The idea of terahertz-time-domain spectroscopy (THz-TDS) is to exploit a single cycle, spectrally broad THz radiation pulse to gain insight into the response of matter. Photoconductive devices and nonlinear crystals are utilized in both the generation as well as the coherent detection of THz radiation. The relatively high cost and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems for commercial applications. Er-doped femtosecond fiber lasers operating at 1.55 μm could offer a viable alternative. In this thesis nonlinear crystals and photoconductive emitters are discussed for excitation in the near infrared (NIR) window of between 800 nm to 1550 nm. The main focus of this thesis is a detailed study of substrate materials for an interdigitated photoconductive antenna. Photoconductive antennas with microstructured electrodes provide high electric acceleration fields at moderate voltages because of small electrode separations. The scalability of these devices allows for large active areas in the mm^2 range, which are sufficient for excitation at large optical powers. In comparison with conventional emitter structures, these antennas have more favourable characteristics regarding THz power, spectral properties, and ease of handling. Depending on the utilized substrate material, photoconductive antennas can then be operated using different excitation wavelengths. By employing substrates with short carrier trapping times these antennas can be operated as THz-detectors. Moreover the design of electrode structures for generating radially and azimuthally polarized THz waves are presented. A second topic deals with the signal analysis and signal interpretation of THz pulses transmitted through several material systems. These experiments show the potential for tomographic and spectroscopic applications. The third part deals with THz emission by frequency mixing in nonlinear organic and inorganic crystals. Hereby the focus is on polaritonic phase matching in GaAs. Furthermore, indications of THz tunability by the excitation wavelength were found by utilizing waveguide structures. However, the observed tuning range is much lower then theoretically predicted. Specific reasons for this are discussed.

Les lasers solides émettant dans l'UV sont l'objet de nombreux efforts de recherche. Ainsi, deux cristaux non linéaires ont été développés et caractérisés pour réaliser la conversion de fréquences laser dans le domaine UV : Ca5(BO3)3F (CBF) et YAl3(BO3)4 (YAB). Concernant les cristaux de CBF, l'influence des gradients thermiques ainsi que de nouveaux flux ont été étudiés en vue d'améliorer la qualité cristalline. La génération du troisième harmonique à 343 nm en type II a été réalisée pour la première fois dans CBF. 300 mW correspondant à un rendement ? (1030 ' 343 nm) = 1,5 % ont été obtenus. L'utilisation de flux à base de LaB3O6 a permis la synthèse de cristaux de YAB par la méthode TSSG. Les propriétés physiques, structurales ainsi que les défauts de ces cristaux ont été caractérisés. De bonnes performances ont été obtenues dans le cadre de la génération du quatrième harmonique à 266 nm puisqu'un rendement de conversion ? (1064 ' 266 nm) = 12,2 % a été atteint. / This PhD study is devoted to the growth and characterization of two promising NLO crystals for UV laser light generation : Ca5(BO3)3F (CBF) and YAl3(BO3)4 (YAB). Concerning CBF, the influence of thermal gradients and new fluxes have been studied in order to improve the crystal quality. The third harmonic generation at 343 nm in CBF is demonstrated for the first time. 300 mW of average power and 1,5 % conversion efficiency from 1030 to 343 nm have been obtained. The use of the flux LaB3O6 allows the growth YAB crystals by TSSG method. Structural and physical properties as well as extended defects and impurities of these crystals have been characterized. Fourth harmonic generation at 266 nm was performed in YAB and 12,2 % conversion efficiency from 532 to 266 nm has been obtained.

