Orientador: Luis Eduardo Evangelista de Araujo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T01:12:06Z (GMT). No. of bitstreams: 1 Carvalho_SilvaniaAlvesde_M.pdf: 47841969 bytes, checksum: 80b56cd870f97d9119c7f4935d7685ad (MD5) Previous issue date: 2006 / Resumo: Neste trabalho estudamos o fenômeno de transparência induzida eletromag-neticamente (EIT) em sistemas de três níveis nas configurações lambda, V e cascata. Nestes sistemas, a transparência de um feixe de prova é induzida por um feixe de acoplamento mais forte. Uma comparação sistemática da largura do sinal de EIT e da dispersão no meio para os três sistemas, usando diferentes valores relativos de comprimento de onda dos dois feixes, é apresentada para meios atômicos alargados inomogeneamente. A dependência da EIT com o ângulo entre os feixes de prova e acoplamento é também investigada. Observamos que, embora os sinais de EIT mais estreitos e profundos, e uma maior inclinação da curva de dispersão sejam obtidos para o sistema lambda, o sistema V é o mais robusto com relação ao ângulo entre os feixes e ao comprimento de onda relativo dos feixes. Por fim, apresentamos os resultados obtidos no experimento realizado em um sistema de 3 níveis na configuração cascata levando em conta os níveis 1S01 1P01 e 1D2em átomos de cálcio, descrevendo o uso inédito da técnica de deteção optogalvânica na observação de EIT. Neste experimento observamos EIT em uma lâmpada de catodo oco de Ca com vapor de fundo de Kr que nos forneceu até 76% de cancelamento da absorção de um laser em 423 nm induzido por um laser amarelo 586 nm forte contrapropagante. EIT neste mesmo sistema cascata considerando uma geometria copropagante dos feixes de laser foi observada / Abstract: In this work we studied the phenomenon of electromagnetically induced trans-parency (EIT) in three level systems in the L, Vee and cascade configurations. In these systems, the transparency of a probe beam is induced by a strong coupling beam. A systematic comparison of the EIT signal width and the dispersion in the medium for the three systems for different relative values of wavelength of the two beams is presented for inhomogeneously Doppler-broadened medium. EIT dependence on the angle between the probe and coupling beams is also investigated. We observe that although the narrowest and deepest EIT signal, and the greatest slope of the dispersion curve are obtained for the L system, the V system is the more robust relative to the angle between the beams and to the relative wavelength of the beams. Finally, we present the results obtained in an experiment in a three level cascade system considering the levels 1S01 1P01 and 1D2 in Ca atoms. We describe, for the first time to our knowledge, the use of the optogalvanic detection technique for observing EIT. In this experiment we observed EIT in a Ca hollow cathode lamp with Kr buffer gas, obtaining up to 76% cancellation of absorption of a blue laser beam (423 nm) induced by a strong and counterpropagating yellow beam (586 nm). EIT in a cascade system considering a copropagating geometry of the laser beams was observed. / Mestrado / Física Atômica e Molecular / Mestre em Física

Orientador: Solange Bessa Cavalcanti / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T09:40:15Z (GMT). No. of bitstreams: 1 LozadaVera_JohnJairo_M.pdf: 3582168 bytes, checksum: b939d006da96c563046ce0074e25f72e (MD5) Previous issue date: 2009 / Resumo: Estuda-se a interação entre pulsos ópticos ultracurtos e materiais com band gap fotônico, considerando-se a propagação de luz através de estruturas unidimensionais, compostas de uma super-rede dielétrica periódica cuja célula unitária consiste em um par de camadas com diferentes índices de refração n1e n2, respectivamente. Em particular considera-se o caso em que n2, por exemplo, é um material com não linearidade X (3). É bem sabido que a largura e a localização dos band gaps dependem fundamentalmente do contraste entre os índices de refração d n = | n1 - n1 | e, atribuindo-se a n2 um índice de refração dependente da intensidade, conseguimos controlar dinamicamente a largura do band gap com o pulso de luz incidente. Portanto, a idéia básica é investigar a dinâmica de propagação nas vizinhanças de um band gap dependente da intensidade para aplicações importantes no projeto de dispositivos fotônicos, como por exemplo limitadores de luz e switches puramente ópticos. Dentro