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Ten-millijoule, thirty-five femtosecond, 800 nm (~40 nm bandwidth) laser pulses are used to study high-order harmonic generation in helium- and neon-filled gas cells of various lengths. Harmonic orders in the range of 50 to 100 are investigated. A semi-infinite cell geometry produces brighter harmonics than cells of sub-centimeter length. In the semi-infinite geometry, the gas occupies the region from the focusing lens to a thin exit foil near the laser focus. Counter-propagating light is used to directly probe where the high harmonics are generated within the laser focus and to investigate phase matching. The phase matching under optimized harmonic generation conditions was found to be unexpectedly good with phase zones many millimeters long. Restricting the laser beam with an 8 mm aperture in front of the focusing lens increases the emission of most harmonic orders observed by as much as an order of magnitude. Optimal harmonic generation pressures were found to be about 55 torr in neon and 110 torr in helium. The optimal position of the laser focus was found to be a few millimeters inside the exit foil of the gas cell. Probing with counter-propagating light reveals that in the case of neon the harmonics are generated in the last few millimeters before the exit foil. In helium, the harmonics are produced over a longer distance. Direct measurement shows that the re-absorption limit for mid-range harmonics in neon has been reached.

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.

L'objet de cette thèse porte sur la conception et la réalisation de deux sources laser non linéaires accordables dans les domaines IR et UV, pour le raccordement de la sensibilité spectrale des détecteurs au moyen du radiomètre cryogénique du laboratoire commun de métrologie (LCM). La source IR est un oscillateur paramétrique optique (OPO) résonant sur les ondes pompe et signal (PRSRO), utilisant un cristal de niobate de lithium à inversion de domaines de polarisation dopé par 5% d'oxyde de magnésium (ppMgCLN). Pompé par un laser Ti:Al2O3 en anneau mono-fréquence et accordable, délivrant 500 mW de puissance utile autour de 795 nm, l'OPO possède un seuil d'oscillation de 110 mW. Une couverture spectrale continue entre 1 µm et 3.5 µm a été obtenue, avec des puissances de l'ordre du mW pour l'onde signal (1 µm à 1.5 µm) et des puissances comprises entre $20$ à $50$ mW pour l'onde complémentaire couvrant un octave de longueur d'onde IR entre 1.7 µm et 3.5 µm. La source UV est obtenue par doublage de fréquence en cavité externe du laser Ti:Al2O3, dans un cristal de triborate de lithium (LiB3O5). Un accord de phase en température à angle d'accord de phase fixé permet l'obtention d'une couverture spectrale comprise entre 390 nm et 405 nm. L'asservissement de la cavité de doublage sur la fréquence du laser Ti:Al2O3 par la méthode de Pound-Drever-Hall, ainsi qu'une adaptation de mode optimale, permet d'obtenir une puissance de 5.64 mW à 400 nm à partir de 480 mW de puissance fondamentale. / The work presented in this PhD dissertation details the strategy adopted to build two non-linear laser sources that are widely in the mid-infrared and blue-UV spectral ranges. These laser sources are needed for the traceability to SI units of coherent light irradiance measurements using a cryogenic radiometer of the using cryogenic radiometer of the Laboratoire commun de métrologie (LCM) .The infrared laser source is an optical parametric oscillator (OPO) resonating on the pump and signal wavelengths (PRSRO) and employing a periodically poled Lithium Niobate non-linear crystal doped with 5% magnesium oxide (ppMgCLN). The PRSRO is pumped by a single-frequency tunable bow-tie ring cavity Titanium-Sapphire laser (Ti:Al2O3) delivering 500 mW output power at 795 nm wavelength, , resulting in a power oscillation threshold of 110 mW. The PRSRO emission could continuously cover the spectral range from 1 µm to 3.5 µm. The level of output power achieved is of the order of 1 mW for the signal wave (1 µm to 1.5 µm) and between 20 mW and 50 mW for idler wave spanning an octave wavelength range (1.7 µm to 3.5 µm).The UV source based on the second harmonic generation on the Titanium-Sapphire tunable laser using an external enhancement cavity containing a critically phase-matched LBO non linear crystal (LiB3O5). Temperature-tuning of the phase-matching condition at a fixed crystal orientation leads to a wide tunability from 390 nm to 405 nm wavelength. The external cavity optical pathlength was actively locked to the laser frequency using a¨Pound-Drever-Hall servo, allowing to extract up to 6 mW power at 400 nm wavelength with a 480 mW pump power. Despite a perfect mode-matching efficiency, the power performance was limited by the poor nonlinear impedance matching of the resonator, due to both the weak nonlinearity of the crystal and the low incoming laser power.