This Master’s dissertation focuses on engineering artificial nanostructures, namely, arrays of metamolecules on a substrate (metasurfaces), with the goal to achieve the desired linear and nonlinear optical responses. Specifically, a simple analytical model capable of predicting optical nonlinearity of an individual metamolecule has been developed. The model allows one to estimate the nonlinear optical response (linear polarizability and nonlinear hyperpolarizabilities) of a metamolecule based on the knowledge of its shape, dimensions, and material. In addition, a new experimental approach to measure hyperpolarizability has also been investigated. As another research effort, a 2D plasmonic metasurface with the collective behaviour of the metamolecules known as hybrid plasmonic-Fabry-Perot cavity and surface lattice resonances was designed, fabricated and optically characterized. We experimentally discovered a novel way of coupling the microcavity resonances and the diffraction orders of the plasmonic metamolecule arrays with the low-quality plasmon resonance to generate multiple sharp resonances with the higher quality factors. Finally, we experimentally observed and demonstrated a record ultra-high-Q surface lattice resonance from a plasmonic metasurface. These novel results can be used to render highly efficient nonlinear optical responses relying on high optical field localization, and can serve as the stepping stone towards achieving practical artificial nanophotonic devices with tailored linear and nonlinear optical responses.

This thesis explores the use of nonlinear crystals from theKTiOPO4(KTP) family with the aim to extend the possibleapplications for laser sources and to gain more knowledge aboutthe material’s benefits and limits. The work focussed onoptical parametric oscillators (OPOs) and optical parametricamplifiers (OPAs), which employ second order nonlinearprocesses. Both devices transfer energy from a laser beam at aparticular wavelength to a different wavelength, which istuneable. In OPOs two new beams at different wavelengths aregenerated, whereas in OPAs an existing weak beam is amplified.The essential part of these devices, which enables theoccurrence of the energy conversion, is a nonlinear crystal. Inthis work the ferroelectric crystals KTP and RbTiOPO4(RTP) have been utilized. By modifying the material’s structure,quasi-phasematching can be obtained, which is a crucialrequirement for achieving efficient energy conversion betweenthe incident and the generated waves. The fabrication ofquasi-phasematched crystals is dependent on the controlledreversion of the material’s spontaneous polarisation,which is accomplished by periodic electric field poling. Nanosecond pulses of more than 200 kW were generated in the“eye-safe”region by employing a double pass OPA.Small signal gains exceeding 75 dB were obtained for anessentially diffraction limited beamwithout spectralbroadening of the seed. By subsequent signal coupling intofibres substantial broadening was accomplished. A systematicmeasurement series of several RTP crystals allowed us toaccurately determine the wavelength and temperature dispersionof the refractive index, which are two essential requirementsfor further employment of this material. The OPOs based on RTPwere widely tuneable by controlling the temperature. It wasalso concluded that RTP behaves similar to KTP in parametricdevices, thus being a material, which can sustain high powers,possesses large nonlinear coefficients and can operate in abroad wavelength region.Efficient Raman oscillation concurrent with parametricoscillation was observed and analysed in several KTP samples.This gave further insight into the processes taking placeinside the material when performing as a frequency converter,if the generated idler lies in the absorption band.This thesis also covers the investigation of afemtosecond optical parametric chirped pulse amplifier.Temporally stretched seed pulses were amplified to 85 µJ,resulting in a gain above 60 dB, and subsequent recompressionresulted in 270 fs pulses. <b>Keywords:</b>nonlinear optics, KTiOPO4, optical parametric oscillator, optical parametricamplifier, RbTiOPO4, quasi-phasematching, electric field poling,stimulated Raman scattering. / NR 20140805

After decades of development, optical fiber technology has reached a high degree of sophistication and maturity, and currently serves as the backbone of today''s internet. Despite its technical versatility and capability, current silica fiber technology still has a significant flaw: since silica fibers only possess very weak second order nonlinearity, it has been impossible to develop a large number of important nonlinear optical devices and instruments, such as optical parametric amplifiers (OPA) and optical parametric oscillators (OPO). In this thesis,we show how to overcome this intrinsic limitation, and introduce second order nonlinearity into silica fiber devices. / Master of Science