do formalismo de Maxwell, experimentos computacionais são feitos considerando a propagação de campos ópticos através de super-redes que possuem não linearidades do tipo Kerr, assim como saturável. Esta última é importante para a descrição da interação com pulsos muito intensos já que neste caso a mudança induzida no índice de refração depende de não linearidades de ordem mais alta e, como consequência, alcança uma saturação. Este modelo é apropriado para descrever materiais tais como vidros dopados com semicondutores (e.g. CdS 1-x Sex) polímeros orgânicos, que possuem propriedades ópticas altamente não lineares. A propagação da onda é resolvida usando uma versão modificada no domínio do tempo do método standard fast Fourier Beam Propagation Method (FFT-BPM) com um passo adaptativo, que pode manejar transmissão, difração e, especialmente, reflexões das ondas electromagnéticas causadas por descontinuidades no índice de refração, com a vantagem de não requerer a introdução de condições de contorno adicionais. / Abstract: The interaction of ultrashort optical pulses with photonic band gap materials has been studied by considering light propagation through one-dimensional photonic band gap structures, composed of a periodic multilayer stack of dielectric materials whose unitary cell consists of a pair of layers with different refractive indices n1and n2, respectively. One of these, say n2, is doped with a X (3)nonlinear material. It is well known that band gaps widths and locations depend fundamentally on the refractive index contrast d n = | n1 - n1 | and, by assigning to n2 an intensity dependent refractive index, one is bound to dinamically control the band gap width with the incident light pulse. Therefore, the basic idea here is to investigate the dynamics of propagation in the neighborhood of such an intensity-dependent band gap for important applications in the design of all-optical photonic devices such as limiters and switches. Within the framework of Maxwell's equations, a numerical investigation is made by considering the propagation of optical fields through multilayers with a Kerr, as well as a saturable, type of nonlinearity. The latter is important for the description of the interaction with high field strength pulses because in this case the field-induced change in the refractive index is influenced by higher-order nonlinearities and, as a consequence, this change becomes saturated. This model is appropriate to describe materials such as semiconductor-doped glasses (e.g. CdS 1-x Sex) and organic polymers, which have high nonlinear optical properties. The wave propagation is solved using a time-domain modified version of the standard fast Fourier Beam Propagation Method (FFT-BPM) with an adaptive step size, which can handle transmission, diÿraction and, especially, reflection of electromagnetic waves caused by discontinuities on the refractive index, with the advantange of not requiring the introduction of additional boundary conditions. / Mestrado / Ótica / Mestre em Física

Orientadores: Flávio Caldas da Cruz, Paulo Clóvis Dainese Júnior / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-28T22:50:49Z (GMT). No. of bitstreams: 1 SaquinaulaBrito_JoseLuis_M.pdf: 4387885 bytes, checksum: 7b7c41672acbabf5abc5b7c398df00c0 (MD5) Previous issue date: 2015 / Resumo: Nos últimos 15 anos os pentes de frequências ópticas baseados em lasers de femtossegundos representaram uma revolução na área de metrología e medidas de precisão, permitindo medir diretamente frequências de várias centenas de THz assim como posibilitando o advento de relógios atômicos ópticos. Estes pentes também têm encontrado importantes aplicações em outras áreas da Física, tais como espectroscopia de alta resolução e precisão, geração de altos harmônicos na região da ultravioleta e raios X moles, ou até na procura de exoplanetas através da calibração de espectrômetros astrofísicos. Neste trabalho, estudamos a geração de pentes de frequências ópticas baseados em moduladores eletro-ópticos e fibras altamente não lineares, com o objetivo de implementar novas configurações, alternativas aos pentes baseados em lasers de femtossegundos. Um objetivo é implementar pentes com a maior largura de banda possível que ao mesmo tempo preservem alta coerência entre as frequências geradas, aproveitando componentes comercialmente disponíveis, desenvolvidos para comunicações ópticas na região espectral