and bimetallic Pd, Pt, Au nanocrystals. As aproof-of-concept application, (Au nanorod core)/(TiO₂ shell) nanostructures wereutilized in dye-sensitized solar cells to function as a scattering layer. The resultantsolar cells exhibited higher power conversion efficiencies with a thinner thickness compared to the traditional TiO₂ solar cells. In addition, I also examined the property of plasmon-enhanced reactive oxygen species generation. Moreover, the TiO₂ shell with a high refractive index can efficiently couple with the plasmon resonance modesof the Au nanorod core, leading to Fano resonances. Fano resonances for both the transverse and longitudinal plasmon modes were simultaneously observed. The longitudinal Fano resonance is tunable by changing the plasmon energy of thenanorod core. In addition, coating with TiO₂ intensifies the transverse plasmon modeof the Au nanorod core.
I believe that my research study will be very helpful for the design and applications of metal/semiconductor nanostructures. The full understanding of the plasmonic and structural evolutions during the preparation processes will be useful for designing metal/semiconductor hybrid nanomaterials with desired compositions and plasmonic properties. The efforts towards the investigations of the preparation, plasmonic properties, and applications of (noble metal core)/(semiconductor shell) nanostructures are important for widening their light-harvesting applications.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Fang, Caihong = 具有表面等離子體激元特性的金屬/半導體核/殼納米結構 / 房彩虹.
Thesis (Ph.D.) Chinese University of Hong Kong, 2014.
Includes bibliographical references.
Abstracts also in Chinese.
Fang, Caihong = Ju you biao mian deng li zi ti ji yuan te xing de jin shu/ban dao ti he/qiao na mi jie gou / Fang Caihong.

Plasmonic properties and the related novel applications are studied on various types of metallic nano-structures in one, two, or three dimensions. For 1D nanostructure, the motion of free electrons in a metal-film with nanoscale thickness is confined in its normal dimension and free in the other two. Describing the free-electron motion at metal-dielectric surfaces, surface plasmon polariton (SPP) is an elementary excitation of such motions and is well known. When further perforated with periodic array of holes, periodicity will introduce degeneracy, incur energy-level splitting, and facilitate the coupling between free-space photon and SPP. We applied this concept to achieve a plasmonic perfect absorber. The experimentally observed reflection dip splitting is qualitatively explained by a perturbation theory based on the above concept. If confined in 2D, the nanostructures become nanowires that intrigue a broad range of research interests. We performed various studies on the resonance and propagation of metal nanowires with different materials, cross-sectional shapes and form factors, in passive or active medium, in support of corresponding experimental works. Finite- Difference Time-Domain (FDTD) simulations show that simulated results agrees well with experiments and makes fundamental mode analysis possible. Confined in 3D, the electron motions in a single metal nanoparticle (NP) leads to localized surface plasmon resonance (LSPR) that enables another novel and important application: plasmon-heating. By exciting the LSPR of a gold particle embedded in liquid, the excited plasmon will decay into heat in the particle and will heat up the surrounding liquid eventually. With sufficient exciting optical intensity, the heat transfer from NP to liquid will undergo an explosive process and make a vapor envelop: nanobubble. We characterized the size, pressure and temperature of the nanobubble by a simple model relying on Mie calculations and continuous medium assumption. A novel effective medium method is also developed to replace the role of Mie calculations. The characterized temperature is in excellent agreement with that by Raman scattering. If fabricated in an ordered cluster, NPs exhibit double-resonance features and the double Fano-resonant structure is demonstrated to most enhance the four-wave mixing efficiency.