Plasmonic metasurfaces enable functionalities that extend beyond the possibilities of classical optical materials and as a result, have gained significant research interest over the years. This thesis aims towards introducing plasmonic metamaterials and metasurfaces, a two-dimensional subset of metamaterials. The thesis also provides insights into the nonlinear optical responses from subwavelength metallic nanostructures manifesting as extraordinary physical phenomena like the second harmonic generation (SHG). The hydrodynamic Drude model is a theory that characterizes electron conduction in a hydrodynamic way to predict optical responses of metals. The thesis discusses the various contributions to the second-order optical nonlinearities from the terms in the hydrodynamic model: Coulomb, convection, and the Lorentz magnetic force. The significance of these terms, specifically the Lorentz magnetic term, is validated in contrast with existing research. The details of the work carried out to achieve a significant contribution to SHG from the Lorentz magnetic term are provided. A dominant Lorentz magnetic force for SHG was achieved through engineering T-shaped aperture arrays milled into a thin gold film. The dimensions of these structures were tuned for fundamental wavelength resonance. The structures exhibit both magnetic and electric field enhancements at the plasmonic resonance. Furthermore, a revised theoretical model is developed to accurately predict both linear and nonlinear optical responses of metamaterials. The model is based on the hydrodynamic Drude model and nonlinear scattering theory. Results from the finite difference time domain simulations performed on the metasurface are presented. It is observed that the T-shaped structure provides 65% greater nonlinear generation from the Lorentz magnetic term than the sum of the other two hydrodynamic terms. The influence of incident beam polarization on SHG conversion efficiency was also investigated. It was discovered that even though the contributions of hydrodynamic (Coulomb and convection) terms are maximum at 0◦ and 90◦, the metasurface shows maximum SHG intensity at 45◦ which indicates a dominant Lorentz magnetic term. Experimental validation was performed using the fabricated metasurface and a good agreement between the experiment and theoretical calculations was observed. Another aspect of the magnetic Lorentz force contribution, Bethe’s aperture theory was evaluated for a circular aperture at off-normal incident light. It is shown that the Lorentz force dominates the SHG by an order of magnitude at angled incidence where the generation is maximized. The angular dependence was observed to match the magnetic and electric dipole interaction effects as predicted from Bethe’s theory. The revised theory developed in this thesis predicts the linear and nonlinear optical responses of metamaterials including their angular dependency. The analysis and numerical calculations for a circular aperture agree well with past experiments. To conclude, the thesis provides an outlook on future developments in the field of nonlinear plasmonic research with regards to the development of highly efficient nonlinear metasurfaces through optimization of the Lorentz contributions. An insight into the recent developments in nanofabrication capabilities, design methodologies, nano-characterization techniques, modern electromagnetic simulations is discussed as avenues for future research in nanophotonic and nanoplasmonic device design and development. / Graduate

Les convertisseurs de fréquences optiques utilisant les processus non linéaires d’ordre deux permettent d’étendre la gamme spectrale accessible aux sources lasers. Celle-ci est en effet limitée par les bandes de gain des milieux lasers disponibles. Par ailleurs, la combinaison cohérente par contrôle actif de la phase est une technique efficace permettant la montée en puissance des sources lasers. Elle nécessite toutefois des modulateurs de phase rapides qui ne sont disponibles commercialement qu’aux longueurs d’onde standard.L’objectif de cette thèse est d'appliquer la combinaison cohérente à des convertisseurs de fréquences en utilisant la relation de phase inhérente au processus non linéaire.Cela permet de contrôler la phase de l’onde générée en agissant sur la phase de l’onde de pompe. C’est ce qu’on appelle le contrôle indirect de la phase.Pour cela, une étude théorique a été menée afin de s’assurer de la compatibilité de la technique de combinaison cohérente par marquage en fréquence avec le contrôle indirect de la phase.La démonstration expérimentale a d’abord été effectuée dans le cas le plus simple, la génération de seconde harmonique, qui met en jeu trois ondes dont deux dégénérées.Enfin, on s’est intéressé au cas de la génération de différence de fréquences dans le moyen infrarouge, où trois ondes sont mises en jeu.Dans ces deux cas, la qualité de mise en phase mesurée est excellente. / Laser emission wavelength is limited by the gain bandwidth of available laser media. Optical frequency converters rely on second order nonlinear processes to overcome this limitation, and give access to new wavelengths outside of the emission range of lasers.Besides, coherent beam combining with active phase control is an efficient technique to power scale laser sources. However, it requires fast phase modulators, some commercially available devices, but only at standard laser wavelengths.The objective of this thesis is to perform coherent combining of frequency converters, thanks to the phase-matching condition required for efficient nonlinear processes to take place.This relation allows for indirect phase control, where the converted-wave phase is tuned through direct phase control of the pump wave.A theoretical study has been carried out to confirm that indirect phase control was compatible with frequency-tagging coherent combining.Then, coherent combining through indirect phase control has been demonstrated experimentally in the simplest case of the second harmonic generators, where two of the three waves involved are degenerate.Finally, coherent combining has been experimentally performed in the non-degenerate case of mid-infrared difference frequency generators.In both experimental demonstrations, an excellent beam combination efficiency has been achieved.

