Terahertz waveguides are the focus of considerable research interest due to their potential for sensing, imaging and communications applications. Two of the most promising designs are the metal wire waveguide and the parallel-plate waveguide. The metal wire waveguide exhibits excellent low loss and low dispersion characteristics. However, the radiation is only weakly coupled to the wire and the beam extends a great distance from the waveguide, which can lead to high bending loss. In my research I show that this large beam extent also gives a high degree of flexibility in the geometry required to couple radiation into the waveguide or between waveguide sections. I also show that the traditional formalism of bending loss is incomplete, and that there is an optimum radius of curvature to reduce loss. The relationship between the beam extent and the radius of the wire presents the possibility of a tapered waveguide to confine the radiation as it propagates. I here present experimental data and simulations results to verify this subwavelength confinement at the tip of a tapered metal wire waveguide, which is of great interest for near-field imaging applications. The parallel-plate waveguide is another design frequently employed due to its low loss and low dispersion characteristics. Resonant structures may also be easily incorporated into the waveguide for sensing and filtering applications. One such structure is a single rectangular groove, which serves as a notch filter with a very narrow linewidth when the transverse-electric (TE) mode of the waveguide is excited, though its physical origin is poorly understood. In this work I present a detailed experimental and theoretical study of the rectangular resonant cavity in a TE-mode parallel-plate waveguide, particularly with respect to its potential as a microfluidic refractive index sensor. This study is extended to include the possibility of two grooves, in both coupled and non-coupled geometries, and their efficacy as multichannel or high-resolution single-channel microfluidic sensors.

Focaliser l’énergie optique en un petit spot de diamètre beaucoup plus petit que la limite de diffraction a longtemps été un sujet très intéressant en photonique. Dans le domaine Térahertz (avec une longueur d’onde de l’ordre de 300 µm) ce défi est particulièrement important pour répondre à l’intérêt croissant de l’imagerie haute résolution et de la spectroscopie des matériaux d’une taille inférieure à l’échelle submillimétrique de la longueur d’onde en espace libre. Dans ma thèse, j’ai étudié le confinement des ondes de surface aux fréquences THz sur des structures métalliques de dimensions sous longueur d’onde. J’ai expérimentalement mesuré le confinement du champ électrique et calculé la relation de dispersion du mode de surface sur une structure métallique déposée sur un substrat diélectrique de faible permittivité. Ces mesures sont obtenues à l’aide d’un banc de mesures THz guidé, développé pendant ma thèse. La mesure est faite en champ proche par une sonde électro-optique micrométrique, librement positionnable. Ces résultats expérimentaux sont complétés par des simulations numériques, obtenues par le logiciel de simulation par éléments finis, Comsol Multiphysics. Les résultats expérimentaux montrent un confinement de λ/20 du mode EM de surface sur une ligne métallique rectangulaire de petites dimensions. / Focusing optical energy into a small spot diameter much smaller than the diffraction limit has long been a very interesting topic in photonics. In Terahertz (with a wavelength of about 300 microns) this challenge is particularly important to meet the growing interest in high-resolution imaging and spectroscopy of materials whose size is smaller than the wavelength in free space. In my thesis, I studied the confinement of surface waves at THz frequencies on metal structures with sub-wavelength dimensions . I experimentally measured the confinement of the electric field and calculated the dispersion relation of the surface mode on a metal structure deposited on a low permittivity dielectric substrate. These measurements are obtained using a guided-wave time domain spectroscopy set-up, developed during my PhD. The measurement is made by a near-field freely positionable electro-optical probe. These experimental results are supplemented by numerical simulations obtained by finite element analysis software Comsol Multiphysics. The experimental results show a confinement of λ/20 of the EM surface mode on a sub-wavelength dimension rectangular metal wire.