博士
國立清華大學
化學系
96
A rapid electrochemical replication technique is developed to fabricate ultra-smooth aluminum foils by exploiting readily available large-scale smooth silicon wafer as the master. Since the adhesion of aluminum on silicon depends on the time of surface pretreatment in water, it is possible to either detach the replicated aluminum from the silicon master without damaging the replicated aluminum and master or integrate the aluminum film to the silicon substrate. Replicated ultra-smooth aluminum foils are used for the growth of both self-organized and lithographically guided long-range ordered arrays of anodic alumina nanochannels without any polishing pre-treatment. An electrochemical nanomolding technique for the large-scale and rapid fabrication of metallic nanostructures has been demonstrated taking advantage of the above method. Here, Nanostructures with features down to 10 nm has been fabricated by fast electrochemical deposition of aluminum on nanopatterned silicon mold followed by mechanical peeling off the aluminum foil from the mold. This high fidelity, non-destructive technique can exploit the mold for repeated use in mass production of nanostructures and also opens up new possibilities in the field of nano-scale design and fabrication. Finally, a large-scale guiding technique has been presented to fabricate long-range order anodic alumina nanochannel arrays based on electrochemical nanomolding. Optical properties of metal nanostructures grown inside the anodic alumina nanochannels have been studied thoroughly. Electromagnetic interactions of the near-, intermediate- and far-zone in an array of metallic nanoparticles are responsible for many of its anomalous plasmonic properties. While this so-called plasmonic coupling has become a focus of many researches lately, its interaction mechanisms still remain concealed, mainly due to the lack of spectroscopic observations from precisely fabricated samples as well as analytical interpretations. Here, I present light scattering spectra of arrays of silver nanoparticles with gaps of sub-10 nm precision, which are fabricated based on the unique self-organizing property of porous alumina templates. I show that their near- and immediate-zone interactions are manifested in the spectra through analytical formulae derived from first principle. The findings provide a profound base to predict unexplored plasmonic properties such as the relationship between the Q factor of the arrays and their structural characteristics. The results are instrumental in the development of extended plasmonic nanostructures, such as surface-enhanced Raman substrates. In a very lucid way, I have also studied the particle plasmon resonace behavior using light scattering spectroscopy in a binary dielectric media where silver nano-rods are embedded partially in Anodic Aluminum Oxide (AAO) matrix and in air. Here I did a systematic experimental study under a controlled variation of the degree of embedding of nano-rods in AAO matrix. I used Finite Difference Time Domain (FDTD) method to calculate the nature of the silver nano-rod resonance at the experimental conditions. The results have been interpreted based on the Drude model.

碩士
國立陽明大學
生醫光電研究所
105
In recent years, various SERS substrates have been developed and applied to the detection of molecules, but fabricating a highly reproductive, simple, and cost effective SERS nanostructure with a significant Raman enhancement is still challenging. In this study, we successfully synthesized two different SERS structures. For first structure, we applied a self-assembly method to immobilize the gold nanoparticles (AuNPs) on a silica beads to form the core-satellite nanostructure, which contained 3D SERS hot spots. Then, we covered the immobilized AuNPs with silver shells to regulate the inter-particle distance to optimize the SERS effects. Furthermore, we accumulated the particles with the nanostructure on filter paper as SERS substrates for SERS detection of malachite green, and then we successfully did SERS detection of malachite green with detection limit of 50 fM. Moreover, we applied fluidic system to detect two different molecules; malachite green and sodium thiocyanate. For the other nanostructure, we used dealloying process to acquire highly porous Au−Ag alloy nanoparticles covered with ultrathin silica shells. These Au−Ag alloy nanoparticles contained more SERS hot spots, and the nanoparticles were more stable and clean on surface in solutions. For the measurement, we accumulated the porous nanoparticles on filter paper as SERS substrates, and we performed SERS detection of adenine.

