Classification of Metallic Targets Using a Walk-Through Metal Detection Portal

Metal detectors have been used for a long time for treasure hunting, security screening, and finding buried objects such as landmines or unexploded ordnance. Walk-through metal detection (WTMD) portals are used for making sure that forbidden or threatening metallic items, such as knives or guns, are not carried into secure areas at critical locations such as airports, court rooms, embassies, and prisons.The 9/11 terrorist act has given rise to stricter rules for aviation security worldwide, and the ensuing tighter security procedures have meant that passengers face more delays at airports. Moreover, the fear of terrorism has led to the adoption of security screening technology in a variety of places such as railway and coach stations, sports events, malls, and nightclubs.However, the current WTMD technology and scanning procedures at airports require that all metallic items be removed from clothing prior to scanning, causing inconvenience. Furthermore, alarms are triggered by innocuous items such as shoe shanks and artificial joints, along with overlooked items such as jewellery and belts. These lead to time- consuming, manual pat-down searches, which are found inconvenient, uncomfortable, and obtrusive by some.Modern WTMD portals are very sensitive devices that can detect items with only small amounts of metal, but they currently lack the ability to further classify the detected item. However, if a WTMD portal were able to classify objects reliably into, e.g., “knives”, “belts”, “keys”, the need for removing the items prior to screening would disappear, enabling a paradigm shift in the field of security screening.This thesis is based on novel research presented in five peer-reviewed publications. The scope of the problem has been narrowed down to a situation in which only one metallic item is carried through the portal at a time. However, the methods and results presented in this thesis can be generalized into a multi-object scenario. It has been shown that by using a WTMD portal and the magnetic polarisability tensor, it is possible to accurately distinguish between threatening and innocuous targets and to classify them into 10 to 13 arbitrary classes. Furthermore, a data library consisting of natural walk-throughs has been collected, and it has been demonstrated that the walk-through data collected with the above portal are subject to phenomena that might affect classification, in particular a bias and the so-called body effect. However, the publications show that, by using realistic walk-through data, high classification accuracy can be maintained regardless of the above problems. Furthermore, a self-diagnostics method for detecting unreliable samples has also been presented with potential to significantly increase classification accuracy and the reliability of decision making.The contributions presented in this thesis have a variety of implications in the field of WTMD-based security screening. The novel technology offers more information, such as an indication of the probable cause of the alarm, to support the conventional screening procedure. Moreover, eliminating the need for removing all metallic items prior to screening enables design of new products for scenarios such as sports events, where conventional screening procedures might be inconvenient, creating thus new business possibilities for WTMD manufacturing companies.The positive results give rise to a variety of future research topics such as using wideband data, enabling simultaneous classification of multiple objects, and developing the portal coil design to diminish signal nonlinearities. Furthermore, the ideas and the basic principles presented in this thesis may be applied to other metal detection applications, such as humanitarian demining

Momentum exchange between light and nanostructured matter

An object\u27s translational and rotational motion is associated with linear and angular momenta. When multiple objects interact the exchange of momentum dictates the new system\u27s motion. Since light, despite being massless, carries both linear and angular momentum it too can partake in this momentum exchange and mechanically affect matter in tangible ways. Due to conservation of momentum, any such exchange must be reciprocal, and the light therefore acquires an opposing momentum component. Hence, light and matter are inextricably connected and one can be manipulated to induce interesting effects to the other. Naturally, any such effect is facilitated by having strongly enhanced light-matter interaction, which for visible light is something that is obtained when nanostructured matter supports optical resonances. This thesis explores this reciprocal relationship and how nanostructured matter can be utilised to augment these phenomena.Once focused by a strong lens, light can form optical tweezers which through optical forces and torques can confine and manipulate small particles in space. Metallic nanorods trapped in two dimensions against a cover glass can receive enough angular momentum from circularly polarised light to rotate with frequencies of several tens of kilohertz. In the first paper of this thesis, the photothermal effects associated with such optical rotations are studied to observe elevated thermal environments and morphological changes to the nanorod. Moreover, to elucidate upon the interactions between the trapped particle and the nearby glass surface, in the thesis\u27 second paper a study is conducted to quantify the separation distance between the two under different trapping conditions. The particle is found to be confined ~30-90 nm away from the surface.The momentum exchange from a single nanoparticle to a light beam is negligible. However, by tailoring the response of an array of nanoparticles, phase-gradient metasurfaces can be constructed that collectively and controllably alter the incoming light\u27s momentum in a macroscopically significant way, potentially enabling a paradigm shift to flat optical components. In the thesis\u27 third paper, a novel fabrication technique to build such metasurfaces in a patternable polymer resist is investigated. The technique is shown to produce efficient, large-scale, potentially flexible, substrate-independent flat optical devices with reduced fabricational complexity, required time, and cost.At present, optical metasurfaces are commonly viewed as stationary objects that manipulate light just like common optical components, but do not themselves react to the light\u27s changed momentum. In the last paper of this thesis, it is realised that this is an overlooked potential source of optical force and torque. By incorporating a beam-steering metasurface into a microparticle, a new type of nanoscopic robot – a metavehicle – is invented. Its propulsion and steering are based on metasurface-induced optical momentum transfer and the metavehicle is shown to be driven in complex shapes even while transporting microscopic cargo

Symmetry and topology at the metasurface

Since the metamaterials ethos of geometry over chemistry was first conceived at the end of the last century, a great deal of effort has been directed towards the conceptual, computational and experimental development of myriad effective electromagnetic media. Taking inspiration from quantum mechanics, here we exploit the possibility of independently controlling the individual elements of an effective polarizability matrix to reveal unique polarisation based phenomena. Firstly, by employing resonant “meta-atoms” to selectively absorb specific polarisation states of THz radiation, while tuning the polarisation conversion efficiency via near-field coupling, Parity Time symmetry breaking has been proposed, based on analytical and numerical modelling, and observed, using THz-Time Domain Spectroscopy, in polarisation space for the first time. We also reveal that anisotropic material as well radiative loss can be highly useful for tailoring the response of resonant metamaterials. Secondly, the possibility of achieving a topologically non-trivial phase within an effectively homogeneous photonic medium is discussed. Originating from the inherent spin-orbit interaction for light, three dimensional metamaterials with chirality and hyperbolicity are shown to be topologically non-trivial, resulting in one-way surface waves that are immune to back-scattering. Building on the effective medium calculations, our predictions are confirmed by numerical studies of realistic meta-structures

New Trends in Quantum Electrodynamics

Quantum electrodynamics is one of the most successful physical theories, and its predictions agree with experimental results with exceptional accuracy. Nowadays, after several decades since its introduction, quantum electrodynamics is still a very active research field from both the theoretical and experimental points of view. The aim of this Special Issue is to present recent relevant advances in quantum electrodynamics, both theoretical and experimental, and related aspects in quantum field theory and quantum optics

Plasmonic effects upon optical trapping of metal nanoparticles

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

Heterogeneous mixtures for synthetic antenna substrates

Heterogeneous mixtures have the potential to be used as synthetic substrates for antenna applications giving the antenna designer new degrees of freedom to control the permittivity and/or permeability in three dimensions such as by a smooth variation of the density of the inclusions, the height of the substrate and the manufacture the whole antenna system in one process. Electromagnetic, fabrication, environmental, time and cost advantages are potential especially when combined with nano-fabrication techniques. Readily available and cheap materials such as Polyethylene and Copper can be used in creating these heterogeneous materials. These advantages have been further explained in this thesis. In this thesis, the research presented is on canonical, numerical and measurement analysis on heterogeneous mixtures that can be used as substrates for microwave applications. It is hypothesised that heterogeneous mixtures can be used to design bespoke artificial dielectric substrates for say, patch antennas. The canonical equations from published literature describing the effective permittivity, ε_eff and effective permeability, μ_eff of heterogeneous mixtures have been extensively examined and compared with each other. Several simulations of homogenous and heterogeneous media have been carried out and an extraction/inversion algorithm applied to find their ε_eff and μ_eff. Parametric studies have been presented to show how the different variables of the equations and the simulations affect the accuracy of the results. The extracted results from the inversion process showed very good agreement with the known values of the homogenous media. Numerically and canonically computed values of ε_eff and μ_eff of various heterogeneous media were shown to have good agreement. The fabrication techniques used in creating the samples used in this research were examined, along with the different measurement methods used in characterising their electromagnetic properties via simulations and measurements. The challenges faced with these measurement methods were explained including the possible sources of error. Patch antennas were used to investigate how the performance of an antenna may be affected by heterogeneous media with metallic inclusions. The performance of the patch antenna was not inhibited by the presence of the metallic inclusions in close proximity. The patch measurement was also used as a measurement technique in determining the ε_eff of the samples

Advanced Raman Spectroscopy of Ultrathin RNiO3 films

The present work aims at investigating the structural properties of ultrathin rare-earth nickelate films by Raman Spectroscopy. Two remarkable cases are studied: LaNiO3 deposited epitaxially on LaAlO3, which shows a metal-to-insulator (MIT) transition but only in the ultrathin film regime, and NdNiO3 deposited epitaxially on NdGaO3 showing an upward shift of its MIT temperature by 130 K but only when deposited along the [111]pc direction of the substrate. The extremely small size of the films and overlap of the film and substrate signatures represent an experimental challenge and require the development of ingenious measurement and analysis strategies. To overcome these limitations, we propose the creation of a multidimensional dataset through depth profile acquisitions, in combination with multivariate analysis tools that were employed to extract the signature of the films. Different analysis strategies were used in both cases to adapt to the specificities of the respective samples. For the LNO films deposited along the [001]pc orientation of LAO, Raman depth profile measurements in combination with a Principal Component Analysis (PCA) allowed us to dissociate the signals from the film and the substrate. The evolution of the LNO peaks does not suggest any phase transition, thus, suggesting that a mechanism unrelated to the MIT of other nickelates is triggering the insulating state. This was further validated by ab initio calculations and TEM imaging. All acquired data point towards the following: as LNO becomes very thin, the surface layer (≈ 2pc u.c.), which is the most rigid part of the structure, imposes its structural and insulating characteristic. In the ultrathin regime this continues to a point where the surface of the film alters the interfacial unit cells of the substrate. For the NNO films deposited along the [111]pc orientation of NGO, depth profile measurements in combination with a Non-negative Matrix Factorisation (NMF) allowed us to dissociate the signals from the film and the substrate. The dissociation was performed at room temperature and the acquired knowledge was then utilised to fit an entire temperature series from 5 to 390 K. Comparing the tendency of the Raman signatures with other rare-earth nickelate allowed to support the proposed position of the film in the phase diagram of nickelates by a structural measurement. More generally, the methodology developed in this work is applicable to other systems and opens new perspectives for application of Raman spectroscopy on ultra-thin films

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

Perspectives on weak interactions in complex materials at different length scales

Nanocomposite materials consist of nanometer-sized quantum objects such as atoms, molecules, voids or nanoparticles embedded in a host material. These quantum objects can be exploited as a super-structure, which can be designed to create material properties targeted for specific applications. For electromagnetism, such targeted properties include field enhancements around the bandgap of a semiconductor used for solar cells, directional decay in topological insulators, high kinetic inductance in superconducting circuits, and many more. Despite very different application areas, all of these properties are united by the common aim of exploiting collective interaction effects between quantum objects. The literature on the topic spreads over very many different disciplines and scientific communities. In this review, we present a cross-disciplinary overview of different approaches for the creation, analysis and theoretical description of nanocomposites with applications related to electromagnetic properties