Synthesis and characterization of plasmonic nanostructures with controlled geometry for photonic applications

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 application

Co3O4 Nanopetals on Si as Photoanodes for the Oxidation of Organics

Cobalt oxide nanopetals were grown on silicon electrodes by heat-treating metallic cobalt films deposited by DC magnetron sputtering. We show that cobalt oxide, with this peculiar nanostructure, is active towards the photo-electrochemical oxidation of water as well as of organic molecules, and that its electrochemical properties are directly linked to the structure of its surface. The formation of Co3O4 nanopetals, induced by oxidizing annealing at 300 \ub0C, considerably improves the performance of the material with respect to simple cobalt oxide films. Photocurrent measurements and electrochemical impedance are used to explain the behavior of the different structures and to highlight their potential application in water remediation technologies

Synthesis and characterization of plasmonic nanostructures with controlled geometry for photonic applications

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 applicationsLo 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 fotonich

Maxwell's color box: Retracing the path of color matching experiments

In his 1860 paper On the theory of compound colours, James Clerk Maxwell described an instrument used to obtain a direct comparison between daylight and a mixture of three selected spectral colors. This investigation was part of Maxwell’s study of human color vision, color perception, and color representation, and it encompasses his main achievements in the field. The working principle underlying this device provided the basis from which color diagrams have been derived, beginning with the standard chromaticity diagram proposed by the International Commission on Illumination in 1931. We describe a reconstruction of Maxwell’s original version of the color box. Constructing and analyzing data obtained with such a replica could serve as a semester project for advanced optics students

Optimal geometric parameters of ordered arrays of nanoprisms for enhanced sensitivity in localized plasmon based sensors

Plasmonic sensors based on ordered arrays of nanoprisms are optimized in terms of their geometric parameters like size, height, aspect ratio for Au, Ag or Au0.5-Ag0.5 alloy to be used in the visible or near IR spectral range. The two figures of merit used for the optimization are the bulk and the surface sensitivity: the first is important for optimizing the sensing to large volume analytes whereas the latter is more important when dealing with small bio-molecules immobilized in close proximity to the nanoparticle surface. A comparison is made between experimentally obtained nanoprisms arrays and simulated ones by using Finite Elements Methods (FEM) techniques

Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays

Combining finite elements method electrodynamic simulations and cost-effective and scalablenanofabrication techniques, we carried out a systematic investigation and optimization of the sensingproperties of non- interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a veryeffective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their maingeometrical parameters (diameter and height) to monitor the plasmonic resonance position and tofindthe configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to thevariation of the refractive index of an embedding medium (bulk sensitivity). The spectral position of theplasmonic resonance can be tuned over a wide range from the visible to the near-IR region (500–1300nm) and state-of-the-art performances can be obtained using the optimized nanodisks; we obtainedlocal and bulk sensitivities of S0=11.9 RIU^-11and Sbulk=662 nm/RIU, respectively. Moreover, theresults of the simulations are compared with the performances of experimentally synthesized non-interacting Au nanodisk arrays fabricated by combining sparse colloidal lithography and hollow masklithography, with the parameters obtained by the sensitivity numerical optimization. An excellentagreement between the experimental and the simulated results is demonstrated, confirming that theoptimization performed with the simulations is directly applicable to nanosensors realized with cost-effective methods, due to the quite large stability basin around the maximum sensitivities

Selective Control of Eu3+ Radiative Emission by Hyperbolic Metamaterials

In recent years the quest for novel materials possessing peculiar abilities of manipulating light at the nanoscale has been significantly boosted due to the strict demands of advanced nanophotonics and quantum technologies. In this framework radiative decay engineering of quantum emitters is of paramount importance for developing efficient single-photon sources or nanolasers. Hyperbolic metamaterials stand out among the best cutting-edge candidates for photoluminescence control owing to their potentially unlimited photonic density of states and their ability to sustain high-k modes that allow us to strongly enhance the radiative decay rate of quantum light emitters. The aim of the present paper is to show how Au/Al2O3 hyperbolic multilayers can be used to selectively control the photoluminescence of coupled Eu3+ emitters. We point out an enhancement of the Eu3+ transitions when they are in the hyperbolic regime of the metamaterials and a significant alteration of the ED and MD branching ratios by changing the emitter–metamaterial distance

Polarized coherent emission outside high-symmetry points of dye-coupled plasmonic lattices

Developing intense, coherent and ultra-fast light sources with nanoscale dimensions is a crucial issue for many applications in nanophotonics. To date, plasmonic nanolasers represent one of the most promising nanophotonic devices capable of this remarkable feature. In the present work we report on the emission properties of two-dimensional Au hexagonal nanodome arrays, fabricated by nanosphere lithography, coupled with a dye liquid solution used as the gain medium. Low-threshold stimulated emission at room temperature is demonstrated by spectral and angle-resolved photolu- minescence measurements performed as a function of the pump fluence. The emission arises with narrow angular divergence in off-normal direction, out of high-symmetry points of the plasmonic lattice. The polarization properties of the stimulated emission are investigated, revealing a strong linear polarization character controlled by the polarization orientation of the pumping beam, while the first-order temporal coherence properties are measured by using a tilted-mirrors Michelson interferometer. Finally, by comparing the results obtained for the plasmonic Au nanodomes arrays with those of purely dielectric nanoarrays, the role of the plasmonic modes and the photonic lattice modes in the emission process is highlighted

Emission Rate Modification and Quantum Efficiency Enhancement of Er3+Emitters by Near-Field Coupling with Nanohole Arrays

The control of the spontaneous emission properties of quantum emitters with limited losses by near-field coupling with plasmons-supporting nanostructures is one of the keys for next-generation high-efficiency and high-coherence plasmonic devices. In the present work, gold nanohole arrays are demonstrated to be an effective plasmonic system for controlling radiative rate and quantum efficiency of the 1540 nm emission of Er3+ ions embedded in silica. Finite element method electrodynamic simulations were used to describe the interaction between dipolar Er3+ emitters and the nanohole arrays. The results are in agreement with those of photoluminescence measurements performed in different coupling configurations. Particularly, we demonstrated that owing to the combination of strong emission enhancement and low level of ohmic losses in the metal, nanohole arrays are able to enhance the far-field photon yield up to 74%. This in turn is related to an extremely high far-field quantum efficiency: more than 90% of the emitted photons reach the far-field for the most efficient configurations investigated in which the extraordinary optical transmission peak of the nanohole array is matched with the Er3+ emission

Lanthanide Ions Sensitization by Small Noble Metal Nanoclusters

Rare-earth ions sensitization is, nowadays, a relevant topic in modern technologies. Noble metal nanoclusters can effectively sensitize lanthanide photoluminecence (PL) via excitation energy transfer (EET). Recent experimental works reported how this process strongly depends on the nanoclusters size and composition, however, a comprehensive understanding of this phenomenon is still lacking. Inspired by the current paradigm on the lanthanide 12antenna complexes, where light is absorbed by the organic ligand, which then converts to a triplet and transfers the excitation to the lanthanide, we propose it also applies to sensitization by metal clusters. To prove this, we studied the optoelectronic features of several MN nanoclusters (M = Au, Ag, Au/Ag mix; N = 12, 20, and 58) at the Time Dependent Density Functional Theory (TDDFT) level, including, via simplified models, the silica matrix and its possible defects, and make considerations on the role these features can have on the EET toward Er3+ ions. Our analysis suggests that PL enhancement is generally more effective when N = 12 and M = Ag or Au/Ag mix, while the worst cases are obtained when M = Au and N = 58. These findings are coherent with prior experimental data and with novel measures that are here presented for the first time. Notably, we recover that the matrix defects can actively take part in the EET and, in some cases, could be (counterintuitively) beneficial for the process efficiency. Globally, this theoretical framework gives a comprensive rationale that can guide the design of new effective rare-earth ion sensitizers based on metal clusters