de 1550 nm. Buscamos implementar dois tipos de pentes de frequências ópticas. Um deles usa um modulador eletro-óptico e gera um pente com pequena largura de banda (10 nm) e espaçamento entre frequências de 25 GHz. O outro pente, gerado com base em fenômenos não lineares em fibras ópticas, fornece maior largura de banda (270 nm) com espaçamento entre frequências de 776 GHz. No caso do pente de frequências gerado pelo modulador (pente eletro-óptico), o processo é devido ao fenômeno eletro-óptico (efeito Pockels) dentro de um cristal de Niobato de Lítio que também forma uma cavidade óptica ressonante. Utilizamos um laser semicondutor contínuo e de frequência única em 1550 nm para gerar um pente (eletro-óptico) com largura espectral de 10 nm com espaçamento de 25 GHz entre as frequências. O outro pente de frequências ópticas é baseado na criação em cascata de produtos da mistura de quatro ondas produzidos a partir de dois lasers semicondutores contínuos, que foram utilizados tanto em onda contínua (cw) ou pulsados, i.e., com modulação de amplitude. Obtivemos espectros com largura de 269 nm (1431 nm ¿ 1700 nm) e espaçamento entre linhas de 6.3 nm (776 GHz). Finalmente, foi alargado o espectro do pente de frequências ópticas gerado pelo modulador eletro-óptico ao usar fibras altamente não lineares. O espectro obtido apresentaram um alargamento modesto, com largura de 23 nm e separação de 25 GHz entre as frequências / Abstract: In the last 15 years, optical frequency combs based on femtosecond lasers have represented a revolution in the area of metrology and precision measurements, making it possible to directly measure frequencies of several hundred terahertz, and affording the advent of optical atomic clocks. These frequency combs today are used in important applications in other areas of Physics, such as high resolution and accuracy spectroscopy, generation of high harmonics in the ultraviolet and soft X-rays region, or even in the search of exoplanets through calibration of Astrophysics spectrometers. In this work, we study the generation of optical frequency combs based on electro-optic modulators and highly nonlinear fibers, with the goal of implementing new configurations, which can be alternative to frequency combs based on femtosecond lasers. One particular goal is to implement frequency combs with the largest possible bandwidth, while still preserving the coherence between the generated frequencies, and taking advantage of commercially available components developed for optical communications, in the 1550 nm spectral region. We were interested in implement two types of optical frequency combs. One of them uses an electro-optical modulator and generates a frequency comb with small bandwidth (10 nm) and 25 GHz frequency spacing. The other comb, generated by nonlinear phenomena in optical fibers, provides greater bandwidth (270 nm) with a frequency spacing of 776 GHz. In the case of the frequency comb generated by the modulator (electro-optical comb), the process is due to the electro-optical phenomenon (Pockels effect) within a Lithium Niobate crystal which also forms a resonant optical cavity. We use a continuous-wave, single frequency semiconductor laser at 1550 nm to generate a frequency comb with a spectral width of 10 nm and 25 GHz frequency spacing. The other optical frequency comb is based on the creation of cascaded four-wave mixing products, produced from two continuous semiconductor lasers that were used both in continuous-wave (cw) or pulsed operation, i.e., with amplitude modulation.We obtained spectra with a width of 269 nm (1431 nm - 1700 nm) and line spacing of 6.3 nm (776 GHz). Finally, we combined both combs by using the highly nonlinear fiber to expand the optical comb spectrum generated by the electro-optical modulator. The resulting spectra showed a modest broadening, with a width of 23 nm and 25 GHz separation between frequencies / Mestrado / Física / Mestre em Física / 1186840 / 134295/2013-7 / CAPES / CNPQ

Orientador: Flávio Caldas da Cruz / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-30T22:47:31Z (GMT). No. of bitstreams: 1 Depetri_WilliamIunes_M.pdf: 11907943 bytes, checksum: 4f7cedccb63e0fd32bea356921cb78f6 (MD5) Previous issue date: 2016 / Resumo: Pentes de frequência ópticos têm sido desenvolvidos para medias de precisão e metrologia, mas tabém vêm ganhando interesse para espectroscopia de precisão de banda larga. Pentes ópticos no infravermelho médio são muito atraentes para espectroscopia molecular, dado que eles permitem acesso a fortes bandas vibracionais de gases na chamada "região de digital molecular". Pentes ópticos são uma tecnologia bem desenvolvida no infravermelho próximo, dado a disponibilidade de lasers de Ti:Safira e Er-fiber, e apesar de desenvolvimentos recentes, sua extensão para o infravermelho médion depende de conversão não-linear de frequência. Aqui, pretendemos estudar a greração de pentes ópticos no infravermelho médio via geração de diferença de frequência e amplificação paramétrica óptica do espectro de um oscilador de femtosegundo no infravermelho próximo. Neste trabalho, analizamos numericamente difereça de frequência e amplificação paramétrica para a geração de pentes de frequência ópticos no infravermelho médio resolvendo no domínio do tempo as equações acopladas para o campo elétrico em mistura de três ondas para pulsos ultracurtos. Uma abordagem tradicional é a utilização de quasi-phase matching em cristais periodicamente polados, o que pode ser bastante eficiente, porém apresenta limitações na largura de banda. Nóis investigamos cristais periódicos com um período fixo, e cristais aperiódicos, ou chirped, tanto com chirp contínuo como discreto. Nós comparamos as configurações nas quais a amplificação paramétrica é otimizada ou não, explorando uma analogia entre conversãpo não-linear em sistemas de dois níveis. Nossos resultados são apresentados como gráficos temporais da propagação dos pulsos, seus respectivos espectros e potências ao longo dos cristais, e comparados com resultados experimentais, com o intuito de ajudar experimentos futuros / Abstract: Optical frequency combs (OFC) have been developed for precision measurements and metrology, but have also gained interest for broadband precision spectroscopy. OFC in the mid-infrared (MIR) are very attractive for molecular spectroscopy, since they allow to access the strong vibrational bands of gases in the so-called molecular fingerprint region. OFC are a well-developed technology in the near-infrared, due to the avilability of Ti:Saphire or Er-fiber lasers, and spite of recent developments, their extension into the MIR relies on nonlinear frequency conversion. Here we intend to study the generation of MIR OFC via difference frequency generation (DFG) and optical parametric amplification (OPA) of spectral portions of a near-infrared femtosecond oscillator. In this work, we numerically analyze DFG and OPA for the generation of optical frequency combs in the mid-infrared by solving the time-domain coupled wave equations for the electric fields in three-wave mixing for ultrashort pulses. A traditional approach is to use quasi-phase matching in periodically poled crystals, which can be quite efficient but may have bandwidth limitations.We investigate periodically poled crystals with a single-grating and aperiodic or chirped crystal with either continuous or discrete chirp. We compare the configurations in which a strong pump pulse has higher or smaller wavelength compared to the signal, cases in which the parametric amplification is enhanced or not. We also analyze these results in the context of adiabatic frequency conversion, which explores an analogy between nonlinear conversion and two-level systems. Our results are presented as time plots for the propagation of the pulses, their corresponding spectra and powers along the crystals and are compared to experimental results, and also intended to support further experiments / Mestrado / Física / Mestre em Física / 132987/2014-7 / CNPQ

La nécessité de synchroniser les différentes étapes des protocoles d’information et de communication quantiques implique l’utilisation de mémoires quantiques. Différents systèmes physiques sont aujourd’hui explorés, parmi lesquels les ions en matrice cristalline, les atomes froids et les vapeurs atomiques. Le protocole de stockage le plus couramment utilisé se fonde sur le phénomène de Transparence Electromagnétiquement Induite (EIT) : une impulsion lumineuse est gravée dans la cohérence Raman entre les deux états fondamentaux d’un système atomique à trois niveaux en Lambda. Bien qu’elle ouvre des perspectives prometteuses, en termes d’efficacité, de fidélité et de temps de stockage, cette technique est néanmoins sensible aux effets déphasants, tels que des gradients de champs magnétiques.Dans ce