This thesis presents studies of novel terahertz photonic devices, including photoconductive optoelectronic devices and guided-wave components, aimed at the development of next-generation terahertz systems. In chapter 2, a scalable interdigitated THz transmitter is designed to increase the output power and compared with a conventional 50 μm coplanar transmitter. In chapter 3, we compare four different receivers with different antenna geometries in terms of bandwidth and sensitivity. Then we describe a photoconductive near-field detector with a subwavelength aperture and its system integration and characterization. In chapter 4, a parallel metal plate waveguide is designed with an integrated step inside the waveguide that can couple to higher order TM modes efficiently from the TEM mode. In this chapter, we also experimentally and numerically study a 2-dimensionally tapered parallel plate waveguide, by which a free-space THz beam can be focused into a deep subwavelength-scale volume. In chapter 5, a parallel thin dielectric film waveguide is used to explore the guiding mechanism of an antiresonant optical reflection waveguide. Cylindrical silica single capillaries and a microstructured capillary, which guide in a similar way, are characterized in terms of mode profiles and attenuation. In chapter 6, we study oblique transmission through freestanding thin nickel films, which are perforated with periodic conical hole arrays. Surface modes can be supported by both metallic surfaces with different nonlinear dispersion curves, which results in spectral interferences in a near-field region when the surface modes couple out of the waveguide into free space.

This thesis focuses on improving the performance of the THz time-domain spectroscopy system using second-order nonlinear crystals for THz generation and detection in terms of bandwidth, sensitivity, and THz field strength. The theories for the THz generation based on optical rectification and detection technique, electro-optical sampling, based on Pockels effect are introduced in Chapter 2. In Chapter 3, some experiments are presented to characterize the performances of the THz system based on a 180 fs Yb:KGW femtosecond laser amplifier operating at 1035 nm. The Yb-based femtosecond laser is becoming increasingly popular due to its robustness, high repetition rate, and high average power. However, the NIR bandwidth of these femtosecond lasers is limited by the gain bandwidth of the gain medium, and achieving pulse durations shorter than 180 fs is challenging. Consequently, the full bandwidth of THz time-domain spectroscopy systems is constrained by such laser systems. In order to broaden the THz bandwidth of such THz time-domain spectroscopy systems, our work in Chapter 4 combines the Yb:KGW femtosecond laser amplifier with an argon-filled hollow-core photonic crystal fiber pulse shaper to spectrally broaden the near-infrared pulses from 3.5 to 8.7 THz, increasing the measured THz bandwidth correspondingly from 2.3 THz to 4.5 THz. This is one of the first works to have broadband THz system based on Yb-based femtosecond lasers in the year of 2018. In Chapter 5, the tilted-pulse-front phase matching in the THz generation and detection scheme is demonstrated using the same surface-etched phase gratings on the front surfaces of the 2 mm-thick GaP generation and detection crystals. This scheme overcomes the THz generation and detection bandwidth limit of thick crystals imposed by the traditional collinear phase matching, while allowing the long nonlinear interaction length. This results in a THz spectral range from 0.1 to 6.5 THz with a peak at 3 THz and a peak dynamic range of 90 dB. In the range between 1.1 and 4.3 THz, the system dynamic range exceeds 80 dB. Based on this contact grating-based THz generation, the next step involves generating high-field THz above 2 THz. For high-field THz generation, the most renowned technique is the tilted-pulse-front technique, which generates high-field THz below 2 THz in a LiNbO₃ crystal. Most nonlinear optics experiments in the THz regime rely on such THz sources. To generate high-field THz above 2 THz, one promising candidate is organic THz crystals. However, most organic crystals require a pump laser with a wavelength exceeding 1200 nm, necessitating a more complex laser system. Additionally, the low damage threshold of these crystals are susceptible to compromise the stability of the measurements. Other techniques, such as air plasma and metallic spintronics, can generate ultra-broadband high-field THz from 0.1 to 30 THz, but the pulse energy within certain frequency windows is relatively low, rendering these THz sources less effective for nonlinearly driving specific optical transitions. On the other hand, semiconductor crystals as THz generation crystals, have a high damage threshold and can achieve good phase matching at wavelength around 800 or 1000 nm. In Chapter 6, high-field THz generation with a peak field of 303 kV/cm and a spectral peak at 2.6 THz is achieved with a more homogenous grating on the surface of a 1 mm-thick GaP generation crystal in a configuration collimating the near-infrared generation beam with a pulse energy of 0.57 mJ onto the generation crystal. The experiments also show that the system operates significantly below the GaP damage threshold and THz generation saturation regime, indicating that the peak THz field strength can approach 1 MV/cm, with a 5 mJ near-infrared generation pulse. This is the first high-field THz source based on semiconductor crystals capable of generating high-field THz above 2 THz. With such a THz source, we can conduct nonlinear optics experiments above 2 THz, including the study of phonon-assisted nonlinearities, coherent control of Bose-Einstein condensation of excitons and polaritons in semiconductor cavities, and saturable absorption in molecular gases.