Metal nanostructures, with their extraordinary optical properties, have attracted great attention in recent years. Subwavelength-scaled metal elements, without involving array effects, have the unique ability to confine or route light at the nano-scale. In this thesis, we provide three topics relating to the manipulation of light using metal nanostructures. We first present a theory to solve the end-face reflection of a subwavelength metal stripe, which is beneficial to the design of optical resonator antennas. Subsequently, we take the advantage of the destructive interference among triple nano-slits to sharpen the focus beam in the near-field at near-infrared wavelengths, which is of interest to the study of near-field optical phase imaging and lithography. Lastly, we demonstrate a rectangular subwavelength aperture quad to convert linearly polarized radiation to a radially polarized beam, which is useful to create a deep-subwavelength focus and for optical trapping.
Graduate

I believe that my research work on the plasmonic spectroscopy of metallic nanostructures has provided an in-depth fundamental understanding of the localized surface plasmon resonance and will have a number of implications for the applications of metallic nanostructures in optics, optoelectronics, and biotechnology.
I will first describe my studies on the plasmonic properties of metallic nanostructures. Specific approaches of modifying the sizes and shapes of Au nanorods have been developed for tailoring their plasmonic properties, including surface plasmon wavelength, absorption, scattering, and extinction cross sections. Single-particle dark-field imaging and spectroscopy have proved that the scattering intensity of overgrown nanorods is larger than that of shortened nanorods from the same starting nanorods. Finite-difference time-domain (FDTD) calculations further show that the scattering-to-extinction ratio increases linearly as a function of the diameter of Au nanorods with a fixed aspect ratio. To obtain a deep understanding on the shape dependence of the localized surface plasmon resonance, I have emplyed FDTD on both Au nanorods and Au nanobipyramids. The results show that, when excited at their LSP wavelengths, Au nanobipyramids exhibit a maximal electric field intensity enhancement that is 3--6 times that of Au nanorods. Au nanorods have been further assembled into chains (end-to-end) and stacks (side-by-side). FDTD calculations have been performed on both Au nanorod chains and stacks with varying gap distances to obtain the dependence of the plasmon shift on the gap distance, which is then used as a plasmonic ruler to estimate the gap distance between assembled nanorods. Moreover, dye--Au nanorod hybrid nanostructures have also been successfully fabricated for the study of the coupling between the transition dipole resonance and the plasmonic resonance. The coupling-induced plasmon shift is found to be strongly dependent on molecular properties, the dye concentration in solutions, and the spacer thickness between dye molecules and the surface of Au nanorods. The coupling can be switched off by means of laser-induced photodecomposition of dye molecules.
Next, I will present my studies on the applications of metallic nanostructures. A SERS substrate has been constructed by assembling silver nanoparticles along silica nanofibers. The enhancement factors are found to be 2 x 10 5 for 4-mercaptobenzoic acid and 4-mercaptophenol, and 7 x 10 7 for rhodamine B isothiocyanate. A novel plasmonic optical fiber device has further been fabricated to detect small changes in the local dielectric environment. For individual Au nanorods, the index sensitivity and figure of merit (FOM) are found to be linearly dependent on the longitudinal plasmon resonance wavelength and reach 200 nm/RIU and 3.8, respectively. For nanorod ensembles, the index sensitivity and FOM of the longitudinal plasmon resonance are found to be 138 nm/RIU and 1.2, respectively.
The study of the plasmonic spectroscopy of metallic nanostructures is of great interest in nanoscale optics and photonics. Metallic nanostructures exhibit rich optical and electrical properties due to their localized surface plasmons (LSPs, collective charge density oscillations that are confined to metallic nanostructures). They can be widely used in a variety of application areas, such as surface-enhanced Raman scattering (SERS), plasmonic sensing, and metal enhanced fluorescence (MEF). In this thesis, a systematic study on the plasmonic spectroscopy of metallic nanostructures has been presented, both theoretically and experimentally.
Ni, Wei hai = 金屬納米結構的等離子體光譜 / 倪衛海.
Adviser: Jianfang Wang.
Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3580.
Thesis (Ph.D.)--Chinese University of Hong Kong, 2008.
Includes bibliographical references (leaves 135-154).
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstracts in English and Chinese.
School code: 1307.
Ni, Wei hai = Jin shu na mi jie gou de deng li zi ti guang pu / Ni Weihai.