Les impulsions laser ultrabrèves nous permettent de suivre en temps réel les phénomènes ultrarapides au sein de la matière à l’échelle microscopique. C’est précisément pour l’invention de la chimie à l’échelle femtoseconde, ou femtochimie, qu’Ahmed Zewail se vit décerner le prix Nobel de chimie en 1999. Depuis les utilisateurs du laser cherchent à augmenter la résolution temporelle, c’est-à-dire réduire la durée des impulsions laser. Aujourd’hui, nous savons générer des flashs lumineux à l’échelle attoseconde dans le domaine spectral de l’extrême ultraviolet (XUV) mais l’efficacité de génération reste faible et le développement de sources laser attosecondes intenses constitue un sujet de recherche très actif sur le plan international.Notre groupe au LOA se concentre sur la génération d’impulsions attoseconde sur miroir plasma en régime relativiste. Pour cela, il cherche à développer une source d’impulsions femtosecondes à forte cadence et fort contraste et suffisamment énergétiques pour atteindre des intensités relativistes (>> 10^18W/cm2) lorsqu’elles sont fortement focalisées sur un plasma surdense. Un plasma surdense réfléchit la lumière incidente et par conséquent agit comme un miroir qui se déplaçant à vitesse relativiste et qui comprime l’impulsion incidente, produisant ainsi un flash attoseconde par cycle optique. En utilisant des impulsions proches d’un cycle optique, il est donc envisageable de générer une seule impulsion attoseconde intense pendant l’interaction.Dans la première partie de mon travail de thèse, j’ai réalisé un compresseur nonlinéaire pour réduire la durée des impulsions issues d’une chaîne à double dérive de fréquence (10mJ, 25fs, 1kHz) à phase enveloppe-porteuse (CEP) stabilisée. En propageant les impulsions du laser à haute intensité dans une fibre creuse remplie de gaz rare, j’ai réussi à générer des impulsions de 1.3 cycle optique avec une puissance crête autour de 1TW avec une CEP stabilisée. Dans un deuxième temps, j’ai mis en forme spatialement et temporellement les impulsions issues du compresseur à fibre pour générer à la fois des impulsions attosecondes intenses et des faisceaux d’électrons énergétiques sur un miroir plasma à gradient de densité contrôlé. Ces expériences nous permis, pour la première fois, de mettre en évidence la production d’impulsions attosecondes isolées dans l’XUV, l’émission corrélée de faisceaux d’électrons énergétiques en régime relativiste ainsi qu’un nouveau régime d’accélération d’électrons à très long gradient plasma. / Very short light pulses allow us to resolve ultrafast processes in molecules, atoms and condensed matter. This started with the advent of Femtochemistry, for which Ahmed Zewail received the Novel Prize in Chemistry in 1999. Ever since, researcher have been trying to push the temporal resolution further and we have now reached attosecond pulse durations. Their generation, however, remains very challenging and various different generation mechanisms are the topic of heated research around the world.Our group focuses on attosecond pulse generation and ultrashort electron bunch acceleration on solid targets. In particular, this thesis deals with the upgrade of a high intensity, high contrast, kHz, femtosecond laser chain to reach the relativistic interaction regime on solid targets. Few cycle driving laser pulses should allow the generation of intense isolated attosecond pulses. A requirement to perform true attosecond pump-probe exeriments.To achive this, a HCF postcompression scheme has been conceived and implemented to shorten the duration of a traditional laser amplifier. With this a peak intensity of 1TW was achieved with near-single-cycle pulse duration. For controlled experiments, a vacuum beamline was developed and implemented to accurately control the laser and plasma conditions on target.During the second part of this thesis, this laser chain was put in action to drive relativistic harmonic generation on solid targets. It was the first time ever that this has been achieved at 1 kHz. By CEP gating the few-cycle-pulses, single attosecond pulses were generated. This conclusion has been supported by numerical simulations. Additionally a new regime to accelerate electron bunches on soft gradients has been detected.

