Nanophotonics of ultrathin films and 2D periodic structures: a combined experimental and theoretical study

Photonics is a key enabling technology for many applications ranging from communications to energy and medicine. Its success is largely relying on our capability to appropriately control light in optical devices. To this end, the understanding of light-matter interaction occurring in the devices is a crucial element for finding effective solutions to the challenges posed by the targeted applications. This thesis is devoted to understand light-matter interaction in periodic nanostructures and ultrathin films and create modelling and design tools for functional optical devices, some of them demonstrated experimentally. We start by investigating the needed theoretical methods for describing the interaction of light with surface periodic nanostructures. We carry out a comprehensive study of the transmission, reflection and dispersion properties of 2D periodic arrays and their stacks, including, the study of more complex structures as well, such as, defects in periodic lattices, random arrays of scatterers and multicomponent lattices, and the calculation of the local density of electromagnetic states in the array. We then show how to use the developed theory to design and understand the behaviours of application-specific devices/structures, made of 2D periodic structures and multilayer stack of thin films. A first device demonstrator consists in periodic arrays of nanoholes performated in a gold film covered with Ge2Sb2Te5 (GST), a phase change material layer.We investigate the effect of GST¿s phase transitions on the transmission resonances of these structures. Wavelength shifts as large as 385 nm are demonstrated in configurations with broad resonances. Additionally, excitation of GST with short pulses allows ultrafast tuning of these resonances in the ps regime without producing any phase transition. Finally, tuning of narrower resonances with shifts of 13 nm is also demonstrated. In a second device demonstrator, a perfect absorber, we show how interference effects, occurring in multilayer thin film structures, can be exploited to achieve nearly 100% absorption. Two perfect absorption regimes are identified: the first one broadband and in the visible; the second one resonant and in the near infrared (NIR) region of the wavelengths. We show that the proposed method enables conceptually simple devices that are easy to fabricate. Moreover, we show that GST constitutes an essential layer for a new class of optical absorbers that can be dynamically tuned. In contrast, previous structures required cumbersome fabrication steps and were not dynamically tunable. In a third device demonstrator, a structure with multilayer thin films is used to design and fabricate an anti-reflective, highly transparent electrode, with world-record low sheet electrical resistance and high optical transmission. In summary, the thesis capitalizes on modelling tools for light-matter interaction at the nano-scale, which are adapted to a general class of device structures and allow us to design optical surfaces based on thin films and nano-structuring with unprecedented performance. This is demonstrated through the design and experimental realization of resonant optical filters with very large tunability, perfect absorbers with very high dynamic range and transparent electrodes with record electro-optical performance.La fotònica és una tecnologia que permetrà implementar noves tecnologies en àrees tan diverses com les comunicacions, l'energia o la medicina. El seu èxit dependrà en gran mesura de la capacitat de controlar la llum en els dispositius òptics. En aquest sentit, entendre com la llum i la matèria interaccionen en aquests dispositius és un dels requisits principals a l'hora de trobar solucions efectives als reptes que ofereixen les diferents àrees d'aplicació de la fotònica. Aquesta tesi té com a objectiu entendre les interaccions entre llum i matèria en estructures periòdiques i capes ultra-primes així com crear eines de disseny i modelat de dispositius òptics, alguns dels quals són també demostrats experimentalment. A la primera part de la tesi s'investiga la teoria necessària per descriure la interacció de la llum en superfícies periòdiques nano-estructurades. Això inclou un estudi detallat de la transmissió, reflexió i dispersió d'estructures periòdiques en 2D o combinacions d¿elles, així com també l'estudi d'estructures més complexes, com ara defectes, estructures aleatòries, i finalment el càlcul de la densitat local d'estats electromagnètics en aquestes estructures. A la segona part de la tesi s'aplica aquesta teoria per dissenyar i entendre el comportament de dispositiu fotònics basats en aquestes estructures 2D per a aplicacions específiques. El primer dispositiu que es demostra consisteix en una estructura periòdica de nano-forats en una capa d'or coberta amb Ge2Sb2Te5 (GST), un material de canvi de fase. S'investiga l'efecte que té un canvi de fase en la capa de GST en les ressonàncies de transmissió d'aquestes estructures i es demostren canvis en la longitud d'ona de ressonància de fins a 385 nm en el cas de ressonàncies amples. A més a més també es demostra com excitant la capa de GST amb polsos ultra-curts aquestes ressonàncies també es poden modificar en una escala de temps de ps sense la necessitat de tenir un canvi de fase. Per últim també es demostren canvis en la longitud d'ona de ressonàncies de fins a 13 nm en dispositius amb ressonàncies estretes. En el segon dispositiu es demostra com els efectes d'interferència que tenen lloc en estructures compostes per vàries capes primes poden ser explotats per tal d'obtenir una absorció de gairebé el 100%. En particular es demostren dos règims d'absorció completa: banda ampla en el visible i absorció ressonant en l'infraroig. Aquest mètode permet fabricar dispositius de manera fàcil. A més a més es demostra com el GST permet crear una nova classe de dispositius amb aborció completa que poden ser sintonitzats dinàmicament, en contrast amb la majoria d'estructures proposades fins al dia d'avui. En la tercera aplicació es dissenya i demostra experimentalment una estructura de vàries capes per a ser usada com a elèctrode transparent amb propietats d'antireflexió, i amb una resistència molt baixa i alta transmissió òptica. En resum, aquesta tesi descriu eines per modelar la interacció entre llum i matèria en l'escala dels nanòmetres per una classe general d'estructures que després són usades per dissenyar superfícies òptiques basades en capes primes i nano-estructuració. En particular això es demostra amb el disseny i realització experimental de filtres òptics ressonants, dispositius amb absorció completa i gran rang dinàmic així com elèctrodes transparents amb unes grans propietats electró-òptiques

