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

Terahertz Faraday rotation in a magnetic liquid: High magneto-optical figure of merit and broadband operation in a ferrofluid

We report on the demonstration of a high figure of merit (FOM) Faraday rotation in a liquid in the terahertz (THz) regime. Using a ferrofluid, a high broadband rotation (11 mrad/mm) is experimentally demonstrated in the frequency range of 0.2–0.9 THz at room temperature. Given the low absorption of the liquid, a high magneto-optical figure of merit (5-16 rad.cm/T) is obtained

Suspended core subwavelength fibers: practical designs for the low-loss terahertz guidance

In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 \mu m suspended solid core and a 900 \mu m suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.Comment: 13 pages, 7 figure

Thin chalcogenide capillaries as efficient waveguides in the mid-IR - THz spectral range

We present chalcogenide glass As2Se3 capillaries as efficient waveguides in the mid-IR and THz spectral ranges. The capillaries are fabricated using a double crucible glass drawing technique. The wall thickness of the glass capillary is properly designed and controlled during drawing, and we are able to produce capillaries with different wall thickness, starting from 12 \mum and up to 130 \mum. Such capillaries show low loss properties in the whole target wavelength region. In the mid-IR range guidance is governed by Fresnel reflection and antiguidance mechanisms (ARROWs), while in the THz spectral range thin walls capillaries guide via total internal reflection

Composite THz materials using aligned metallic and semiconductor microwires, experiments and interpretation

We report 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 (0.1-20 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, we show 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

Solid-state-biased coherent detection of ultra-broadband terahertz pulses

Significant progress in nonlinear and ultrafast optics has recently opened new and exciting opportunities for terahertz (THz) science and technology, which require the development of reliable THz sources, detectors, and supporting devices. In this work, we demonstrate the first solid-state technique for the coherent detection of ultra-broadband THz pulses (0.1–10 THz), relying on the electric-field-induced second-harmonic generation in a thin layer of ultraviolet fused silica. The proposed CMOS-compatible devices, which can be realized with standard microfabrication techniques, allow us to perform ultra-broadband detection with a high dynamic range by employing probe laser powers and bias voltages much lower than those used in gas-based techniques. Eventually, this may pave the way for the use of high-repetition-rate ultrafast lasers and commercially available electronics for the coherent detection of ultrashort THz pulses