mémoire, j’étudie tout d’abord le stockage d’impulsions lumineuses classiques par EIT dans une vapeur d’hélium métastable à température ambiante. Les résultats expérimentaux obtenus sont en accord avec les simulations numériques des équations de Maxwell-Bloch complètes du système et montrent notamment l’existence d’une phase supplémentaire acquise par l’impulsion restituée en configuration désaccordée. Cette phase s’explique par la propagation du faisceau sonde dans un milieu dispersif. Dans une deuxième partie, je mets expérimentalement en évidence, dans le même système, une nouvelle forme de stockage basée sur le phénomène d’Oscillations Cohérentes de Population (CPO), par nature plus robuste aux effets déphasants que l’EIT. Les simulations numériques permettent d’analyser plus précisément les mécanismes à l’œuvre dans une mémoire CPO et, notamment, l’influence de la phase relative entre les faisceaux signal et de couplage sur les efficacités de stockage. / The need to synchronise quantum information and communication protocols implies the use of quantum memories. Different physical systems are investigated nowadays, among which ions in crystals, cold atoms and atomic vapours. The most common protocol is based on the Electromagnetically Induced Transparency (EIT) phenomenon: a light pulse is engraved in the Raman coherence of both ground states of an atomic Lambda–type three-level system. Though it opens promising perspectives, with respect to efficiency, fidelity and storage time, this technique is, however, sensitive to dephasing effects such as magnetic field gradients.In this thesis, I first study the storage of classical light pulses via EIT in a room- temperature metastable helium vapor. The obtained experimental results agree with the numerical simulation of the complete Maxwell-Bloch equations of the system. In particular, the existence of an extra phase acquired by the retrieved pulse is demonstrated in the detuned configuration, which can be explained by the propagation of the signal beam in the medium. In the second part, I experimentally isolate, in the same system, a new storage protocol based on the Coherent Population Oscillation (CPO) phenomenon, which is by nature more robust than EIT to dephasing effects. The numerical simulations allow us to precisely analyse the mechanisms involved in a CPO memory and, in particular, the influence of the relative phase between the signal and coupling beams on the storage efficiencies.

The author explores absorptive and refractive optical nonlinearities at 1.06 [mu]m in bulk, semi-insulating, undoped GaAs with a particular emphasis on the influence of the native deep-level defect known as EL2. Picosecond pump-probe experimental technique is used to study the speed, magnitude, and origin of the absorptive and refractive optical nonlinearities and to characterize the dynamics of the optical excitation of EL2 in three distinctly different undoped, semi-insulating GaAs samples. Intense optical excitation of these materials leads to the redistribution of charge among the EL2 states resulting in an absorptive nonlinearity due to different cross sections for electron and hole generation through this level. This absorptive nonlinearity is used in conjunction with the linear optical properties of the material and independent information regarding the EL2 concentration to extract the cross section ratio [sigma][sub p]/[sigma][sub e] [approx equal]0.8, where [sigma][sub p](e) is the absorption cross section for hole (electron) generation from EL2[sup +] (EL2[sup 0]). The picosecond pump-probe technique can be used to determine that EL2/EL2[sup +]density ratio in an arbitrary undoped, semi-insulating GaAs sample. The author describes the use of complementary picosecond pump-probe techniques that are designed to isolate and quantify cumulative and instantaneous absorptive and refractive nonlinear processes. Numerical simulations of the measurements are achieved by solving Maxwell equations with the material equations in a self-consistent manner. The numerical analysis together with the experimental data allows extraction of a set of macroscopic nonlinear optical parameters in undoped GaAs. The nonlinearities in this material have been used to construct three proof-of-principle nonlinear optical devices for use at 1.06 [mu]m: (1) a weak beam amplifier, (2) a polarization rotation optical switch, and (3) optical limiters.