Design of an Electromagnetic Coil Array for Wireless Endoscope Capsule Localization

Wireless capsule endoscopy is a technique that visualizes mucosa of gastrointestinal tract. The first wireless capsule endoscope was developed in 2001, and since then, the tech-nology has been in constant development. With its reduced amount of discomfort, ability to visualize the whole gastrointestinal tract and many interesting future prospects, capsule en-doscopy is challenging the conventional procedure, in which a camera module at the end of a tube is inserted to the gastrointestinal tract. In general, the method can be used to diagnose many diseases, such as cancers, Celiac disease and Crohn’s disease. In order to achieve all the future prospects of the technology, for example, active steering of the capsule, biopsy and drug-delivery with the capsule, challenge of detecting the cap-sule’s position and orientation inside human needs to be overcome. Therefore, the localiza-tion challenge is considered in this thesis. The aim is to select an optimal method for localiza-tion based on current research, and to model and develop a prototype that could be utilized in capsule localization. The designed sensor array is evaluated with the help of finite element method -based modelling, sensitivity analysis and practical experiments. Based on the studied literature, an active magnetic field strength -based localization tech-nique was selected for further analysis. According to the literature, the method provides high accuracy and could also be utilized in other purposes, for example, wireless charging and ac-tive locomotion of the capsule. In addition, the method does not suffer from attenuation of the fields within a human body. Therefore, theoretical basis of the magnetic field strength -based localization was presented, and four electromagnetic coil arrays were designed for sensitivity analysis. Contrary to many developed arrays seen in the literature review, all the designed systems were planar, in order to develop a system that could be fitted, for instance, inside a hospital bed. Based on the sensitivity analysis, an array with relatively large sensitivity and optimal number of measurement channels was selected for practical study. In addition, pos-sible markers that could be fitted inside a regular sized endoscopic capsule were numerically modelled. It was found that a resonated solenoid marker with ferrite core causes the largest voltage compared to other modelled targets, and therefore it was constructed for experi-ments. The whole array was built and equipped with electronics and measurement devices to be able to perform testing. Two channels of the array were selected for example measurements, and the results were analysed and compared with modelled values and sensitivity patterns. The system was tested at multiple different heights and positions, and the effect of changing the marker’s orientation with respect to the array was analysed. It was found that the system gives a reasonable response when the marker is oriented along the excitation magnetic field. With this orientation, it was possible to measure significant voltages caused by the marker even at distance of 25 cm from the array. However, when the marker was oriented so that the excitation field could not properly excite it, the measured voltages got smaller. In addition, at certain orientation, the measured voltages did not seem reliable because voltage caused by the marker could not been discriminated from the noise of the system. The study indicates that the method is suitable for localization of resonated solenoid sample with ferrite core, but further improvements are needed to make the system work at each position and orientation of the marker. In addition, an inversion algorithm that estimates marker’s position and orien-tation based on the measured voltage needs to be integrated to the system

Characterisation of Single Biomolecules With Optoplasmonic Resonators

Biomolecules can be detected through induced changes in the optical whispering-gallery mode (WGM) resonance in a circularly symmetric dielectric. The spatial and temporal confinement of light in a WGM is further complemented by coupling to the localised surface plasmons (LSPs) of metallic nanoparticles attached to the WGM resonator. LSP-WGM hybridisation allows for the optical readout of single-molecule surface reactions on gold nanoantennae, the mechanisms for which are not yet fully understood from a theoretical perspective. The specificity of this modality is, moreover, a subject of intense research. In this thesis, we propose three strategies for characterising molecules with light. The first strategy is a prototype polarimeter that differentiates chirality based on a signal-reversible Faraday effect in a magneto-optical WGM microcavity. Thermal tuning integrated into the resonator minimises geometrical birefringence, in turn maximising Faraday rotation to optimise chiral sensitivity. There we endeavour to resolve single-molecule chirality. Without engineering reconsiderations, however, the polarimeter is found to be limited to bulk chiral analysis. The second strategy is an (optoplasmonic) LSP-WGM resonator with chiral gold nanoantennae. Signals from the molecules conjointly show a correlation with the molecular weight and diffusivity of detected DL-cysteine and poly-DL-lysine. Aside from these features, the sensing site heterogeneity on the chiral gold nanoparticles impedes chiral discrimination. The third strategy is a novel reaction scheme adapted to the optoplasmonic sensor. Aminothiol linkers functionalise the gold surface via amine-gold anchoring, setting up cyclical interactions with thiolated analytes by thiol/disulfide exchange. Unexpected perturbations in the LSP-WGM resonance are observed, such as linewidth oscillation without resonance shift attributed to optomechanical coupling between LSPs and the vibrational modes in a given analyte. This offers a new form of spectroscopy wherein single biomolecules could be characterised by their mass, size, and composition through monitoring secondary parameters of the optoplasmonic resonance.European Commissio

Application of disposable chiral plasmonics for biosensing and Raman spectroscopy

This thesis explores the capabilities of disposable chiral plasmonic metafilm assays, termed Disposable Plasmonic Assays, as a promising platform for biosensing and surface-enhanced Raman spectroscopy. The sensing and Raman properties of these metafilms arise from the excitation of surface plasmons when exposed to incident light. These plasmonic properties strongly depend on the geometric characteristics of the constituent nanostructures found in the metafilms. Specifically, the primary nanostructure employed throughout this research is the chiral 'shuriken' star, which generates chiral electromagnetic fields exhibiting greater chiral asymmetry than circularly polarized light. Monitoring changes in the resonance positions of the characteristic optical rotatory dispersion spectra produced by the Disposable Plasmonic Assays allows for the observation of surface binding events. By measuring resonance shift data and through the utilisation of various gold film functionalisation techniques, these assays are demonstrated as versatile, label-free biosensing platforms capable of specifically detecting a wide range of target proteins and virus particles from complex solutions. Furthermore, the multiplexing performance of these assays is showcased, enabling the detection of multiple different antigens and virions in a single experiment. These results highlight the potential of plasmonic metafilms as rapid and disposable point-of-care immunoassays for diagnostic applications. In addition to biosensing, the chiral geometry of Disposable Plasmonic Assays is exploited for the chiral discrimination of metal nanoparticles and small molecules using Surface Enhanced Raman Spectroscopy (SERS). By linking helicoid shaped gold nanoparticles to the metafilm surface via a dithiol linker, the chiral properties of both nanoparticles and metafilms combine, resulting in the creation of differential electromagnetic 'hotspot' regions based on their symmetry combinations. The electromagnetic intensity in these regions corresponds to the SERS signal obtained from the achiral dithiol linker molecule, facilitating a deeper understanding of the chirally dependent SERS phenomenon. These findings serve to validate and explain the differential SERS data obtained enantiomers of biomolecules and drug molecules from silver modified Disposable Plasmonic Assays

Modelling of plasmonic systems:advanced numerical methods and applications

Metallic nanostructures interact in complex ways with light, forming the subject of plasmonics and bringing novel physical phenomena and practical applications. The fundamental and practical importance of plasmonics necessitates the development of a multitude of simulation techniques. Surface integral equation (SIE) is a numerical method which is particularly suited for simulating many plasmonic systems. In this thesis, we develop SIE-based numerical methods for plasmonics and use them to study plasmonic systems of interest. Electric and magnetic surface currents are the basic quantities calculated in SIE, and it is appealing to directly compute various physical quantities directly using them. We develop a formalism to compute optical forces and torques, polarisation charges and multipole moments using the surface currents for better accuracy and efficiency. Numerical simulation is all about finding the right balance between accuracy and computational cost. SIE allows to choose this tradeoff in computing the integrals for the simulation matrix. We study the effect of the integration routine on the accuracy of the matrix and propose an optimised recipe for evaluating the integrals. Although this recipe incurs an overhead, we show how it becomes necessary in computing some physical quantities and simulating some systems, and how it allows simulations using a coarser discretisation. One drawback of SIE is that it can only simulate domains for which the response of each domain can be expressed in terms of the Green's function for the domain. Only homogeneous and periodic domains could be dealt with till now, limiting its applicability. We extend SIE to simulate nanostructures embedded in the layers of a stratified medium to partly overcome this restriction, paving the way for further improvements. SIE has the ability to model complex and realistic geometries. We exploit this feature to study the effect of fabrication-induced rounding on nanorods and gap antennae. We show how rounding results in blue shift of resonances, migration of charges from corners to edges to faces, and reduced coupling between nanostructures. The surface current-based formalism to calculate optical forces and torques permits their computation for particles in close proximity. We use this to study the internal forces in compound plasmonic systems, and show the presence of strong internal forces between their components. We also demonstrate surprising features such as force and torque reversal, and circular polarisation-dependent behaviour in achiral systems. We then numerically investigate the possibility of using optical torques to orient and rotate plasmonic nanostructures, relying on surface plasmon resonance, retardation effects and circular polarisation. Polarisation charges contain useful information about the behaviour of plasmonic systems, but there are difficulties in understanding and visualising them. We discuss the complex nature of polarisation charges and suggest various techniques to visualise them in complicated systems in a manner which is easy to understand without loss of information. Finally, we utilise the ability of SIE to compute accurate near fields to study the Raman enhancement in multi-walled carbon nanotubes on coating with metal, and the analogous quenching of Raman signal from silicon substrates

Giving Metamaterials a Hand

The focus of this thesis is the interaction of electromagnetic fields with chiral structures in the microwave regime. Through this study, which focuses on three regimes of electromagnetic interactions, I aim to develop a deeper understanding of the consequences and manifestations of chiral interactions The structures are on the order of, or smaller than, the wavelength of the probing radiation. As the structures are chiral, they have broken inversion symmetry, and exist in two states where one is the mirror image of the other. The results in this thesis can have impacts on future optical communications technologies and methods of sensing biological molecules. To begin with, the manipulation of the circular polarisation of a propagating beam by bilayer chiral metasurfaces is investigated. The metasurfaces consist of two layers of stacked crosses with a twist between top and bottom layers, forming chiral metamolecules. A broad frequency region of dispersionless polarisation rotation appears between two resonances, due to alignment between electric and magnetic dipoles. The dependence of this effect on the layer separation is studied for two similar metasurfaces. Evanescent chiral electromagnetic fields are the focus of the next chapter. An array of chiral antennas produces chiral near-fields at their resonant frequency. Aligned and subwavelength helices placed within this field interact differently depending on the handedness of the field with respect to the handedness of the helices. This difference in interaction strength is measured for the helices and an effective medium model where multipolar interactions are forbidden. Comparison of these two systems leads to the conclusion that the contribution to a chiral interaction from multipolar modes is minimal, in contrast to previous publications. The third study concentrates on the electromagnetic wave bound to an "infinitely long" metal helix. The helix has infinite-fold screw symmetry, and this leads to interesting features in the energy-dispersion of the waves it supports. The broad frequency range of high, tunable, dispersionless index is interpreted using a geometrical approach, and the factors that limit the bandwidth explained. A modified geometry is suggested for increased bandwidth. The final part of the thesis is dedicated to future work, based on the results presented thus far. Three suggestions for future study are presented, including chiroptical signals from higher-order chiral arrangements, the effect of reflecting surfaces next to chiral objects and the possible use of orbital angular momentum for chiroptical measurements.Engineering and Physical Sciences Research Council (EPSRC