L’optique non linéaire est un outil très puissant pour l’étude des propriétés photoniques de molécules, de matériaux et de nanostructures. La taille et la forme des nanoparticules de métaux nobles (NMNPs) influencent fortement leurs propriétés optiques non linéaires du second ordre. Dans cette thèse, nous proposons une étude systématique de l'influence de la surface de nanoparticules sur leurs valeurs de première hyperpolarisabilité bêta. Des nanoparticules en poudre d’argent (de diamètres 7 nm) ainsi que des solutions colloïdales sur NMNPs -avec différentes compositions, tailles et formes -ont été synthétisés : des nanosphères d'argent (de diamètres 10 nm), des nanosphères d’or (de diamètres 3,0; 11,6; 15,8; 17,4; 20,0 et 43,0 nm), des nanobâtonnets d’or (de rapports d'aspect 1,47; 1,63 et 2,30), des nanobâtonnets d’argent (de rapports d'aspect 5,0; 6,3; 7,5; 8,2 et 9,7), des nanofleurs de platine (de diamètres 7,0; 8,0; 10,0; 14,0; 20,0 et 31,0 nm) ainsi que des nanoprismes d'or (d’une longueur de côtés de 47,5 à 112,3 nm). La diffusion harmonique de la lumière (HLS) à 1064 nm est utilisée pour étudier la génération du second harmonique des NMNPs colloïdaux, et d'en déduire leurs valeurs de première hyperpolarisabilité bêta. Pour les nanosphères et les nanorods étudiés dans ce travail, nous démontrons que leurs valeurs de bêta présentent une forte dépendance avec leur surface, qui est le paramètre dominant dans l'évolution des valeurs de bêta. Par ailleurs, la rugosité de la surface des particules ainsi que la forme des irrégularités des nanofleurs sont responsables de valeurs exceptionnellement élevées de bêta. En outre, nous démontrons expérimentalement, pour la première fois dans la littérature, que les valeurs de bêta des nanoprismes présentent non seulement une dépendance linéaire par rapport à la surface, mais sont également sensibles aux courbures des sommets du triangle. / Nonlinear optics is well known to be a highly powerful tool to investigate the photonic properties of molecules, materials and nanostructures. Size and shape of noble metal nanoparticles (NMNPs) strongly influence their second-order nonlinear optical properties. In this PhD thesis, we propose a systematic investigation of the influence of the nanoparticle surface area on their first hyperpolarizability beta values. Powdery-silver nanoparticles (diameters 7 nm) and colloidal solutions on NMNPs with different composition, sizes and shapes have been synthesized, i.e. silver nanospheres (diameters 10 nm), gold nanospheres (diameters 3.0; 11.6; 15.8; 17.4; 20.0 and 43 nm), gold nanorods (aspect ratios 1.47; 1.63 and 2.30), silver nanorods (aspect ratios 5.0; 6.3; 7.5; 8.2 and 9.7), platinum nanoflowers (diameters 7.0; 8.0; 10.0; 14.0; 20.0 and 31.0 nm) and gold nanoprisms (edge length tuned from 47.5 to 112.3 nm). Harmonic light scattering (HLS) at 1064 nm is used to investigate the second harmonic generation from colloidal NMNPs, and to infer their first hyperpolarizability tensor beta. For the nanospheres and nanorods investigated in this work, we demonstrate that their beta values display a strong dependence with their surface area, which is the dominant parameter in the evolution of beta values. Otherwise, particle surface corrugation and shape irregularities of nanoflowers are responsible for exceptionally high beta values. Moreover, we report for the first time in the literature that the beta values of nanoprisms display not only a linear dependence with respect to the surface area, but are also sensitive to the sharpness of the triangle vertices.

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(理学) / 甲第24401号 / 理博第4900号 / 新制||理||1700(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 石田 憲二, 教授 田中 耕一郎 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM