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This thesis describes the experimental implementation of novel intersecting cavity terahertz optical parametric oscillators based on bulk and periodically-poled magnesium oxide-doped lithium niobate. Both collinear and non-collinear phase-matching geometries have been demonstrated and injection-seeding has been implemented in devices using periodically-poled material to reduce threshold and increase the down-conversion efficiency. A comprehensive characterisation of the original intracavity terahertz OPO was undertaken, which revealed the parameters having the greatest impact on OPO efficiency (idler mirror reflectivity and cavity length) and led to a better understanding of the losses in the system. During the characterisation process, generation of further terahertz radiation at the same frequency as that generated by the parametric process was observed and identified as being a result of difference frequency generation (DFG) between the parametrically-generated idler and terahertz waves. This phenomenon had previously only been observed when periodically-poled materials were employed in the system. The effect of this additional DFG process has been analysed in terms of the enhancement of the terahertz field on the basis of the coupled wave equations and physically measured quantities. The use of periodically-poled lithium niobate has been a major part of the research presented in this thesis. A comprehensive study of the modified phase-matching conditions was carried out and both collinear and novel hybrid non-collinear phase-matching geometries were identified. Several computer models were developed to assess the performance of any given grating design in these different geometries and the effects of temperature tuning and pump wavelength variation were also investigated using the models. Experimental studies confirmed the viability of the modelling approach but material limitations (particularly the early onset of crystal damage) limited the outcomes of the experiments. A detailed comparison of the poled and bulk materials was made to highlight the present drawbacks of the poled material. Finally, injection seeding was used to improve the efficiency of the collinear phase-matched PPLN OPOs. When seeding was used the depletion of the pump pulse was increased to the point of being measurable, reaching an upper level of 10%. Coupling constraints placed on the seed laser limited the amount of depletion attained. The potential for injection seeding to be used in the hybrid non-collinear phase-matching scheme was also identified but not realised during the course of this work. Were this technique successful, the tuning range of the intersecting cavity terahertz OPO could be extended to encompass the sub-1THz region, something that has previously been limited by the available idler cavity angles.

Os estados emaranhados da luz são de grande importância para protocolos de comunicação quântica. Uma das principais fontes que vem sendo estudada no Laboratório de Manipulação Coerente de Átomos e Luz - LMCAL é o oscilador paramétrico ótico (OPO) no qual, através de processos paramétricos não lineares de segunda e terceira ordem (x(2) e x(3)), são produzidos feixes intensos que apresentam correlações quânticas. Recentemente, o LMCAL vem explorando o processo de mistura de quatro ondas (fenômeno derivado da susceptibilidade de terceira ordem x(3)) como fonte geradora de feixes emaranhados. Inicialmente, foi realizado a partir de células de rubídio e agora, em colaboração com o grupo de pesquisa da Profa. Michal Lipson da Universidade de Columbia, em chips de nitreto de silício (Si3N4); permitindo assim possibilidades de modulação ultra-rápida, confinamento de luz em volumes muito reduzidos, além da ótica não-linear do OPO. O presente projeto visa estudar as propriedades quânticas da luz nos OPOs em chips de silício, permitindo que sistemas muito eficientes em informação clássica possam ser usados também para implementação de protocolos de informação quântica. / Entangled States of light beams are of great importance for quantum communication protocols. One of the most relevant source of such states which is being studied at the Laboratory of Coherent Manipulation of Atoms and Light - LMCAL (in portuguese) is the Optical Parametric Oscillator (OPO) which through second and third order nonlinear parametric processes (x(2) and x(3)) produces intense fields that have quantum correlations. Recently, LMCAL is exploring four-wave mixing (FWM), a third-order nonlinear parametric process, as a source of entangled beams. Initially, on rubidium cells and now, in collaboration with Prof. Michal Lipson from the Columbia University, on silicon nitride (Si3N4) chips; opening a new avenue for ultrafast modulation, light confinement in reduced light volumes, as well as the nonlinear optics of the OPO. This project is intended to study quantum properties of light of on-chip OPOs in order to achieve the integration of these highly efficient devices for implementations of quantum information protocols.

Résumé: Cette thèse est le fruit de plusieurs années de travail passées à développer un spectromètre de génération de fréquence somme dans le laboratoire LASMOS. Ce spectromètre comprend essentiellement deux lasers accordables, un échantillon non linéaire et un monochromateur. Mélanger les deux faisceaux laser accordables sur l’échantillon non linéaire résulte en la génération d'une onde supplémentaire de fréquence égale à la somme des deux fréquences incidentes (SFG). Détecter le signal SFG et accorder une, ou les deux fréquences, permettent la caractérisation de la réponse non linéaire de l'échantillon. Dans notre montage, le premier faisceau laser est accordable dans l'infrarouge de 2,5 jusqu'à 20 0m et le deuxième est accordable dans la région visible de 400 à 700 nm. La génération des deux faisceaux laser accordables est faite par des oscillateurs paramétriques optiques (OPO) qui sont construits autour de cristaux de LiNbO3, d’AgGaS2 et de BBO. Une étape supplémentaire de génération de fréquence différence (DFG) étend l’accordabilité de 10 à 20 0m. L’étape de la DFG est basée sur un tandem de cristaux non linéaire de KTP et de CdSe. Une structure en train d’impulsions est nécessaire pour pomper optiquement le système de l'OPO. Le train d’impulsions peut être engendré par la technique de verrouillage des modes. Nous avons focalisé notre travail sur le verrouillage des modes de l'oscillateur Nd:YAG par le miroir non linéaire à doublage de fréquence (FDNLM). Cette dernière technique combinée avec un absorbeur saturable à deux photons (comme une plaquette de GaAs) permet d’obtenir des impulsions optiques courtes et stables à partir d’un laser Nd:YAG pulsé et pompé par des lampes flashes à bas taux de répétition. Des impulsions aussi courtes que 12 ps sont émises à un taux de répétition de 100 MHz. Par ce résultat, nous nous approchons de la limite physique de la durée de l’impulsion dans un oscillateur Nd:YAG. Cette technologie « tout d’état solide » présente les mérites suivants: stabilité photochimique, temps de réponse rapide, applicabilité à une gamme spectrale relativement large et simplicité. Une approche théorique complète également notre étude pour déterminer le domaine d'opération de ce dispositif. Abstract: This thesis describes the efforts of several years devoted to developing a Sum-Frequency Generation spectrometer in the LASMOS laboratory. The Sum- Frequency Generation spectrometer relies mainly on two tunable lasers, a nonlinear sample and a monochromator. Mixing the tunable laser beams on the nonlinear sample will result in the generation of an additional beam at the sum frequency (SFG) of the incident beams. Detecting the SFG signal and tuning one of the incident frequencies or both frequencies permit the characterization of the nonlinear response of the sample. In our setup, the first laser beam is tunable in the IR spectral region from 2.5 up to 20 0m and the second one is tunable in the visible spectral region from 400 to 700 nm. The generation of both tunable laser beams is carried out by the optical parametric oscillators (OPO) built around a LiNbO3, an AgGaS2 and a BBO nonlinear crystals. An additional difference-frequency generation (DFG) stage based on a tandem of KTP and CdSe crystals is built to extend the tunability up to 20 0m. A pulse-train structure is necessary for pumping the OPO. Pulse trains can be generated by the mode-locking technique. In our work, we focused on the mode locking of the Nd:YAG oscillator using a frequency-doubling nonlinear mirror (FDNLM). This technique combined with a two-photon saturable absorber (for example: a GaAs platelet) can generate short and stable optical pulses from a flash-pumped Nd:YAG laser oscillator at a low repetition rate. Pulses as short as 12 ps are generated at 100 MHz repetition rate. With this result, we approach the physical limit of pulse duration in the Nd:YAG laser. This all-solid-state technology presents the following merits: photochemical stability, fast response time, applicability to a wide spectral range, and simplicity. A theoretical model is used to analyze the domain of operation of this device.

In this study we demonstrate the use of third harmonic (TH) and second harmonic (SH) generation in imaging dental sections. Teeth are the hardest and most indestructible part in human body. The TH and SH greatly facilitate observation of porous structures and collagen within the dental sections, respectively. Strong SH has been found on various biological specimens, such as collagen, potato starch, and skeletal muscles. These materials all possess periodical nano-structures that are often referred as (nonlinear) bio-photonic structures. In particular, collagen is an extra-cellular structural protein and is a major component of bone, cartilage, skin, and other tissues. Collagen fibrils have a triple-helical structure and it is believed that this structure enables collagen to generate SH signal from a wide range of wavelengths in the infrared region. For comparison, microtubule structures within dentin, due to its large index mismatch with surrounding, can be clearly seen with THG imaging. The THG also facilitate observation of prismatic structures in enamel. The successful construction of a multi-photon laser scanning microscope that can operate in both reflection and transmission modes is the key for this study. A femtosecond, sync-pumped optical parametric oscillator (OPO) is used to generate second and third harmonics from dental sections. Dental sections have large index of refraction¡]n~1.68¡^and scatter visible light severely. The employment of excitation wavelength at 1260 nm greatly reduces scattering and absorption within the sample. Its corresponding SH and TH wavelengths are at 630 nm and 420 nm, respectively. Additionally, 3-D structural views are also reconstructed from the optically sectioned images by the use of specialized 3D image processing software.

Os estados emaranhados da luz são de grande importância para protocolos de comunicação quântica. Uma das principais fontes que vem sendo estudada no Laboratório de Manipulação Coerente de Átomos e Luz - LMCAL é o oscilador paramétrico ótico (OPO) no qual, através de processos paramétricos não lineares de segunda e terceira ordem (x(2) e x(3)), são produzidos feixes intensos que apresentam correlações quânticas. Recentemente, o LMCAL vem explorando o processo de mistura de quatro ondas (fenômeno derivado da susceptibilidade de terceira ordem x(3)) como fonte geradora de feixes emaranhados. Inicialmente, foi realizado a partir de células de rubídio e agora, em colaboração com o grupo de pesquisa da Profa. Michal Lipson da Universidade de Columbia, em chips de nitreto de silício (Si3N4); permitindo assim possibilidades de modulação ultra-rápida, confinamento de luz em volumes muito reduzidos, além da ótica não-linear do OPO. O presente projeto visa estudar as propriedades quânticas da luz nos OPOs em chips de silício, permitindo que sistemas muito eficientes em informação clássica possam ser usados também para implementação de protocolos de informação quântica. / Entangled States of light beams are of great importance for quantum communication protocols. One of the most relevant source of such states which is being studied at the Laboratory of Coherent Manipulation of Atoms and Light - LMCAL (in portuguese) is the Optical Parametric Oscillator (OPO) which through second and third order nonlinear parametric processes (x(2) and x(3)) produces intense fields that have quantum correlations. Recently, LMCAL is exploring four-wave mixing (FWM), a third-order nonlinear parametric process, as a source of entangled beams. Initially, on rubidium cells and now, in collaboration with Prof. Michal Lipson from the Columbia University, on silicon nitride (Si3N4) chips; opening a new avenue for ultrafast modulation, light confinement in reduced light volumes, as well as the nonlinear optics of the OPO. This project is intended to study quantum properties of light of on-chip OPOs in order to achieve the integration of these highly efficient devices for implementations of quantum information protocols.

Every gas molecule has a unique absorption spectrum that can be captured using optical spectroscopy to identify an unknown sample's composition. Frequency combs systems can provide an extremely broad mid-infrared spectrum that is very useful for molecular detection. A degenerate optical parametric oscillator (OPO) was built to generate the down-converted and shifted frequency comb spectrum. This system utilizes an ultra-short pulse 1.56µm pump laser and a never before used orientation patterned gallium-phosphide crystal. Periodically polled lithium niobate (PPLN), Gallium Arsenide (GaAs) and Gallium Phosphide are all crystals used to accomplish this task. GaP, in comparison to PPLN, has (i) a larger nonlinear coefficient, (ii) much deeper infrared transparency, and (iii) smaller group dispersion – to allow for achieving broad spectral coverage. GaP also has a larger band gap than GaAs; therefore it can still be pumped with a standard telecom C-band laser. An octave-wide spanning frequency comb system was achieved and the characterization of the system is presented. This system is specifically designed to be compact and portable for initial experimental testing in the applications of medical breath analysis and combustion gas investigation.

