Terahertz Fiber Sensing

Terahertz fibers used for optical-sensing applications are introduced in this chapter, including the dielectric wires, ribbons and pipes. Different analyte conformations of the liquid, solid particle, thin film and vapor gas are successfully integrated with suitable fibers to perform high sensitivities. Based on the optimal sensitivities, analyte recognitions limited in traditional terahertz spectroscopy are experimentally demonstrated by the terahertz fiber sensors. Using the cladding index-dependent waveguide dispersion and high fractional cladding power of terahertz wire fiber, 20 ppm concentration between polyethylene and melamine particles can be distinguished. When the evanescent mode field of a terahertz ribbon fiber is controlled by a diffraction metal grating, subwavelength-confined surface terahertz waves potentially enable the near-field recognition for nano-thin films. Resonance waveguide field surrounding the terahertz pipe fiber is able to identify the macromolecule deposition in subwavelength-scaled thickness, approximately λ/225. For inner core-confined resonance waveguide field inside the terahertz pipe fiber, low physical density of the vaporized molecules around 1.6 nano-mole/mm3 can also be discriminated

Photonic Crystal Millimetre Wave and Terahertz Waveguides and Functional Components

This work discusses both the theoretical and experimental guidance of low-loss single-mode millimetre-wave (mmW) and terahertz (THz) waves within microstructured photonic crystal fibre or waveguides, as well as functional components which can be built upon them. The aim of this work is to provide good interconnects for mmW and THz system. The interconnects are desired to be low loss, single mode, low dispersion, as well as easy to fabricate and integrate. In this work, photonic crystal structures, which can easily manipulate the wave-behaving photons by artificially changing its geometrical and material properties, are used in the proposed mmW and THz waveguides. The proposed photonic crystal waveguides includes cylindrical Bragg fibres and flat hollow photonic crystal integrated waveguides. The geometrical differences between Bragg fibres and photonic crystal integrated waveguides make them work better for different challenges. The former are promising for long distance guidance of signals due to its ultra-low loss, while the latter are strong candidates for compact and multilayer packaging applications since its flatness and other exceptional properties. The thesis has three primary themes. The first them is about the design principles, analysis, and fabrication and measurement of low-loss asymptotically single-mode THz Bragg fibres. A design principle for manipulating the photonic bandgap of Bragg fibres, which is called as the generalized half-wavelength condition, is proposed. Based on the design principle, an ultra-low loss THz Bragg fibre with single mode and low dispersion is proposed, verified by the simulation. Considering practical fabrication challenges, a sub-THz Bragg fibre is fabricated using 3D printing technology and characterized to be one of the lowest loss waveguide at around 300 GHz. The mode transition and filtering in the fabricated sub-THz Bragg fibre is investigated, disclosing the mechanisms of asymptotically single-mode operation pattern of Bragg fibres. The second theme is about the design, fabrication and measurement of single-mode mmW flat and hollow photonic crystal integrated waveguides with low loss and zero group velocity dispersion. The hollow photonic crystal integrated waveguides comprise of air-core line-defect photonic crystal structures sandwiched by a pair of metallic parallel plates. Two different types of photonic crystals are used in the designs, namely hexagonal lattice array of air holes in dielectric slab and Bragg reflectors that consist of periodic arrangement of dielectric layers and air layers. Therefore, two types of hollow photonic crystal integrated waveguides are designed. The designs are fabricated and verified at Ka-band by measurements. The hollow photonic crystal integrated waveguides possess the merits of both substrate integrated waveguide and photonic crystal waveguide, but eliminates their drawbacks, making them strong candidates for compact and multilayer mmW and THz system-in-package applications. The third theme is about the design and simulation of mmW and THz functional components built upon the previously designed microstructured photonic crystal fibres and flat waveguides. The functional components that have been designed include waveguide bends, power splitters or combiners, cavity, h-plane horn antenna, and circular Bragg fibre horn antenna. This theme aims to demonstrate the expansibility and flexibility of the proposed microstructured photonic crystal fibres and flat waveguides as promising platforms for designing mmW and THz functional components. Though each theme discusses the theoretical analysis and/or experimental measurements of distinct phenomena, they are deeply related within the overall theme of engineering low-loss single-mode fibres or waveguides and their integration into mmW or THz systems

Design and Analysis of Advanced Photonic Devices for Electromagnetic Transmission and Sensing