abstract: Photonic integrated circuit (PIC) in the visible spectrum opens up new opportunities for frequency metrology, neurophotonics, and quantum technologies. Group III nitride (III-N) compound semiconductor is a new emerging material platform for PIC in visible spectrum. The ultra-wide bandgap of aluminum nitride (AlN) allows broadband transparency. The high quantum efficiency of indium gallium nitride (InGaN) quantum well is the major enabler for solid-state lighting and provides the opportunities for active photonic integration. Additionally, the two-dimensional electron gas induced by spontaneous and polarization charges within III-N materials exhibit large electron mobility, which is promising for the development of high frequency transistors. Moreover, the noncentrosymmetric crystalline structure gives nonzero second order susceptibility, beneficial for the application of second harmonic generation and entangled photon generation in nonlinear and quantum optical technologies. Despite the promising features of III-N materials, the investigations on the III-N based PICs are still primitive, mainly due to the difficulties in material growth and the lack of knowledge on fundamental material parameters. In this work, firstly, the fundamental nonlinear optical properties of III-N materials will be characterized. Then, the fabrication process flow of III-N materials will be established. Thirdly, the waveguide performance will be theoretically and experimentally evaluated. At last, the supercontinuum generation from visible to infrared will be demonstrated by utilizing soliton dynamics in high order guided modes. The outcome from this work paves the way towards fully integrated optical comb in UV and visible spectrum. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

In this thesis, research starts by questioning Berry curvature dipole's role in electronic properties in solids. Strongly inspired by the recent studies, we discover a more profound interpretation of the Berry curvature dipole. It is demonstrated that the anomalous correction to the electron acceleration is proportional to the Berry curvature dipole and is responsible for the Non-linear Hall effect recently discovered in materials with broken inversion symmetry. This allows uncovering a deeper meaning of the Berry curvature dipole as a non-linear version of the Drude weight that serves as a measurable order parameter for broken inversion symmetry in metals. Later, we introduce the Quantum Rectification Sum Rule in time-reversal invariant materials is derived by showing that the integral over frequency of the rectification conductivity depends solely on the Berry connection and not on the band energies or relaxation rates. In the final part of the thesis, we use the Keldysch-Floquet formalism to obtain non-perturbative predictions of the optical responses in solids, mainly focusing on the clean limit response of systems with broken time-reversal symmetry.

L’explosion de la demande en données a imposé de nouvelles exigences en terme de débit de transmission qui sont de plus en difficiles à satisfaire sans accroître considérablement les consommations énergétiques dans les centres de données, points névralgiques des réseaux de télécommunications. Dans ce contexte, la photonique silicium est considérée comme la solution la plus adaptée pour répondre de ces problématiques en remplaçant les interconnexions métalliques par des liaisons optiques à base de silicium. Le modulateur électro-optique constitue l’un des composants clés de ces liaisons optiques. Cependant, la centrosymétrie du silicium empêche l’exploitation de l’effet Pockels, un phénomène d’optique non linéaire très efficace dans la conception de modulateurs à très grande bande passante et à faible consommation énergétique. Cette limitation peut être néanmoins contournée lorsque des contraintes mécaniques sont appliquées au silicium de façon à briser sa symétrie d’inversion. Plusieurs travaux théoriques et expérimentaux ont alors été entrepris récemment pour mettre en évidence et quantifier l’effet Pockels induit par contraintes dans le silicium. Mais la nature semi-conductrice du silicium rend l’analyse de l’effet Pockels profondément complexe et cela a soulevé une controverse quant à sa réelle existence dans le silicium contraint. En effet, l’influence des porteurs libres dans le silicium et aux interfaces engendrent un fort signal de modulation, noyant la signature de l’effet Pockels. Pour enrayer les effets de porteurs, la solution apportée par le travail de thèse a été d’étudier le signal de modulation à hautes fréquences (> 5 GHz). Plusieurs études hyperfréquences de l’effet Pockels ont donc été menées dans des structures photoniques en silicium contraint et seront présentées dans ce manuscrit de thèse. Les premières études ont été réalisées sur une plate-forme SOI et les résultats expérimentaux ont permis de mettre en évidence la présence d’un signal de modulation électro-optique à hautes fréquences et dont l’intensité dépend clairement de l’orientation cristallographique du silicium et de l’amplitude de la contrainte appliquée sur celui-ci. Sur la base d’un modèle théorique décrivant le tenseur de susceptibilité électrique du second ordre χ(²), un modèle multiphysique a été développé et a permis de décrire de manière très précise à la fois les résultats expérimentaux et la distribution spatiale du χ(²) dans des guides d’onde silicium contraints. Ces travaux ont également permis de montrer que les faibles intensités des champs électriques appliqués dans les guides d’onde silicium, dues à la distribution des porteurs, sont en grande partie responsable de la faible efficacité de modulation par effet Pockels. Une seconde étude a donc été menée sur une plate-forme SOI modifiée et permettant la conception de circuits électriques plus performants avec des champs électriques générés plus intenses. Les résultats expérimentaux obtenus montrent une amélioration d’un facteur 20 de l’efficacité de modulation par effet Pockels en comparaison des premières études. De plus, le modèle multiphysique a de nouveau permis de décrire ces résultats, renforçant donc davantage sa validité. L’ensemble de ces travaux ouvrent notamment comme perspectives la possibilité d’obtenir un diagramme de l’œil électro-optique dans la mesure où une contrainte plus importante est appliquée aux guides d’onde silicium. De plus, le modèle décrivant le tenseur de susceptibilité électrique du second ordre χ(²) peut également être exploité pour décrire le phénomène de génération de seconde harmonique en optique guidée dont l’existence reste encore ambiguë à l’heure actuelle. / The explosion of data demand imposed new requirements in terms of data transmission rate that are more and more difficult to meet without greatly increasing the power consumption in data centres, hot spots of telecommunications networks. In this context, silicon photonics is considered the most adapted solution to address these complex issues by replacing metallic interconnects by silicon-based photonic links. The electro-optic modulator is one major building block in such photonic links and ensure the conversion of data carried by an electric signal to an optical one. However, silicon being a centrosymmetric material, it cannot exhibit the Pockels effect, a very valuable optical nonlinear phenomenon used in most high-speed and low power consumption modulators. This limitation is nonetheless relaxed by applying deformations to the silicon lattice by means of stress in order to break its inversion symmetry. Numerous theoretical and experimental studies were reported to demonstrate and quantify the Pockels effect. But, the semiconductor nature of silicon tremendously complicate the analysis of the Pockels effect, which existence was questioned in strained silicon and source of controversy. Indeed, free carriers in silicon waveguides and at the interfaces induce a strong modulation signal, thereby screening Pockels effect. To stem the influence of free carriers, the work done in the thesis consisted in studying high frequency-based modulation signal (> 5 GHz). Various microwave studies were then performed in strained silicon photonic structures and will be presented in the following thesis manuscript. First studies were achieved on a SOI platform and the obtained experimental results demonstrated the presence of a weak high-frequency electro-optic modulation signal which intensity clearly depends on the silicon cristallographic direction and the level of stress applied to silicon. Based on a theoretical model describing the second-order nonlinear electric susceptibility χ(²), a multiphysic model has been developed and successfully described both experimental results and the spatial distribution of χ(²) within strained silicon waveguides. These studies also showed that the weak intensity of the applied electric fields, due to the free carriers distribution, are responsible for the weak measured Pockels-based modulation efficiencies. A second study has then been carried out on a modified SOI platform allowing the design of more efficient electric circuits inducing stronger electric fields. An improvement by a factor of 20 was observed on the obtained experimental results compared to the previous ones. Moreover, the multiphysic model could again describe those results, proving its reliability. As outlooks, electro-optic eye diagram of complex electric signals could be obtained at the condition of stronger stress applied to silicon waveguides. Furthermore, the model describing the second-order nonlinear susceptibility χ(²) can also be exploited to depict the second harmonic generation in strained silicon waveguides, which existence is still not clear for the moment.

Les ondes térahertz (THz), situées entre l'infrarouge et les micro-ondes dans le spectre électromagnétique, correspondent aux fréquences caractéristiques de nombreux mouvements moléculaires et permettent ainsi de caractériser des molécules complexes par spectroscopie dans le domaine temporel. Cette thèse a pour objectif d'étudier les champs THz émis par une source constituée d'une impulsion laser à deux couleurs générant un plasma par ionisation de l'air. En raison de l'asymétrie temporelle du champ laser, un courant électronique présentant une composante basse-fréquence dans la gamme THz est formé dans le plasma par conversion non linéaire et produit un champ secondaire comprenant une composante THz. Les effets non linéaires intervenant dans la génération du rayonnement THz sont l'effet Kerr à basse intensité (< 10¹³ W/cm²) et les photocourants à plus haute intensité (> 10¹³ W/cm²), au-dessus du seuil d'ionisation. Ce dernier mécanisme, qui génère le plus de rayonnement THz, est principalement étudié dans ce manuscrit. Si la puissance crête de l'impulsion laser est suffisamment élevée, des filaments de lumière peuvent être formés par combinaison de l'effet Kerr focalisant et de la formation d'un plasma défocalisant. Le phénomène de filamentation laser permet ainsi de créer des ondes THz à distance. En modulant l'impulsion laser, il est aussi possible de modifier les champs et spectres THz associés. En particulier, nous étudions les effets d'une dérive de fréquence et de la combinaison de multi-impulsions sur l'efficacité de conversion laser-THz. Nous consacrons en outre une large part de nos études à l'influence de l'augmentation de la longueur d'onde laser sur le rendement en énergie de l'émission THz. / The terahertz waves (THz), located between the infrared and the microwaves in the electromagnetic spectrum, correspond to the characteristic frequencies of numerous molecular motions and thus make it possible to characterize complex molecules by time-domain spectroscopy. This thesis aims to study the THz fields emitted by a source formed by a two-color laser pulse generating a plasma by air ionization. Due to the time asymmetry of the laser field, an electric current having a low-frequency component in the THz range is formed in the plasma by nonlinear conversion, generating a secondary field including a THz component. The nonlinear effects involved in the generation of THz radiation are the Kerr effect at low intensity (< 10¹³ W/cm²) and the photocurrents at higher intensity (> 10¹³ W/cm²), above the ionization threshold. This latter mechanism, which generates the most THz radiation, is mainly studied in this manuscript. If the peak power of the laser pulse is sufficiently high, light filaments can be created by combining the focusing Kerr effect and the defocusing action of the plasma. So, the filamentation process can produce THz waves remotely. By modulating the laser pulse, it is possible to modify the associated THz fields and spectra. In particular, we study the effects of pulse chirping and multi-pulse combination. We also devote a large part of our studies to the influence of increasing the laser wavelength on the THz energy yield.