Ce travail est consacré à l’étude de la génération de deuxième harmonique (SHG) dans des guides d’ondes de Nitrure d’éléments III. Un des buts principaux de ce travail, était d’identifier les origines des pertes à la propagation dans les guides d’ondes GaN et de fortement les réduire dans des guides présentant des possibilités d’accord de phase, pour améliorer l’efficacité de la SHG. Nous avons fait un progrès très important dans cette direction et avons fabriqué des guides d’ondes plans de GaN épitaxiés sur des substrats de saphir avec des pertes à la propagation inférieure à 1dB/cm dans le visible. Dans ces guides d’ondes à faibles pertes, il a été possible d’obtenir un processus de SHG efficace en utilisant l’accord de phase modal. Nous avons obtenu 2% de conversion entre une pompe dans le proche infrarouge et un harmonique dans le visible, ce qui correspond à une efficacité de conversion normalisée de 0,15%W-1cm-2. Les pertes à la propagation et l’efficacité de conversion obtenues sont les meilleurs résultats rapportés jusqu'ici pour des guides d’ondes plan en GaN. De plus, nous avons étudié des guides d’ondes de Nitrure d’éléments III épitaxiés sur des substrats de Si, dont la fabrication demande de relever plusieurs défis, mais qui ouvrent des possibilités intéressantes. La première est la possibilité de graver sélectivement les nitrures ou le Si, ce que nous avons utilisé pour développer une plate-forme permettant la fabrication d’objets suspendus comme des micro-disques, des guides d’ondes et des micro-disques couplés à un guide d’ondes. Cette plate-forme a permis la première démonstration de SHG doublement résonante en utilisant un accord de phase modal entre des modes de galerie du micro-disque. Bien que toutes les expériences que nous avons exécutées aient été faites dans une région spectrale limitée, l’étude numérique présentée dans ce manuscrit démontre la grande adaptabilité de cette plate-forme basée sur la possibilité de faire varier la composition des guides d’ondes AlGaN de GaN pur à AlN pur. La deuxième possibilité liée à l’épitaxie de nitrures d’éléments III sur Si, est qu'en la combinant avec des techniques de report, on peut jouer avec des guides nitrures d’éléments III sur SiO2. Nos résultats numériques révèlent le potentiel complet des guides d’ondes d’AlGaN en démontrant qu’en utilisant différentes combinaisons de mode et en jouant sur la composition et la géométrie des guides d’ondes, il est possible d’obtenir un signal de deuxième harmonique dans l’ultra-violet, le visible ou le proche-infrarouges. Ces résultats montrent aussi, que pour améliorer encore l’efficacité de la SHG, on doit fabriquer des guides d’ondes canaux présentant un isolement optique parfait du substrat de Si et une inversion de polarité précisément placée dans le cœur du guide d’ondes. Dans une telle structure on pourrait profiter simultanément du confinement de la puissance, de l’accord de phase modal et d’un recouvrement optimisé des modes en interaction. Dans ce cas, nos calculs montrent que l’efficacité de conversion pourrait atteindre 100%W-1cm-2. Au cours de ce travail nous avons pu tester des guides canaux et des guides présentant une inversion de polarité dans le cœur. La qualité des flancs des guides canaux s’est avérée être tout à fait encourageante, mais leur performance non linéaire sont restées très limitées, principalement à cause de fortes pertes à la propagation dues au couplage avec le substrat absorbant et à la forte rugosité de surface des couches inversées. Les structures utilisant les techniques de report, n'ont pu être