In this thesis, we report the investigation of advanced photonic devices for electromagnetic transmission and biochemical sensing in the terahertz and optical regimes. The choice of material for designing a terahertz device is deemed to be one of the most crucial factors. First, we consider materials that are frequently used in making terahertz devices. We experimentally demonstrate the optical, thermal, and chemical properties of various chosen glasses, polymers, and resin to select the optimal material for terahertz. Second, we perform a broad review on terahertz optical fibres—this includes various fibre categories, their guiding mechanisms, fabrication methodologies, possible experimental methodologies, and applications. Third, we analyse and demonstrate the design of various fibre structures for terahertz transmission and sensing, and then perform experiments on a hollow core antiresonant fibre. We demonstrate successful fabrication of an asymmetrical Zeonex fibre using a novel fabrication method. This is carried out by using a tabletop horizontal extruder designed for producing polymer filaments. The fabricated fibre is then experimentally investigated for terahertz transmission and gas sensing. Fourth, we study optical fibre based surface plasmon resonance biosensors for operation in the optical regime. Theoretical studies are undertaken to obtain the best possible sensor in consideration of performance, experimental feasibility, and fabrication. One of the optimized sensors is then fabricated as a possible candidate for possible realworld sensing applications. Finally, we study metasurface planar devices for achieving high sensitivity and quality factor in the terahertz regime. We first demonstrate a tunable graphene metasurface that can achieve multi-band absorption and high refractometric sensing. Later, we demonstrate on an all-dielectric metasurface that reports highest Q-factor in the terahertz regime. We fabricate and experiment on the dielectric metasurface and find good agreement with the simulation.Thesis (Ph.D.) -- University of Adelaide, School of Electrical & Electronic Engineering, 202

Fabrication and Characterization of Fiber Optical Components for Application in Guiding, Sensing and Molding of THz and Mid-IR Radiation

Le domaine du térahertz (THz) se réfère aux ondes électromagnétiques dont les fréquences sont comprises entre 0.1 et 10 THz, ou encore pour des longueurs d'onde entre 3 mm et 30 μm. Ce type de radiation, qui se situe entre les ondes radios et la lumière infrarouge, possède des propriétés uniques. Les ondes térahertz peuvent passer à travers diverses substances amorphes, plusieurs matériaux synthétiques et des textiles, mais aussi des diélectriques non polaires tels des matériaux à base de pâtes et papiers, qui sont aussi partiellement transparents aux ondes térahertz. Plusieurs biomolécules, protéines, explosifs ou narcotiques, possèdent aussi des lignes d'absorption caractéristiques (entre 0.1 and 2 THz) tels des « codes barres » permettant de les identifier. Il y a deux avantages principaux à l'utilisation des ondes THz: d'une part elles peuvent pénétrer des matériaux normalement opaques à d'autres fréquences, et d'autre part, et elles permettent une haute sélectivité chimique. Par ailleurs, les ondes THz possèdent une basse énergie (1 THz = 4.1 meV), soit un million de fois plus faible que les rayons X, et ne causent pas d'effets photo-ionisant néfastes aux tissus biologiques. Ceci offre un avantage majeur autant pour l'imagerie de tissus biologiques que dans un contexte médical opérationnel, pour lesquels diverses substances doivent être exposées à la radiation THz. Dans le cadre de cette thèse, j'ai travaillé sur trois sujets de recherche principaux. Le premier sujet concerne le développement de nouvelles méthodes de fabrication et de caractérisation térahertz de couches minces composites contenant une matrice de microfils alignés en métal (alliage d'étain) ou en verres semi-conducteurs de chalcogénures (As2Se3). Les matrices de microfils sont fabriquées par la technique d'empilement-et-étirage de fibres employant de multiples étapes de co-étirage de métaux et verres semi-conducteurs à basse température de fusion, avec les polymères. Les fibres sont ensuite empilées et comprimées ensembles pour former des couches minces en composites (i.e. couches de métamatériaux). La transmission optique à travers ces couches minces de métamatériaux a été effectuée sur toute la plage des térahertz (0.120 THz) en combinant les mesures d'un spectromètre infrarouge à transformée de Fourier (FTIR) ainsi qu'un spectromètre térahertz résolu dans le temps (THz-TDS). Les couches de métamatériaux comportant des microfils de métal démontrent de fortes propriétés polarisantes, alors que ceux contenant des microfils semi-conducteurs permettent un large contrôle de l'indice de réfraction tout en étant insensibles à la polarisation incidente. Grâce----------Abstract The terahertz (THz) range refers to electromagnetic waves with frequencies between 100 GHz and 10 THz, or wavelengths between 3 mm and 30 μm. Light between radio waves and infrared has some unique properties. Terahertz waves pass through a variety of amorphous substances, many synthetics and textiles, but also nonpolar dielectric materials, like paper-based materials and cardboard, are transparent to the terahertz waves. Many biomolecules, proteins, explosives or narcotics also have unique characteristic absorption lines, so-called spectral “fingerprints”, at frequencies between 0.1 and 2 THz. There are two main advantages of the terahertz radiation: penetration of conventionally opaque materials on the one hand, and their non-ionising nature on the other hand. Particularly, THz waves have low photon energies (1 THz = 4.1 meV), one million times weaker than X-rays, and will not cause harmful photoionization in biological tissues. This has advantages both for imaging biological materials and in operational contexts where different objects have to be exposed to THz radiation. Within the scope of this work I would like to address three main research topics. In Chapter 2, I describe fabrication method and THz characterization of composite films containing either aligned metallic (tin alloy) microwires or chalcogenide As2Se3 microwires. The microwire arrays are made by stack-and-draw fiber fabrication technique using multi-step co-drawing of low-melting-temperature metals or semiconductor glasses together with polymers. Fibers are then stacked together and pressed into composite films. Transmission through metamaterial films is studied in the whole THz range (0.120 THz) using a combination of FTIR and TDS. Metal containing metamaterials are found to have strong polarizing properties, while semiconductor containing materials are polarization independent and could have a designable high refractive index. Using the transfer matrix theory, it was shown how to retrieve the complex polarization dependent refractive index of the composite films. We then detail the selfconsistent algorithm for retrieving the optical properties of the metal alloy used in the fabrication of the metamaterial layers by using an effective medium approximation. Finally, we study challenges in fabrication of metamaterials with sub-micrometer metallic wires by repeated stack-and-draw process by comparing samples made using 2, 3 and 4 consecutive drawings. When using metallic alloys we observe phase separation effects and nano-grids formation on small metallic wires