In this work, we discuss a description of quantum properties of light beams produced by an Optical Parametric Oscillator (OPO) above threshold, pumped by a 780 nm Titanium Sapphire laser, resonant with the D2 line of Rb with a noise spectrum of a coherent beam. The twin beams have wavelengths around 1560nm, in the telecommunication band. The states emitted by the OPO under study are approximately Gaussian (1), which allows us to describe them in terms of first and second order moments. As we analyze the noise spectrum of the fluctuations of each beam, the first order moments are null, which allows us to describe the state of the system in terms of a covariance matrix. We present the behavior of the noise spectrum and the correlations between the reflected pump, signal and idler beams. In addition, we applied different entanglement criteria to study the system. We present a prediction of the bipartite entanglement using Duan criterion (2). For tripartite correlations, we use the Furusawa criterion (3). We performed a study of bipartition pump and sum quadrature. As well, we applied the PPT criterion (4) for continuous variables, independently transposing each beam under study. The PPT criterion is necessary and sufficient to demonstrate entanglement between bipartitions for Gaussian states. In order to explore the behavior of the system using the steering criterion in the model developed by Reid (5), we theoretically study the criterion of inference for bipartite quadratures: between the twin beams or between one of the twin beams and the pump beam. In addition, we analyzed the inference between the three modes, through the bipartition pump mode and combination of sum quadrature of the twin beams. We develop a quantum teleportation protocol of a coherent input state, composed by the Titanium Sapphire beam whose Bell measurement is performed using the pump reflected by the OPO as an entangled state for the signal and idler beams as a first possibility to implement the protocol. Furthermore, we propose a protocol in which the signal beam assists in the process of teleportation. Thus, the fidelity of the system increases and exceeds the limit of non-cloning. We will describe the first measurements of correlations between the beams emitted by the OPO for this system. The tripartite entanglement characterization is the first step in the implementation of quantum optical protocols using a source of entangled states that is compatible with both Rubidium atomic systems and the telecommunications region, thus forming a quantum network. / Nesse trabalho apresentamos uma descrição das propriedades quânticas dos feixes de luz produzidos por um Oscilador Paramétrico Ótico (OPO) acima do limiar, bombeado por um laser de Titânio Safira com comprimento de onda de 780nm, linha D2 do Rb, com espectro de ruído de um feixe coerente. Os feixes gêmeos possuem comprimentos de onda na região de 1560 nm que compreende a janela de transmissão de fibras óticas. Os estados emitidos pelo OPO em estudo são aproximadamente gaussianos (1), o que nos permite descrevê-los em termos de momentos de primeira e segunda ordem. Como analisamos o espectro de ruído das flutuações de cada feixe, os momentos de primeira ordem são nulos, o que nos permite descrever o estado do sistema em termos de uma matriz de covariância. Apresentamos então o comportamento do espectro de ruído e das correlações entre os feixes de bombeio refletido, sinal e complementar. Ademais, aplicamos diferentes critérios de emaranhamento para estudar o sistema. Apresentamos uma previsão do emaranhamento bipartido utilizando o critério de Duan (2). Para as correlações tripartidas, utilizamos o critério de Furusawa (3). Realizamos um estudo da bipartição bombeio e quadratura soma. E, aplicamos o critério PPT para variáveis contínuas (4), transpondo independente cada feixe em estudo. O critério PPT é necessário e suficiente para demonstrar emaranhamento entre bipartições para estados gaussianos. Com o objetivo de explorar qual o comportamento do sistema frente ao critério de \"steering\" no modelo desenvolvido por Reid (5), estudamos teoricamente o critério de inferência para quadraturas bipartidas: entre os feixes gêmeos ou entre um dos feixes gêmeos e o feixe de bombeio. Finalizamos essa análise caracterizando a inferência entre os três modos, através da bipartição modo do bombeio e combinação das quadraturas soma dos feixes gêmeos. Apresentamos a formulação de um protocolo de teletransporte quântico de um estado coerente de entrada formado pelo feixe do Titânio Safira, cuja medida de Bell é realizada utilizando o bombeio refletido pelo OPO como estado emaranhado para os feixes sinal e complementar, como uma primeira possibilidade de implementar o protocolo. Também propomos um protocolo em que o feixe sinal auxilia no processo de teleportação. Dessa forma, a fidelidade do sistema aumenta e supera o limite da não clonagem. Descreveremos as primeiras medidas de correlações entre os feixes emitidos pelo OPO para esse sistema. A caracterização de emaranhamento tripartido nessa nova configuração é o primeiro passo para a implementação de protocolos de ótica quântica utilizando uma fonte de estados emaranhados que é compatível tanto com sistemas atômicos de Rubídio quanto com a região de telecomunicações, formando assim uma rede quântica de transferência e aprisionamento de informação.

No presente trabalho, descrevemos a construção de um oscilador paramétrico ótico (OPO) com meio atômico de ganho de susceptibilidade X(3) , e a caracterização de seu limiar de oscilação. O processo base para a construção deste OPO é a Mistura de Quatro Ondas (4WM), que acontece em meios não lineares tipo X(3) , como é o caso dos isótopos de 85 Rb e 87 Rb. Realizou-se uma revisão da teoria atômica do rubídio, do 4WM e da teoria clássica do OPO. Obtivemos e caracterizamos o processo de 4WM em função de três parâmetros experimentais: a dessintonia do feixe de bombeio em relação ao pico do crossover da transição 5S 1/2 (F = 2)-> 5P 1/2 (F \' ) do 85 Rb na linha D 1 , da potência do bombeio e da temperatura da célula de rubídio. Encontrou-se uma ótima região de frequências em torno a Delta = 0.77GHz para a construção do OPO com os átomos de rubídio como meio não linear. Nesta região obteve-se uma amplificação máxima de 450% para o feixe de prova com absorção nula e uma intensidade do conjugado alta, para o feixe de prova com frequência Delta= w+ 3GHz (anti-Stokes). A elevada amplificação observada permite, em princípio, um limiar de oscilação menos abrupto: o surgimento de oscilação aparece de forma menos sensível a variações da potência de bombeio. Com efeito, para a dessintonia Delta = 0.38GHz o limiar é suave. A caracterização do 4WM e do limiar de oscilação do OPO é importante para a possível geração de estados não gaussianos. Um limiar suave permite, em princípio, operação e estudo muito próximo do limiar, região em que há previsões indicando a geração de estados não gaussianos que são relevantes para aplicações em informação quântica. / In the present work, we describe the construction of a optical parametric oscillator (OPO) with atomic gain medium with susceptibility X(3) and the caracterization of the oscilation threshold. The basic process for the construction of this OPO is the Four Wave Mixing (4WM), a process that takes place in nonlinear media type X (3) , as is the case of the isotopes of 85 Rb and 87 Rb. A review of the atomic theory of rubidium, 4WM and the classical theory of the OPO was carried out. The 4WM process was obtained and characterized as a function of three experimental parameters: the pump beam detunning with respect to the transition crossover peak 5S 1/2 (F = 2) ->5P 1/2 (F \' ) of 85 Rb D1 line, the pumping power and the temperature of the rubidium cell. A good region of frequencies around Delta= 0.77GHz was found for the construction of the OPO with the rubidium atoms as a non-linear medium. In this region, it was obtained a maximum amplification of 450% for the prove beam with zero absorption and a high intensity for the conjugate beam with frequency Delta= w+ 3GHz (anti-Stokes). The high observed amplification allows, in principle, an oscillation threshold less abrupt: the oscillation appears in a less sensitive way with variations in pumping power. In fact, for the detunning Delta= 0.38GHz the threshold is smooth. The characterization of the 4WM and oscillation threshold of the OPO is important for the possible generation of non-Gaussian states. A smooth threshold allows, in principle, operation and study very close to this region, where there is predictions indicating the generation of non-Gaussian states that are relevant for applications in quantum information.