Ce travail de thèse a été réalisé dans le cadre d'un contrat avec l'ANR (ANR-09-VERS-013 « THID ») et porte sur le développement d'une nouvelle génération de tags Chipless à bas coûts fonctionnant dans le domaine THz, pour des applications d'identification et/ou authentification unitaire des articles commerciaux, des papiers d'identités, des personnes pour le contrôle d'accès... Les structures proposées, constituées d'un empilement périodique de couches diélectriques d'indices de réfraction différents, utilisent les propriétés particulières des cristaux photoniques 1D de présenter une réponse électromagnétique entrecoupée de bandes interdites photoniques (BIP). Toute perturbation de la périodicité de la structure engendre des pics dans les bandes interdites qui sont utilisés pour coder une information binaire. Cette structuration particulière des matériaux permet donc de manipuler précisément une signature électromagnétique. Pour des raisons liées à l'industrialisation (facilité de fabrication en masse) et aussi de coût, nous avons retenu des matériaux de base déjà couramment utilisés dans l'industrie papetière : le papier et le polyéthylène. Le choix de ces matériaux, qui doivent allier contraste d'indice élevé et faible absorption, représente une étape cruciale dans ce travail. Ainsi, à partir des résultats expérimentaux obtenus par spectroscopie THz dans le domaine temporel (THz-TDS) sur un grand nombre de matériaux, nous avons pu concevoir deux familles de tags sur la base de ces différents matériaux. Par ailleurs, nous avons développé deux méthodes de codage d'une information binaire, toutes deux basées sur l'absence ou la présence de pics dans une BIP, pics dont la position et le nombre dépendent bien évidemment des défauts de périodicité introduits. Pour des applications liées à l'identification, des capacités de codage de près de 20 bits ont été démontrées. Nous avons aussi montré que la richesse d'information contenue dans la réponse électromagnétique de ces Tags THz peut être utilisée pour les applications liées à l'authentification unitaire, en utilisant comme critère de discrimination le coefficient d'autocorrélation. Nous avons ainsi pu évaluer les performances d'un test d'authentification basé sur ce critère dans différents domaines d'analyse : temporel, fréquentiel et temps-fréquence. Nous avons montré qu'une étude du spectrogramme (combinant temps et fréquence) est ainsi bien plus pertinente qu'une étude dans les seuls domaines temporel ou fréquentiel. / This thesis work deals with the development of a new generation of low-cost Chipless tags operating in the THz frequency domain, it has been supported by the french national agency for research (ANR-09-VERS-013 « THID » ). It covers a wide area of applications such as the identification and/or unitary authentication of commercial items, identity papers, access control…To manufacture these tags, we proposed to use a periodic stack of dielectric material layers with different refractive index and whose thickness is of the order of the wavelength, commonly known as a one dimensional photonic crystal. The electromagnetic signature of such a structure exhibits photonic bandgaps (PBG), i.e. frequency windows in which light propagation is prohibited. We suggested modifying the periodicity of the crystal to create defect levels (peaks) for example in the 1st PBG to encode binary information. This particular structure allows to precisely tuned an electromagnetic signature. To ensure a mass and cost effective industrialization, we retained basic materials which are widely used in the pulp and paper industry: paper and polyethylene. The choice of these materials, which must combine high index contrast and low absorption, represents the first and a crucial step in this work. We characterize a wide range of materials using classical THz time domain spectroscopy (THz-TDS) and we propose two families of tags based on paper and polyethylene. Furthermore, we developed two methods to encode binary information, both based on the absence or presence of peaks in a PBG, peaks whose number and position depend on the introduced defects of periodicity. In a real identification test, a coding capacity of nearly 20-bit has been demonstrated. We also showed that the information contained in the electromagnetic response of these THz tags can be used for other applications related to the unitary authentication and by using the correlation coefficient as criterion for discrimination of the different signatures. Therefore, we evaluate the performance of an authentication test based on this criterion in various analysis domains: time, frequency and time-frequency. We showed that a study of the spectrogram (combining time and frequency representation) is much more relevant than a study in the only time or frequency domain.