This thesis reports experimental study of surface plasmon excitations--localized surface plasmons (SPs) and propagating surface plasmon polaritons (SPPs)--and their interactions with dipole emitters CdSe/ZnS (core/shell) nanocrystals. This study will contribute to potential applications of SP-enhanced fluorescent sensors and fast SPP-waveguided electronics. Our angle-dependent, polarization-related extinction spectra show that SPs in 2D nanodisk arrays are not only related to the intrinsic properties of individual nanoparticles, but also dependent on the dipole-dipole interactions among them. SP resonance peaks are red-shifted with increasing incidence angle. As the nanodisk center-to-center distance decreases within sub-wavelengths, coupling to waveguide modes and diffracted evanescent wave modifies the transmission. The out-of-disk-plane dipole surface plasmon resonance is used to couple to nanocrystals and to test the conventional assumption that dipole emission rates are homogeneous in time-resolved photoluminescence (PL) measurements of ensemble samples. Our new finding is that the spontaneous emission rate of dipole emitters deposited on a 2D gold nanodisk array depends on the detection angle and polarization. At the band-edge emission wavelength of nanocrystals, the out-of-incidence-plane, s-polarized PL measurements are detection angle-independent, and the in-plane-of-incidence, p-polarized PL measurements show an additional decay caused by SP-enhanced emission. In planar gold films we perform reflectivity measurements in the Kretschmann-Raether (KR) configuration and determine the frequency- and momentum-dependent SPP resonance. In hybrid samples of planar gold films and semiconductor nanocrystals, the coupling between the dipole emitters and SPPs can generate SPP emission through an inverted KR hemisphere prism. For the first time we observed a decay rate increase of SPP emission as a function of nanocrystals emission wavelength in gold films with silica separation layers, as compared to free-space dipole emission detected in the front of the metal surface. Simulations based on the theory of Ford and Weber show that this increase is primarily due to energy transfer of perpendicular dipoles into lossy surface waves. Our results of polarization-selective and angle-dependent SP-enhanced emission can be used to optimize and tune the performance of light sources or fluorescent sensors. The study of SPP emission will lead to efficient energy transfer in fast plasmonic device applications. Keywords: Au, CdSe/ZnS nanocrystals, SP, SPP emission, nanodisk arrays, time-resolved single photon counting.

Recent scientific progress has resulted in the development of sophisticated hybrid nanostructures composed of semiconductor and metal nanoparticles. These hybrid structures promise to produce a new generation of nanoscale optoelectronic devices that combine the best attributes of each component material. The optical response of metal nanostructures is dominated by surface plasmon resonances which create large local electromagnetic field enhancements. When coupled to surrounding semiconductor components, the enhanced local fields result in strong absorption/emission, optical gain, and nonlinear effects. Although hybrid nanostructures are poised to be utilized in a variety of applications, serious hurdles for the design of new devices remain. These difficulties largely result from a poor understanding of how the structural components interact at the nanoscale. The interactions strongly depend on the exact composition and geometry of the structure, and therefore, a quantitative comparison between theory and experiment is often difficult to achieve. Colloidal semiconductor quantum dots are strong candidates for integration with metal nanostructures because they have a variety of desirable optical properties, such as tunable emission and long term photostability. However, one potential drawback of colloidal quantum dots is the intermittency in their fluorescence (commonly referred to as “blinking”). Blinking was first observed over a decade ago, yet there is still no complete theory to explain why it occurs. In spite of the lack of a full theoretical explanation, multiple methods have been used to reduce blinking behavior, including modifying quantum dot interfaces and coupling quantum dots with metal nanostructures. This thesis focuses on studying the coupling between colloidal quantum dots and metal nanoparticles in simple model systems. Atomic force microscopy nanomanipulation is used to assemble the hybrid structures with a controlled geometry. The experimental studies report for the first time the modified fluorescence decay, emission intensity, and blinking of a single quantum dot coupled to a single Au nanoparticle. Since the geometry of the structure is known, these studies provide reliable information on the interparticle coupling, and quantitative experimental results are shown to be consistent with classical electrodynamic theories.
text