In der vorliegenden Doktorarbeit wird ein Dauerstrich Optisch Parametrischer Oszillator (cw OPO) vorgestellt, der speziell für die hochauflösende Dopplerfreie Molekülspektroskopie und Metrologie entwickelt wurde. Der kontrollierte Zugang zu jeder beliebigen Wellenlänge im breiten Emissionsspektrum von OPOs wie auch das präzise Abstimmen seiner Ausgangsfrequenz über zu untersuchende molekulare und atomare Übergänge stellten lange Zeit Probleme dar, deren Lösung die Grundzielsetzung dieser Arbeit war. Das im Laufe dieser Arbeit entwickelte System hat diese Ziele vollständig erreicht, was durch verschiedene Messungen und Anwendungen demonstriert wurde. Zu diesem Zweck wurde ein neues OPO-Design mit einem Intracavity-Etalon entwickelt und aufgebaut, wobei der OPO auf dem Konzept eines einfach-resonanten cw OPOs mit resonanter Pumpwelle basiert. Die OPO-Ausgangsstrahlung zeigt sehr gute Langzeitstabilität und Spektraleigenschaften, welche durch direkte Frequenzvergleichsmessungen mit einem optischen Methan-Frequenzstandard im Infraroten bestimmt wurden. Eine Idler-Linienbreite von 12 kHz und ein Modensprung-freier Betrieb des OPOs über einen Zeitraum von einigen Tagen wurde beobachtet. Außerdem wurde gezeigt, dass ein OPO zu einer hochstabilen optischen Referenz phasengelockt und somit seine Frequenz sehr genau kontrolliert und durchgestimmt werden kann. Als erste erfolgreiche Anwendung eines OPOs in der Dopplerfreien Spektroskopie wurde ein Aufbau zur Frequenz-Modulationsspektroskopie in Methan realisiert. Weiterhin, wurde der entwickelte cw OPO mit einem femtosekunden optischen Frequenzkamm kombiniert, um eine neue Idee für eine kohärente Verbindung zwischen dem sichtbaren und dem infraroten Spektralbereich zu realisieren. Als erste Demonstration dieser Technologie wurde ein direkter absoluter Frequenzvergleich zwischen einem Jod-stabilisierten Laser bei 532 nm und einem Methan-stabilisierten Laser bei 3390 nm durchgeführt. / This thesis presents a continuous-wave optical parametric oscillator (cw OPO), specially developed for high-resolution Doppler-free molecular spectroscopy and metrology. The basic objective was to solve the long-standing problem of controlled access to any desired wavelength in the wide emission range of OPOs, including the ability to precisely tune the output frequency over the molecular and atomic transitions of interest. The system implemented during this work fully achieves these goals and its performance was demonstrated in various measurements and applications. For this aim, a new design for the OPO cavity with an intracavity etalon was implemented, extending the concept of a cw singly resonant OPO with resonated pump wave. The newly developed device demonstrates very good long-term stability and spectral properties, which were determined in direct beat frequency measurements with a methane infrared optical frequency standard. Thus, an idler radiation linewidth of 12 kHz and mode-hop-free operation of the OPO over several days were observed. Furthermore, it was shown that an OPO can be phase locked to a highly stable optical reference and thus much more precisely controlled and tuned. As the first successful application of OPOs in Doppler-free spectroscopy, a frequency modulation spectroscopy setup for detection of sub-Doppler resonances in methane was implemented. Furthermore, the developed cw OPO was integrated with a femtosecond optical frequency comb to realize a new concept for a coherent link between the visible and infrared spectral ranges. As a first demonstration of this technique, a direct absolute frequency comparison between an iodine stabilized laser at 532 nm and a methane stabilized laser at 3390 nm was performed.