Interactions between polyhedral permanent magnets

With the current trend toward industrial automation, efficient energy generation, and electric motor vehicles, permanent magnets are seeing more widespread use than ever before. They permeate our world, enabling sound generation through loudspeakers, mass data storage in the server farms keeping us online, and even the vibration motors in our pockets notifying us of new messages. Never before have permanent magnets seen such widespread use, and thus it is paramount to understand the interactions between them. The primary aim of this thesis is to investigate and model the magnetic fields produced by generalised polyhedral permanent magnets, and the forces and torques between them. To achieve this aim, two main objectives were identified. The first objective was to analytically solve the magnetic charge model field equations for arbitrary polyhedral permanent magnets with a relative permeability of unity. This was performed using two unique approaches, leading to two unique but equivalent sets of field solutions, with the first being more effective when the field is calculated at few points, and the second being more effective when the field is calculated at many points. These field solutions were implemented in MATLAB code with a focus on computation efficiency, thus reducing calculation time. The solutions may also be used to numerically integrate over the surface of another magnet to accurately estimate the force and torque imparted. The second objective was to derive a methodology to model the field due to a polyhedral permanent magnet with non-unity relative permeability. This was done by applying a surface mesh to a magnet, and allowing the ‘magnetic charge’ on each surface element to vary based on the permeability and magnetic field passing through the element. This was derived in such a way that the field is calculated only once, with no iteration required. Rather, a matrix equation is solved to give the surface charge distribution, leading to calculations of the magnetic fields, forces, and torques based on the previous objective. This was again implemented in MATLAB code with focus on computation efficiency, leading to fast calculations. This thesis begins with a short prologue, giving a brief historical overview of the development of magnetism as a physical science. Chapter 1 follows, outlining the theory used for modelling magnets and giving a review of relevant literature. Chapters 2 and 3 outline two new methods for calculating the magnetic field produced by general ideal polyhedral permanent magnets, each with benefits and drawbacks over the other. In addition, Chapter 2 found that a pair of pyramid frustum magnets produce a larger mutual force than a pair of cuboidal magnets, suggesting further investigation into frustum magnets. Chapter 4 applies the methodology from Chapter 3 to a planar array of frustum magnets, finding no significant benefit over traditional cuboidal planar arrays. Chapter 5 explores magnetic permeability, deriving a methodology to calculate magnetic fields, forces, and torques imparted by linear magnetic materials of polyhedral geometry. Finally, the thesis is concluded in Chapter 6, summarising the preceding chapters and outlining potential future work to follow this thesis. The primary outcome of this thesis is the development of a new methodology which can accurately and quickly compute the magnetic fields, forces, and torques imparted by magnetic materials of polyhedral geometry. The methodology allows for materials with constant non-unity relative permeability, more accurately reflecting permanent magnet materials and magnetic behaviour. Moreover, other geometries may be accurately approximated by polyhedra and the methodology applied, allowing the fast and accurate approximation of any current-free magnetostatic system.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

The development of metasurfaces for manipulating electromagnetic waves

The work outlined in this thesis focuses on the development and fabrication of metasurfaces for manipulating electromagnetic waves, with the potential for applications in imaging and holography. Metasurfaces are the Two-Dimensional counterpart of metamaterials, which are artificial materials used to invoke electromagnetic phenomena, not readily found in nature, through the use of periodic arrays of subwavelength ‘meta-atoms’. Although they are a new and developing field, they have already secured a foothold as a meaningful and worthwhile focus of research, due to their straight-forward means of investigating fundamental physics, both theoretically and experimentally - owing to the simplicity of fabrication - whilst also being of great benefit to the realisation of novel optical technologies for real-world purposes. The main objective for the complete manipulation of light is being able to control, preferably simultaneously, the polarisation state, the amplitude, and the phase of electromagnetic waves. The work carried out in this thesis aims to satisfy these criteria, with a primary focus on the use of Geometric phase, or Pancharatnam-Berry phase. The first-principles designs are then used to realise proof-of-concept devices, capable of Circular Conversion Dichroism; broadband simultaneous control of phase and amplitude; and a high-efficiency, broadband, high-resolution hologram in the visible-to-infrared