Hollow Core Photonic Bragg Fibers for Industrial Sensing Applications

Le rôle de plus en plus important des senseurs optiques dans une multitude d’applications scientifiques et industrielles, incluant la détection biologique, le diagnostic médical, l’industrie alimentaire, le contrôle des procédés et le monitoring environnemental, a mené à un regain de vitalité dans les efforts de recherche et développement dans ce domaine. Pour ces senseurs, la fibre optique peut être une technologie prometteuse en raison de ses nombreux avantages tels que la portabilité, la protection face l’interférence électromagnétique, la possibilité de les utiliser dans des environnements explosifs, ou encore celle d’avoir une mesure quantitative et qualitative continue. Aujourd’hui, une vaste gamme de senseurs à fibre optique a été proposée et développée. Cependant, la plupart de ces senseurs fonctionnent sur la base d’un couplage évanescent des modes de réflexion totale interne (RTI) à proximité de l’analyte. Ceci comporte plusieurs désavantages, tels que le faible chevauchement du mode avec l’analyte, une distance de sensibilité plus petite, le besoin d’apporter des modifications complexes à la fibre, ainsi qu’une faible robustesse mécanique de la fibre. Afin de surmonter ces limitations et simplifier l’implémentation pratique, dans cette thèse, nous proposons l’utilisation de fibres de Bragg à coeur creux opérant dans des plages fréquentielles distinctes (le visible et les térahertz) pour effectuer de la réfractométrie dans des analytes liquides et de surface. Nous mènerons des analyses théoriques et des caractérisations expérimentales des guides d’onde proposés et nous étudierons leur potentiel d’applications dans une variété de domaines industriels. Dans la première partie de la thèse, nous explorerons la capacité d’utiliser des fibres de Bragg à coeur creux dans le visible pour simultanément détecter la partie réelle et imaginaire de l’indice de réfraction d’analytes liquides. Ce senseur de Bragg est constitué d’un large coeur creux (diamètre d’environ 0.7 mm) entouré de couches alternantes de polymethyl methacrylate (PMMA)et de polystyrènse (PS) qui agissent essentiellement comme des réflecteurs de Bragg. Nous utiliserons ce senseur pour le monitoring de la concentration de liquides de refroidissement commerciaux. La stratégie de détection s’appuie sur une double mesure de la position spectrale du centre de la bande interdite et sa transmission en amplitude. Les deux mesures sont hautement sensibles à l’indice de réfraction de l’analyte qui sera introduit dans le coeur creux du guide d’onde. Ceci permettra de déterminer la concentration des liquides de refroidissement.----------Abstract The expanding role of optical sensors in numerous scientific and industrial applications，including biosensing, medical diagnostics, food industry, process control, and environmental monitoring, has led to a growth of research and development efforts in this field. Optical fibers can be used as a very promising platform for these applications, due to many appealing properties such as compactness, high degrees of integration, safety in explosive environments and potential to provide real time and remote analysis. To date, a wide range of fiber-optic sensors have been proposed and developed. Most of these sensors, however, use an evanescent coupling of total internal reflection guided modes to the test analytes, which typically suffers from many disadvantages, such as poor modal overlap with the analytes, limited probing length, as well as complicated fiber modifications and poor mechanical robustness in fiber structures. In order to circumvent these limitations and simplify the practical sensing implementation, in this thesis, we study using hollow-core Bragg fiber sensors operating in different frequency ranges (i.e., visible and terahertz range) for bulk refractometry of liquid analytes and surface sensing applications. We then carry out the theoretical and experimental characterizations of the proposed sensors, and study their applications in various industrial fields. In the first part of the thesis, we explore the capability of using the hollow-core Bragg fibers operating in the visible range to simultaneously monitor both the real and imaginary parts of liquid analyte refractive index. The Bragg fiber sensor features a large hollow core (diameter: ~0.7mm) surrounded by an alternating polymethyl methacrylate (PMMA)/polystyrene (PS) multilayer as a Bragg reflector. We then apply this fiber sensor to monitor the concentrations of various commercial cooling oils. The sensing strategy relies on a two-channel sensing modality that simultaneously interrogates the bandgap center position of the Bragg fiber as well as the fiber transmission amplitude at the bandgap center. Both measurands are highly sensitive to the complex refractive index of the analyte filled in the fiber core, thus enabling efficient determination and cross correlation with the concentration of cooling oils. The presented fiber sensor inherently integrates optical detection with microfluidics without any fiber modifications, thus allowing for real time monitoring of the concentrations of many industrial fluids, such as heat transfer fluids, sawing fluids, and other industrial dilutions with sub-1%v accuracy