testées car elles ont cassé en cour de fabrication. L'obtention de guides optimisés exige de progresser encore pour réaliser des couches de confinement optique plus épaisses et/ou d’adapter la technique de report à ces matériaux. / This work is dedicated to the study of the second harmonic generation (SHG) in III-Nitride waveguides. One of the main goals of this work, was to identify the origins of the propagation losses in GaN waveguides, and to strongly reduce them in waveguides presenting some phase matching possibilities, in order to improve the SHG efficiency. We have made a very important progress in this direction, and fabricated by hetero-epitaxy GaN planar waveguides on sapphire substrates with propagation losses below 1dB/cm in the visible spectral region. These low-loss waveguides were used for the demonstration of an efficient second harmonic generation process using modal phase matching. We obtained 2% of power conversion from the near-infrared to the visible spectral regions with a normalized efficiency of 0.15%W-1cm-2. The obtained propagation losses and conversion efficiency are the best-reported results so far for GaN planar waveguides. In addition, we have studied epitaxial III-nitride waveguides on Si substrates, which are very challenging to fabricate, but opens new interesting opportunities. The first one is the possibility to etch selectively the nitrides or the Si. The selective chemical etching was used to develop a platform allowing the fabrication of suspended objects such as micro-disks, waveguides and micro-disks coupled to a waveguide. This platform has allowed the first demonstration of doubly resonant SHG using phase matching between the whispering gallery modes of a micro-disk. Although all the experiments we performed were done in a limited spectral region, the numerical study presented in this manuscript demonstrates the large adaptability of this platform based on the possibility of varying the composition of AlGaN waveguides from pure GaN to pure AlN. The second opportunity of epitaxial III-nitrides layers on Si is the possibility to combine them with report technologies to obtain III-nitride waveguides on SiO2. Our numerical results reveal the full potential of AlGaN waveguides by demonstrating that using different mode combinations and playing with waveguides composition and geometry, it is possible to obtain a second harmonic signal in the ultraviolet, the visible or the near-infrared spectral regions. These results also demonstrate, that to further improve the SHG efficiency, one has to fabricate ridge waveguides presenting a perfect optical isolation from the Si substrate and a polarity inversion precisely positioned in the core of the waveguide. In these structures one could benefit simultaneously from the power confinement, the modal phase matching and an optimized overlap of the interacting modes. In this case, we calculate that the conversion efficiencies could be as high as 100%W-1cm-2. Both ridge waveguides and polarity inversion were tested in this work. The quality of the ridges was quite encouraging, but their nonlinear performance remained limited mainly because of the high propagation losses due to the coupling with the absorbing substrate and to the roughness of the surface of the epitaxial inverted layers. The structures fabricated using the report technique, haven’t been tested, as they were broken during their fabrication. Getting fully optimized waveguides requires further progresses in realizing thicker optical buffer layers and/or adapting the report technique to these materials.

Since its inception, the field of nonlinear optics has only increased in importance as a result of a growing number of applications. The efficiency of all parametric nonlinear optical processes is limited by challenges associated with phase-matching requirements. To address this constraint, a variety of approaches, such as quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have historically been used to phase-match interactions. However, the methods demonstrated to date suffer from the inconvenience of only being phase-matched for one specific arrangement of beams, typically co-propagating along the same axis. This stringency of the phase-matching requirement results in cumbersome optical configurations and large footprints for integrated devices. In this thesis, we show that phase-matching requirements in parametric nonlinear optical processes may be satisfied for all orientations of input and output beams when using zero-index media: a condition of omnidirectional phase matching. To validate this theory, we perform experimental demonstrations of phase matching for five separate FWM beam configurations to confirm this phenomenon. Our measurements constitute the first experimental observation of the simultaneous generation of a forward- and backward-propagating signal with respect to the pump beams in a medium longer than a free-space optical wavelength, allowing us to determine the coherence length of our four-wave-mixing process. Our demonstration includes nonlinear signal generation from spectrally distinct counter-propagating pump and probe beams, as well as the excitation of a parametric process with the probe beam's wave vector orthogonal to the wave vector of the pump beam. By sampling all of these beam configurations, our results explicitly demonstrate that the unique properties of zero-index media relax traditional phase-matching constraints, and provide strong experimental evidence for the existence of omnidirectional phase matching in zero-index media. This property can be exploited to facilitate nonlinear interactions and miniaturize nonlinear devices, and adds to the established exceptional properties of low-index materials.

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This research investigates the realization of parametric amplification in superconducting circuits and structures where nonlinearity is provided by Josephson junction (JJ) elements. We aim to develop a systematic analysis over JJ-based devices toward design of novel traveling-wave Josephson parametric amplifiers (TW-JPA). Chapters of this thesis fall into three categories: lumped JPA, superconducting periodic structures and discrete Josephson transmission lines (DJTL). The unbiased Josephson junction (JJ) is a nonlinear element suitable for parametric amplification through a four-photon process. Two circuit topologies are introduced to capture the unique property of the JJ in order to efficiently mix signal, pump and idler signals for the purpose of signal amplification. Closed-form expressions are derived for gain characteristics, bandwidth determination, noise properties and impedance for this kind of parametric power amplifier. The concept of negative resistance in the gain formulation is observed. A design process is also introduced to find the regimes of operation for gain achievement. Two regimes of operation, oscillation and amplification, are highlighted and distinguished in the result section. Optimization of the circuits to enhance the bandwidth is also carried out. Moving toward TW-JPA, the second part is devoted to modelling the linear wave propagation in a periodic superconducting structure. We derive closed-form equations for dispersion and s-parameters of infinite and finite periodic structures, respectively. Band gap formation is highlighted and its potential applications in the design of passive filters and resonators are discussed. The superconducting structures are fabricated using YBCO and measured, illustrating a good correlation with the numerical results. A novel superconducting Transmission Line (TL), which is periodically loaded by Josephson junctions (JJ) and assisted by open stubs, is proposed as a platform to realize a traveling-wave parametric device. Using the TL model, this structure is modeled by a system of nonlinear partial differential equations (PDE) with a driving source and mixed-boundary conditions at the input and output terminals, respectively. This model successfully emulates parametric and nonlinear microwave propagation when long-wave approximation is applicable. The influence of dispersion to sustain three non-degenerate phased-locked waves through the TL is highlighted. A rigorous and robust Finite Difference Time Domain (FDTD) solver based on the explicit Lax-Wendroff and implicit Crank-Nicolson schemes has been developed to investigate the device responses under various excitations. Linearization of the wave equation, under small-amplitude assumption, dispersion and impedance analysis is performed to explore more aspects of the device for the purpose of efficient design of a traveling-wave parametric amplifier. Knowing all microwave characteristics and identifying different regimes of operation, which include impedance properties, cut-off propagation, dispersive behaviour and shock-wave formation, we exploit perturbation theory accompanied by the method of multiple scale to derive the three nonlinear coupled amplitude equations to describe the parametric interaction. A graphical technique is suggested to find three waves on the dispersion diagram satisfying the phase-matching conditions. Both cases of perfect phase-matching and slight mismatching are addressed in this work. The incorporation of two numerical techniques, spectral method in space and multistep Adams-Bashforth in time domain, is employed to monitor the unilateral gain, superior stability and bandwidth of this structure. Two types of functionality, mixing and amplification, with their requirements are described. These properties make this structure desirable for applications ranging from superconducting optoelectronics to dispersive readout of superconducting qubits where high sensitivity and ultra-low noise operation is required.