Les polaritons inter-sous-bandes, observés pour la première fois il y a une quinzaine d’années, sont des quasi-particules dont de nombreuses propriétés restent encore à découvrir. La recherche dans ce domaine se focalise actuellement sur la réalisation de condensats de Bose-Einstein. Une telle découverte pourrait révolutionner l’optoélectronique du moyen infra-rouge jusqu’au THz ouvrant la voie à l’instauration de nouveaux concepts de sources lumineuses,de détecteurs ou de systèmes logiques en couplage fort. Dans cette quête, le choix de la cavité résonnante est critique. Dans ce manuscrit nous proposons d’utiliser des cavités métal-isolant-métal (M-I-M) avec un réseau dispersif sur le métal supérieur. Ce type de cavité,conservant un confinement élevé entre les deux plans métalliques, offre de nombreuses possibilités d’ajustement de la résonance de cavité : via la géométrie de la cavité ( épaisseur de la cavité, période et recouvrement du réseau) ainsi que par le couplage de la lumière avec la cavité (vecteur d’onde incident). Les cavités M-I-M dispersives ouvrent donc un nouveau champ d’exploration des polaritons inter-sous-bande. Dans un premier temps nous avons introduit ces cavités dans le domaine du THz afin d’étudier les phénomènes de relaxation polariton-polariton. Un système expérimental dédié à cette exploration a été conçu pour mesurer la réflectivité des polaritons THz avec une fine résolution en angle. Dans une second temps, des capteurs moyen infrarouge en couplage fort avec une cavité M-I-M dispersive ont été conçus, fabriqués et mesurés dans le but d’explorer la génération de photo-courant à partir de polaritons et d’utiliser le couplage fort pour dissocier l’ énergie de détection de l’énergie d’activation. Cette seconde étude s’inscrit dans l’objectif de pompage électrique des polaritons ISB. Parallèlement à l’étude des polaritons, nous avons également participé au développement de techniques(interféromètre Gires-Tournois et revêtement anti-réflection) pour compresser les impulsions optiques de lasers à cascade quantique THz. / After fifteen years of intersubband polaritons development some of the peculiar properties of these quasi-particles are still unexplored. A deeper comprehension of the polaritons is needed to access their fundamental properties and assess their applicative potential as efficient emitters or detectors in the mid-infrared and THz.In this manuscript we used Metal-Insulator-Metal (MI-M) cavities with a top metal periodic grating as a platform to deepen the understanding of ISB polaritons.The advantages of M-I-M are twofold : first they confine the TM00 mode, second the dispersion of the cavity -over a large set of in-plane wave-vectors- offers various experimental configurations to observe the polaritons in both reflection and photo-current. We reexamined the properties of ISB polaritons in the mid-infrared and in the THz using these resonators. In the first part, we explore the implementation of dispersive M-I-M cavities with THz intersubband transitions. In the THz domain, the scattering mechanisms of the THz ISB polaritons need to be understood. The dispersive cavity is a major asset to study these mechanisms because it provides more degrees of freedom to the system. For this purpose, we fabricated a new experimental set-up to measure the polariton dispersion at liquid Helium temperature. After the characterization of the polaritons in reflectivity, a pump-probe experiment was performed on the polaritonic devices. The second part of this manuscript presents the implementation of M-I-M dispersive cavities with abound-to-quasi-bound quantum well infrared photo detector designed to detect in strong coupling. Beyond electrical probing of the polaritons, the strong coupling can disentangle the frequency of detection from the thermal activation energy and reduce the dark current at a given frequency. In parallel to the exploration of THz polaritons, we developed two techniques (Gires-Tournois Interferometer and Anti-reflection coating) in order to shorten the pulses of THz quantum cascade lasers with metal-metal waveguides.

The region between the infrared and microwave region in the electromagnetic spectrum, the Terahertz (THz) gap, provides an exciting opportunity for future wireless communications as this band has been under utilised. This doctoral work takes a two-pronged approach into closing the THz gap with low-dimensional materials. The first attempt addresses the need for a compact THz source that can operate at room temperature. The second approach addresses the need to build optical elements such as filters and modulators in the THz spectrum. Terahertz quantum cascade lasers (THz QCLs) are one of the most compact, powerful sources of coherent radiation that bridge the terahertz gap. However, their cryogenic requirements for operation limit the scope of the applications. This is because of the electron-electron scattering and heating of the 2-dimensional free electron gas which leads to significant optical phonon scattering of the hot electrons. Theoretical studies in laterally confined QCL structures have predicted enhanced lifetime of the upper state through suppression of the non-radiative intersubband relaxation of carriers, which leads to lower threshold, and higher temperature performance. Lithographically defined vertical nanopillar arrays with electrostatic radius less than tens of nm offer a possible route to achieve lateral confinement, which can be integrated into QCL structures. A typical gain medium in a QCL consists of at least 100 repeat periods, with a thickness of 6-14 micron. For practical implementation of the top-down approach, restrictions are imposed by aspect ratios that can be achieved in present dry-etching systems. Typically, for sub-200 nm radius pillars, the thickness ranges from 1-3.5 micron. It is therefore necessary to work with THz QCLs based on 3-4 quantum well active regions, so as to maximise the number of repeat periods (hence gain) within an ultra-thin active region. After an introductory chapter, Chapter 2 presents a theoretical treatise on the realistic electrostatic potential in a lithographically defined nanopillar by scaling from a single quantum well (resonant tunnelling diode) to a THz QCL. Chapter 2 also discusses, the effect of lateral confinement on the intersubband states and the plasmonic mode in a THz QCL. One of the key experimental challenges in scaling down from QCLs to quantum-dot cascade lasers is the electrical injection into the nanopillars. This involves insulation and planarisation of the high aspect-ratio nanopillar arrays. Furthermore, the choice of the planarising layer is critical since it determines the loss of any optical mode. This experimental challenge is solved in Chapter 3. Chapter 4 presents the electro-optic performance of low-repeat period QCLs with an active region thickness that is less than 3.5 micron. Another topic of recent interest in the THz optics community is plasmonics in graphene. This is because the bound electromagnetic modes (plasmons) are tightly confined to the surface and can also be tuned with carrier concentration. Plasmonic resonance at terahertz frequencies can be achieved by gating graphene grown via chemical vapour deposition (CVD) to a high carrier concentration. THz time domain spectroscopy of such gated monolayer graphene shows resonance features around 1.6 THz superimposed on the Drude-like frequency response of graphene which may be related to the inherent poly-crystallinity of CVD graphene. Chapter 5 discusses these results, as an understanding of these features is necessary for the development of future THz optical elements based on CVD graphene. Chapter 5 finally describes how the gate tunability of THz transmission through graphene can be exploited to indirectly modulate a THz QCL. Chapter 6 presents ideas from this doctoral work, which can be developed in future to address the issues of enhanced temperature performance of THz QCLs and to realise realistic THz devices based on graphene.