Plasmonics provides many opportunities of sensing and detection since it combines the nanoscale spatial confinement and the optical temporal resolution. The wireless nature of photonic investigation, moreover, is very desirable for biomedical applications. Plasmonic metals, however, are difficult to pattern with great nanoscopic precision, and traditional approaches were time-consuming, non-scalable, stochastically-manufactured, or highly-limiting in the pattern designs. In this work, wafer-scalable nanofabrication methods are presented for various plasmonic structures for biomedical sensing applications. The fabrication steps have ready counterparts in commercial semiconductor foundries and therefore can be directly applied for mass production.

The fabrication and measurement of extraordinary transmission (EOT) are discussed in Chapter 2. Fabrication options are available for substrates like silicon-on-sapphire and silicon-on-glass, so that the devices can be mechanically robust for user-friendliness. The metal layer can also be varied for EOT applications in different ranges of wavelengths. The EOT nanostructures can be fabricated to be polarization-sensitive, and the concept of fluorescence-based EOT assays is demonstrated.

The fabrication and applications of surface-enhanced Raman spectroscopy (SERS) are then discussed. With a hybrid approach, the top-down designing defines uniform SERS nanostructures on a chip, while the bottom-up process of thermal reflow increases the fabrication precision beyond the lithography resolution limit. Based on the thiophenol study, an enhancement factor greater than 1010 can be achieved. The first Raman spectrum of tracheal cytotoxin is demonstrated without any special sample preparation, and thrombin binding could be easily resolved through chip functionalization. The binding dynamics of ethyl mercaptan, which is similar to the highly toxic gas of hydrogen sulfide, can be detected with a good resolution in time at a low concentration.

With a few more steps of fabrication, the plasmonic structures can be integrated into systems that do not call for laboratory infrastructures. A built-in micro-channel on a chip can make the device useful without dedicated support of a microscope or additional microfluidic structures. The nanostructures can also be transferred onto flexible substrates for better conformity onto various surfaces. Finally, the SERS structures can be transferred onto a fiber tip for in-field or through-the-needle applications, especially when combined with a portable Raman-scope.

By treating free electrons in metallic nanostructures as incompressible and irrotational fluid, Plasmon hybridization (PH) method can be used as a very useful tool in interpolating the electric magnetic behaviors of complex metallic nanostructures. Using PH theory and Finite Element Method (FENI), we theoretically investigated the optical properties of some complex nanostructrus including coupled nanoparticle aggregates and nanowires. We investigated the plasmonic properties of a symmetric silver sphere heptamer and showed that the extinction spectrum exhibited a narrow Fano resonance. Using the plasmon hybridization approach and group theory we showed that this Fano resonance is caused by the interference of two bonding dipolar subradiant and superradiant plasmon modes of E1u symmetry. We investigate the effect of structural symmetry breaking and show that the energy and shape of the Fano resonance can be tuned over a broad wavelength range. We show that the wavelength of the Fano resonance depends very sensitively on the dielectric permittivity of the surrounding media. Besides heptamer, we also used plasmon hybridization method and finite element method to investigate the plasmonic properties of silver or gold nano spherical clusters. For symmetric clusters, we show how group theory can be used to identify the microscopic nature of the plasmon resonances. For larger clusters, we show that narrow Fano resonances are frequently present in their optical spectra. As an example of asymmetric clusters, we demonstrate that clusters of four identical spherical particles support strong Fano-like interference. This feature is highly sensitive to the polarization of the incident electric field due to orientation-dependent coupling between particles in the cluster. Nanowire plasmons can be launched by illumination at one terminus of the nanowire and emission can be detected at the other end of the wire. With PH theory we can predict how the polarization of the emitted light depends on the polarization of the incident light. Depending on termination shape, a nanowire can serve as either a polarization-maintaining waveguide, or as a polarization-rotating, nanoscale half-wave plate. We also investigated how the properties of a nearby substrate modify the excitation and propagation of plasmons in subwavelength silver wires.

Vertically aligned nanocomposite (VAN) thin films are a promising thin-film platform that allows the combination of a highly desired material with another complementary oxide. Traditionally, VANs have been limited to combining an oxide with another oxide which has shown a wide range of functionality, and, by adjusting the different growth parameters, it has led to the tuning of their physical properties. While VANs have already shown to be an effective platform with immense potential, further enhancement of physical properties can be performed by replacing one of the oxides with a metal forming metal-oxide VANs.

In this dissertation, by the inclusion of the 3d transition metals, e.g., Fe and Co, into various oxide matrices, such as La_0.5Sr_0.5FeO₃, BaZrO₃, and BaTiO₃, strong, highly anisotropic, ferromagnetic properties have been achieved. By varying the growth parameters, tunable physical properties, mainly coercivity and anisotropic ratio, have been demonstrated. Furthermore, in the case of Co-BaZrO₃, a multi-layer stack has been successfully grown and demonstrated a tailorable magnetoresistance. Additionally, a novel system by combining Fe pillars into a BaTiO₃ matrix has been demonstrated. This new system allows for the combination of the room temperature Fe ferromagnetic properties with the ferroelectric properties of BaTiO₃, allowing for coupling between the two with coercivity tuning and tailorable ferromagnetic properties.

Lastly, it has been shown a possible framework by adding additional metals into the existing metal-oxide VAN platform. By adding the third phase, another metal, it opens up a new avenue to induce additional functionality while creating a method to introduce coupling between the different metals and physical properties.

Investigation of laser-matter interaction has been an important research topic which is closely related to applications in various fields including industry, military, electronics, photonics, etc. With the advent of ultrafast transmission electron microscope (UTEM), in situ investigation of the interaction between pulsed laser and nanostructured materials becomes accessible, with unprecedented spatial and temporal resolution. Here, we studied two categories of materials with the help of UTEM, namely, energetic materials and plasmonic nanostructures. The results demonstrate that UTEM provides a novel and convenient way for the investigation the structural and morphological change of energetic materials under external stimuli at nanoscale. Also, UTEM makes it possible to visualize the light-induced welding between plasmonic nanostructures at real time, which helps to reveal more details about the mechanisms involved. Furthermore, we studied the formation of some novel structures by combing different gold and silver nanostructure.

Tese no âmbito do Doutoramento em Engenharia Electrotécnica e de Computadores, ramo de especialização em Telecomunicações, orientada pelos Professores Doutores Mário Gonçalo Mestre Veríssimo Silveirinha e Tiago André Nogueira Morgado e apresentada no Departamento de Engenharia Electrotécnica e de Computadores da Faculdade de Ciências e Tecnologia da Universidade de Coimbra.
Nanophotonics is a field of research dedicated to study the interactions of nanosized-objects with light. One of the goals of nanophotonics is to enable the miniaturization of optical components at a competitive scale with microelectronics. There are several rewards in using light based technologies, such as building photonic circuits that are not only smaller but faster and more efficient than the electronic counterparts, new solar cells that have enhanced energy absorption, nano-optical sensors able to detect ultralow concentrations of molecules in chemical solutions, amongst many others. My work aims to contribute to this field of research by exploring new mechanisms to accomplish an efficient spatial confinement of light. This thesis is devoted to the analytical and numerical study of three different ways to confine light in the nanoscale. First, we investigate light trapping in open plasmonic resonators (metaatoms) with different shapes. It is found that in some conditions complexshaped dielectric cavities may support discrete light states screened by volume plasmons that in the limit of a vanishing material loss have an infinite lifetime. The embedded eigenstates can be efficiently pumped with a plane wave excitation when the meta-atom core has a nonlinear response, such that the trapped light energy is precisely quantized. Then, we investigate how the spatial dispersion effects, e.g., caused by the electron-electron interactions in a metal, affect these trapped eigenstates in three-dimensional open plasmonic resonators. Heuristically, one may expect that the repulsive-type electron-electron interactions should act against light localization, and thereby that they should have a negative impact on the formation of the embedded eigenstates. Surprisingly, it is found that the nonlocality of the material response creates new degrees of freedom and relaxes the requirements for the observation of trapped light. In particular, a zero-permittivity condition is no longer mandatory and the same resonator shell can potentially suppress the radiation loss at multiple frequencies. The possibility to trap and guide light in wire metamaterials is also investigated. Specifically, we investigate the guided modes supported by a metamaterial slab formed by two mutually orthogonal and nonconnected sets of parallel metallic wires. It is demonstrated that the wire medium slab has a peculiar comb-like dispersion diagram. In the continuum approximation, the metamaterial supports a diverging number of guided mode branches that accumulate near the light line due to a strong hyperbolic response in the static limit. In a realistic system, the number of guided modes branches is finite and is determined by the density of wires. Remarkably, the guided modes may be characterized by a fast field variation along the transverse direction, which can be exploited to detect subwavelength particles or defects. Lastly, we investigated topological trapped states in photonic crystals. We show that in one-dimensional periodic systems the number of bands below a band gap determines the topological Chern number of an extended system with a synthetic dimension. It is theoretically and numerically demonstrated that in real-space the Chern number gives the number of gapless trapped state branches localized at the interface of the photonic crystal, when its geometry is continuously displaced by one lattice period. Furthermore, we introduce a novel class of topological systems with inversion-symmetry and fractional (non-integral) Chern numbers. It is proven that the non-integral topological number arises due to the discontinuous behaviour of the Hamiltonian in the spectral domain. We introduce a bulk-edge correspondence that links the number of edge-states with the fractional topological number.
A nano-fotónica é uma área de investigação dedicada ao estudo das interacções da luz com objectos nanométricos. Um dos objectivos da nanofotónica é possibilitar a miniaturização de componentes ópticos para uma escala competitiva com a microelectrónica. Existem vários benefícios em usar tecnologia fotónica, como a construção de circuitos fotónicos com pequenas dimensões que não são apenas mais rápidos mas também mais eficientes do que as suas contrapartes eletrónicas, novas células solares com uma maior absorção energética, sensores nano-ópticos capazes de detectar concentrações extremamente baixas de moléculas em soluções químicas, entre outros. O objectivo principal do meu trabalho é contribuir para esta área de investigação, explorando novos mecanismos de confinamento espacial da luz de forma eficiente. Esta tese é dedicada ao estudo analítico e numérico de três mecanismos diferentes de confinar a luz à nano-escala. Em primeiro lugar, é investigado o aprisionamento da luz em ressoadores plasmónicos abertos (meta-átomos) de diferentes geometrias. É mostrado que, em certas condições, cavidades dieléctricas de geometrias complexas podem suportar estado fotónicos discretos que, no limite em que as perdas materiais são nulas, possuem tempos de vida infinitos. Estes estados surgem devido à acção dos plasmões de volume suportados pela camada plasmónica exterior do meta-átomo e podem ser excitados eficientemente por uma onda plana quando o núcleo do ressoador possui uma resposta não-linear. Demonstra-se que a energia aprisionada no núcleo do ressoador é precisamente quantizada. Depois, é investigado o impacto dos efeitos de dispersão espacial, causados por exemplo pelas interacções electrão-electrão em metais, nos estados próprios embebidos suportados por ressoadores abertos plasmónicos tridimensionais. Heuristicamente, seria de esperar que as interacções repulsivas electrão-electrão agissem de maneira deteriorante no mecanismo de localização de luz e, portanto, tivessem um impacto negativo na formação dos estados próprios embebidos. Surpreendentemente, é mostrado neste trabalho que a dispersão não-local do material que encapsula o meta-átomo dá origem a novos graus de liberdade e relaxa os requisitos necessários ao aprisionamento da luz. Em particular, a condição que exige que o material da cápsula exiba uma permitividade exactamente igual a zero deixa de ser obrigatória, passando a ser possível que a mesma cápsula suprima a perda por radiação em várias frequências. É estudada de seguida a possibilidade de aprisionar e guiar luz em metamateriais de fios metálicos. Especificamente, investigamos os modos guiados suportados por um metamaterial formado por dois planos de fios metálicos mutuamente ortogonais. É demonstrado que o meio de fios tem um diagrama de dispersão peculiar, semelhante a um pente. No limite em que o material é visto como um meio contínuo (homogeneizado), o metamaterial suporta um número divergente de “ramos” de modos guiados que se acumulam junto à linha da luz devido à forte resposta hiperbólica do metamaterial no limite estático. Num sistema realista, o número de ramos é finito e determinado pela densidade de fios. Curiosamente, os modos são caracterizados por uma variação do campo rápida na direcção transversal, que pode ser explorada na detecção de partículas e defeitos de dimensão sub-lambda. Por último, são investigados modos de luz topologicamente aprisionados em cristais fotónicos. São estudadas as propriedades topológicas de sistemas periódicos unidimensionais, e é mostrado que o número de bandas abaixo do hiato de frequências determina o número de Chern de um sistema extendido com uma dimensão sintética. É demonstrado teórica e numericamente que, no espaço-real, o número de Chern determina o número de estados aprisionados na interface de um cristal fotónico no intervalo de frequências da banda não-propagante, quando a sua geometria sofre uma deslocação contínua de um período de estrutura. Além disso, é introduzida uma nova classe de sistemas topológicos com inversão de simetria e números de Chern fraccionários. É provado que o número topológico fraccionário é devido às descontinuidades do Hamiltoniano no domínio espectral. É introduzida uma correspondência volume-interface que liga o número de estados de interface com o número topológico fraccionário.
Instituto de Telecomunicações

碩士
國立臺灣海洋大學
機械與機電工程學系
100
Nanostructure-based surface plasmon resonance sensors are capable of sensitive and label-free detection for biomedical applications. However, nanostructures with higher sensitivities and high-throughput, low-cost fabrication techniques are the main issues which should be addressed. In this thesis, we utilized three kinds of methods, thermal-annealing-assisted template stripping, and template-stripping with UV gel and nanoimprinting, to fabricate bi-layer metallic grating structures, which fulfills the mentioned requirements. We studied the effect of the structure parameters of bi-layer gold grating structures and bi-layer Al/Au bimetallic grating structures on refractive index sensitivity. The bi-layer grating structures with a 500 nm period, various slit widths, from 60 to 180 nm, and various metal thicknesses, from 50 to 100 nm, were made. We found that a transverse magnetic-polarized wave in these gold nanostructures generated sharp and asymmetric Fano resonances in transmission spectra. The full width at half-maximum bandwidth decreased with the decrease of the slit width, the decrease of the ratio of gold film in hetero Al-Au film, the increase of metal thickness and the decrease of the distance between layers. The narrowest bandwidth was 6 nm. Compared to single-layer nanoslit arrays, the proposed structure has a similar wavelength sensitivity but narrower bandwidth. In addition, it has a higher intensity sensitivity up to 33344 %/RIU and reaches a figure of merit up to 103.5. The current structure can achieve a detection limit of 5.99 × 10-6 RIU when the intensity resolution is 0.2%. We furtherconducted an antigen-antibody interaction experiment in aqueous environment to verify the detection sensitivity in surface binding event.