Finite element characterisation of plasmonic waveguides in terahertz and optical frequencies

In recent years plasmonic devices have become an interesting area of research due to the sub-wavelength confinement and propagation of radiation, allowing the design of very compact structures. Compact structures are necessary to make smaller integrated optical circuits. Due to the use of metals, plasmonic guides usually show more losses compared to the conventional dielectric guides. Therefore, plasmonic waveguides are not normally used for long distance transmission. However, they are promising for inter-chip or intra-chip communication and also have seen a lot of sensor applications. There has been considerable interest in exploiting the frequency bands in the terahertz regime to open up new frontiers of research across a diverse range of applications. An array of opportunities for creating novel technologies using this frequency band had remained largely unexplored and undeveloped for a considerable period of time due to the lack of suitable sources, as well as lack of guiding and detecting devices. This thesis describes the design, analysis and optimisation of plasmonic devices in optical and terahertz frequencies. A fully vectorial H-field based finite element method has been used in the research reported in this thesis to reveal the modal characteristics of different plasmonic structures. A six layer planar contra-directional nano-coupler has been analysed at optical frequency. Three different modes of propagation were considered to study the characteristics of different properties of the structure, including the coupling length. A design approach has been proposed to make the coupler low loss as well as smaller in length. For the terahertz plasmonics, a rectangular metallic hollow core guide was considered at terahertz frequency. Several modes were considered for the modal analysis of the structure. Modal analysis was performed by changing metal, introducing different dielectric coating in the hollow core, changing the thickness of the metal and dielectric layers and changing the dimensions of the guide. A dispersion analysis was also performed. The criteria for designing very low loss, compact and low dispersion guide have been presented for the structure at the end of the study

Application des fibres creuses à cristal photonique pour la réalisationde résonateurs micro-ondes et de guides d’onde térahertz

The domain terahertz (THz) is a little studied frequency range. The difficulty to generate and detect in this domain has long prevented its development compared to the optical domain (> 100 THz) and microwaves ( 100 THz) et des micro-ondes (< 100 GHz). L’étude proposée dans cette thèse concerne les guides d’onde THz basés sur l’adaptation des structures des fibres optiques à cœur creux composées d’un cristal photonique. Les faibles pertes qu’il est possible d’atteindre avec ce genre de fibres optiques ont été démontrées. L’intérêt de cette adaptation est donc de retrouver les avantages de ce genre de structures au domaine THz. Un chapitre introductif présente plus en détail le domaine THz et les contraintes de ce domaine. Un état de l’art des guides d’onde THz ainsi que le banc de spectroscopie THz dans le domaine temporel développé pour l’étude des guides d’onde sont aussi présentés. Le deuxième chapitre est consacré aux modèles développés en optique pour expliquer le confinement dans les fibres basées sur les cristaux photoniques ainsi que l’adaptation de ces cristaux aux domaines micro-ondes et THz. Le dernier chapitre porte sur l’optimisation et la réalisation de résonateurs micro-ondes et de guides d’onde THz basés sur des cristaux photoniques à bandes interdites adaptés

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Novel Specialty Optical Fibers and Applications

Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications