archives@tulane.edu / The sharing of quantum resources between multiple parties allows for the creation of quantum networks. Traditional four-wave mixing creates twin correlated beams of light. More complex four-wave mixing schemes can create a multitude of correlated beams for use in quantum communication, helping pave the way toward future quantum networks. These correlations can be seen in the intensity-difference squeezing between output modes. In this dissertation, we examine a variety of multi-mode quantum systems. I begin in chapters 2 and 3 by using the noise figure, which compares the signal-to-noise ratios of output modes to input modes, to calculate intensity-difference squeezing and make predictions about phase-sensitivity. In chapter two, I analyze a dual-pump four-wave mixing system yielding three output modes for cases in which a single seed, two asymmetric seeds, and two symmetric seeds are used. In chapter 3, I perform similar calculations for three different cascaded four-wave mixing configurations. Various intensity-difference squeezing combinations are compared for two variations of two cascaded four-wave mixing cells and for three cascaded four-wave mixing cells. Chapter 4 describes a dual pump four-wave mixing scheme with four output modes created experimentally and chapter 5 shows that when only one input mode is seeded this process is phase-insensitive. Interestingly, I find that when only two of the input modes are seeded the system becomes phase-sensitive. Finally, in chapter 6, I describe the simulated and experimental results of using a deep neural network to improve the bit error rates in a classical free-space optical on-off keying scheme, that will eventually be expanded into the quantum regime. / 1 / Sara K Wyllie

The family of III-V semiconductors is of high significance in photonics for two main reasons. First, not only they are the most practical material platforms for active photonic devices but also they are suitable for monolithic integration of passive and active photonic devices. Second, some III-V compounds exhibit high values of second and third-order nonlinear coefficients – the property useful in all-optical signal processing and wavelength conversion. This Ph.D. thesis explores the above perspectives with two candidates from the group III-V family, namely AlGaAs and InGaAsP. The dissertation consists of two main parts. The first part is dedicated to the theoretical modelling of nonlinear bianisotropic AlGaAs metasurfaces, while the second part focuses on the experimental studies of the nonlinear optical performance of InGaAsP waveguides. Concerning the first part, due to the high confinement of light supported by the Mie resonances, AlGaAs nanoantennas and metasurfaces with both high refractive index and high nonlinear susceptibility have found a unique place in planar nonlinear optics, where not only the presence of high intensity of light is of significant matter, but also the optically thin thickness of the entities releases the device from phase matching. We first describe the linear optical properties of AlGaAs meta-atoms and metasurfaces such as relatively high scattering cross-sections and the bianisotropic effect. Also, we derive and explain all required analytic formulas for this purpose. Bianisotropic metasurfaces with magnetoelectric coupling and asymmetric optical properties have sparked considerable interest in linear meta-optics. However, further in this thesis, we explore the nonlinear features of bianisotropic AlGaAs metasurfaces. In particular, we explore a second-harmonic generation in a bianisotropic AlGaAs metasurface based on the multipolar interference inside the meta-atoms and the nonlinear polarization current. We theoretically demonstrate that it is possible to obtain several orders of magnitude secondharmonic power differences for the forward and backward illuminations by adjusting the geometrical parameters of the meta-atoms in such a way that quasi-bound states in the continuum (quasi-BICs) are achievable. This research paves the way for the generation of directional higher-order waves. Concerning the second part, the research is focused on exploring nonlinear material platforms for monolithic integration of active and passive devices on the same chip. In this regard, we explore InGaAsP/InP waveguides of different geometries. First, we provide the theoretical background such as the nonlinear Schrodinger equation and four-wave mixing (FWM) equations in a nonlinear waveguide, then we solve the set of FWM equations using MATLAB to observe the qualitative behavior of the signal, idler, and the pump inside a nonlinear waveguide. Furthermore, we design and employ two waveguide geometries i.e. half-core and nanowire waveguides. We first design these waveguides so that achieving zero group velocity dispersion is possible through a suitable material composition and certain geometrical dimensions. However, for the rest of the work, we continued with the waveguides of different dimensions compared to the designed ones (due to some limitations in fabrication). We demonstrate self-phase modulation (SPM) and FWM for the half-core waveguides. For the case of the nanowire waveguides, we also demonstrate the FWM effect. We measured and extracted the effective value of the nonlinear refractive index of InGaAsP/InP waveguides to be n2 = 1.9 × 10−13 cm2/W through the relation between the idler and the pump power when the phase mismatch is negligible. Finally, we experimentally observe the two-photon absorption effect in our waveguides through the nonlinear characteristics of input and output powers of the waveguides from which the two-photon absorption coefficient of 19 cm/GW is calculated.

The nonlinear optical response of low-temperature (LT) grown GaAs were studied using four-wave mixing techniques. Through measurements of the four-wave mixing response as a function of pulse delay and photon energy, a strong optical response was identi ed associated with the fundamental band gap exciton. These experiments therefore demonstrated the importance of the exciton in understanding the ultrafast nonlinear optical response of LT-GaAs despite the absence of any evidence of the exciton in past linear absorption studies in this material. Measurement of the fourwave mixing response as a function of pulse delay and the polarization states of the two excitation pulses shows that the dominant contribution to the exciton signal is tied to excitation-induced dephasing. Four-wave mixing experiments in which the sample is exposed to an additional laser pulse indicate that the exciton signal may be strongly diminished due to a combination of screening and a reduction in the total dephasing time. The short temporal duration of the above e ect provides evidence of an ultrashort (< 100 fs) electron trapping time in this system tied to arsenic related defects introduced during low-temperature growth. These ndings are of importance to the understanding of the optical properties of LT-GaAs and will aid in the development of optoelectronic devices using this material system.

Photonic integration and all-optical signal processing are promising solutions to the increasing demand for high-bandwidth and high-speed communication systems. III-V semiconductor materials, specially AlGaAs, have shown potentials for photonic integration and efficient nonlinear processes due to their low nonlinear absorption, flexibility at controlling the refractive index, and mature fabrication technology. In this thesis, we report the designs of AlGaAs microring resonators optimized for efficient four-wave mixing. Four-wave mixing (FWM) is a nonlinear optical phenomenon which can be used to realize many optical signal processing operations such as optical wavelength conversion and optical time division multiplexing and demultiplexing. Our designed AlGaAs microring resonators are expected to have good optical confinement, transmission characteristics, and efficient coupling between the ring and waveguide. Here we also present our fabrication efforts to fabricate the microring resonators device and the insights gained in the process. The microring resonators devices have a potential to be used in optical communication networks for all-optical signal processing operations.

Two experimental methods, optical limiting and degenerate four-wave mixing, are employed to study the nonlinear optical properties of various novel fullerenes structures. Optical limiting refers to decreased transmittance of a material with increased incident light intensity. Detailed measurements of the wavelength-dependence of fullerene optical limiters have illustrated several key features of reverse saturable absorption. Most important among these is the requirement of weak but non-negligible ground state absorption. We have shown that the optical limiting performance of C₆₀ can be extended into the near infrared range by appropriate modifications of the structure such as higher cage fullerenes or derivatization of the basic C₆₀ molecule. The higher cage fullerene C₇₆ shows improved optical limiting behavior compared to C₆₀, for wavelengths higher than 650 nm, but becomes a weak limiter in the 800 nm range. C₈₄, even at high concentrations in [alpha]-chloronaphthalene, does not reach the good performance of C₆₀, but instead shows weak optical limiting in the 800 nm range. We also demonstrate that by attaching various groups to the C₆₀ molecule, we can extend the optical limiting performance in the near infrared regime. The C₆₀ derivatives studied, (C₆₀ cyclic ketone, C₆₀ secondary amine, C₆₀CHC₆H₄CO₂H, and C₆₀C₄H₄(CH₃)CH₂O₂C(CH₂)CO₂H), have a similar characteristic: the attached groups cause a symmetry-breaking of the C₆₀ sphere and, therefore, there are new allowed transitions that appear as absorption features up to 750 nm. The optical limiting measurements show that these materials, even for low input energies, have an exceptionally strong optical limiting response in the 640 to 750 nm spectral region. For wavelengths higher than 800 nm, however, they become transparent and no optical limiting is observed. Excited state absorption cross-sections obtained from analysis of the optical limiting data reveal that the C₆₀ derivatives have a maximum triplet-triplet absorption cross-section at 700 nm, which is shifted from the 750 nm value for the C₆₀ molecule. For the first time, optical limiting measurements are performed on five separate C₈₄ isomers. These intriguing results show that the optical limiting behavior is strongly dependent on the cage symmetry. It is also found that the most abundant isomer does not have the strongest optical limiting performance, but is in fact one of the weaker optical limiters of the isomers isolated so far. The endohedral metallofullerenes are a unique class of fullerene materials and consist of one or more metal atoms encapsulated inside the buckyball cage. An important characteristic of these materials is the charge-transfer from the dopant atoms to the fullerene cage, which has a high electron affinity. The charge-transfer is similar to the optical excitation in a material, but although the electrons are placed in the lowest unoccupied molecular orbital (LUMO), there are no holes produced in the highest occupied molecular orbital (HOMO). This is an important analogy, since it has been previously shown that optical excitation enhances the nonlinear optical properties of a material. The nonresonant degenerate four-wave mixing experiments performed on the endohedral metallofullerene Er₂@C₈₂, at 1064 nm, show that the third order nonlinear susceptibility value is increased by orders of magnitude relative to the empty cage fullerenes, thus, confirming the charge-transfer process from the encapsulated atoms to the fullerene cage. We obtain a value [gamma]_xyyx⁽³⁾( &#173 [omega]; [omega], [omega], &#173 [omega])= &#173 8.65 &#215 10⁻³² esu for the molecular second order hyperpolarizability, which is almost three orders of magnitude larger than the values reported in literature for an empty cage fullerene. / Ph. D.

Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.

The third-order nonlinear optical (NLO) properties of indium tin oxide (ITO) thin films, Fe3+, Mn3+, Co2+ 5,10,15,20-tetrakis-4hydroxytetraphenyl)porphyrin (TPP) films, and a series of ethynyl-linked azobenzene oligomers were investigated using degenerate four wave mixing (DFWM) with 100 fs laser pulses. To measure the NLO of ITO thin films, A DFWM method for measuring thin films on thick substrates was refined for the characterization of films less than 100 nm thick and applied to ITO films ~25 nm thick. It was found that the third-order nonlinear susceptibility of ITO, χ(3)ITO, is purely electronic at 900 - 1300 nm (11000 - 7700 cm-1) and has a value of (2.16 ± 0.18) x 10-l8 m2 V-2. The χ(3)IT0 value reaches (3.36 ± 0.28) x 10-l8 m2 V(sup>-2 at 1500 nm (6700 cm-1) due to two-photon absorption by free carriers (electrons). Ultrafast electron relaxation was also observed. The ~100 fs lifetime of this process could reflect electron scattering in the conduction band. This DFWM method was also used to investigate the two-photon properties of ~500 nm thick electropolymerized films of Fe3+, Mn3+, and Co2+ TPP in the near-IR spectral region. Metalloporphyrins with strong charge transfer (CT) transitions inthe linear absorption spectra also show enhanced two-photon absorption. (Metalloporphyrin two-photon absorption cross section, δ, increases >10 times over that for the metal free porphyrin.) This effect was attributed to a two-photon induced charge transfer between the metal ion's d orbitals and the π-system of the porphyrin. Correlationof one- and two-photon absorption properties of transition metal porphyrins suggests a new and simple approach to improve organic materials for photonic applications. Finally, a series of oligomers consisting of ethynyl-linked azobenzene units was prepared using Pd-catalyzed cross coupling. The linear and nonlinear optical properties of the oligomers were investigated. The molecular second hyperpolarizability, γ, followed the power law γσ n2.12±0.05 (n is number of repeat units) for unusually large molecular lengths. The exceptional exciton delocalization length exceeds 360 conjugated bonds (>49 nm) and is attributed to the rigidity of the conjugated backbone.

The preparation of single photons in a pure quantum state is a subject of great interest in physics, enabling the control of light at an unprecedented level. The ease with which certain degrees of freedom of photon states, such as polarisation, can be manipulated, along with the inherent resilience of photons to decoherence, makes them well suited for use as qubits. Recent rapid developments in the transmission and processing of quantum information, as well as the likely technological impact of potential real-world applications such as quantum cryptography and quantum computation, mean that the demand for high performance single photon sources is likely to increase in the near future. One approach to producing single photon states, which are known to be in a well-defined spatio-temporal mode without destructively measuring them, is to take advantage of nonlinear optics. Nonlinear processes can be used to realise frequency conversion by generating a single, correlated pair of photons from an intense pump laser source. The detection of one of the photons from a pair can then be used to indicate the presence of the other photon in the pair, a procedure known as heralding. This thesis describes the development of a source of heralded single photons at 1550 nm, generated directly in the core of a photonic crystal fibre (PCF). By taking advantage of low loss fibre components for the required spectral filtering of the generated photon state, a heralding fidelity of 52% was achieved. The source was designed to be used with a picosecond pulsed fibre laser, making it relatively low cost and maintenance free. With 148 mW of average pump power a heralded output photon rate of 6.4 × 104 s-1 was observed, demonstrating the brightness of the source. The purity of the generated single photons was established by measuring non-classical interference, with a visibility of 70%, between the photons output from this source and a source based on a PPLN waveguide. The fabrication of a series of birefringent PCFs for the generation of spectrally pure state photons at 1550 nm is also discussed. These PCFs will be useful for incorporation in the next generation of high performance, fibre-based photon sources.

O emaranhamento é uma propriedade intrinsecamente quântica, que pode ser empregada em protocolos de comunicação quântica de informação, como criptografia quântica e teletransporte, e um recurso importante no processamento quântico de informação, visando a implementação de algoritmos mais eficientes que os atualmente empregados, de uso limitado em problemas ditos insolúveis, nos quais, as etapas de processamento crescem exponencialmente com o tamanho dos dados de entrada. O Processo de Mistura de Quatro Ondas [4WM: Four Wave Mixing] na configuração duplo- em vapor de Rubídio tem-se mostrado como uma fonte robusta de estados fortemente emaranhados [Boyer et al., 2008b]. No presente trabalho é caracterizado o ganho paramétrico G dos campos Stokes e anti-Stokes no processo de 4WM associado à linha D 1 do Rubídio. Os parâmetros estudados foram a dessintonia do campo de bombeio , sua intensidade I B e a temperatura da célula de Rubídio T Rb . Além da caracterização do ganho, estudaram-se também a absorção eletromagneticamente induzida e as intensidades dos feixes conjugados. Obtiveram-se ganhos de até 450% ajustáveis via sintonização destes parâmetros. Os elevados ganhos em meios atômicos permitem geração de emaranhamento sem uso de cavidade e aceitam operação multimodo. Consequentemente podem ser usados na realização de medidas ultraprecisas e ensejam a possibilidade de explorar regimes até agora inatingíveis pelos Osciladores Paramétricos Óticos [OPO] baseados em cristais (nos quais os ganhos são da ordem de 4%). O OPO tem sido estudado em situações acima ou abaixo do limiar, mas na transição rareiam resultados experimentais. Os resultados obtidos foram comparados com a literatura, e o próximo passo é usá-los em cavidades abertas para construir um OPO que seja facilmente operado acima ou abaixo do limiar, e estudar o limite entre vácuo enmaranhado e feixes intensos. Espera-se observar estados emaranhados não gaussianos gerados neste sistema e estudar suas aplicações em processamento de informação com variáveis contínuas da luz. / Entanglement is an intrinsically quantum property that can be used in quantum communication protocols, such as quantum cryptography and teleportation. It is a basic piece for quantum information processing, aiming at the implementation of more efficient algorithms than those currently used. These new algorithms are important to attack the so called nonsolvable problems, in which, the processing steps grow exponentially with the size of the input data. On the other hand, the Four Wave Mixing Process in the double- level configuration in Rubidium vapor has been shown to be a robust source of strongly entangled fields [Boyer et al., 2008b]. In the present work the parametric gain G of the Stokes and anti-Stokes fields in the 4WM process associated with the D 1 line of Rubidium is characterized. The parameters studied were the detuning and intensity I B of the pumping field and the temperature of the Rubidium cell T Rb. Besides the gain characterization, the electromagnetically induced absorption and the intensities of the conjugated fields were also studied. Adjustable gains up to 450% were achieved via the tuning of those parameters. The high gains in atomic media allow the generation of entanglement without cavity, and permit multi-mode operation. Therefore, they can be used to carry out ultra-precise measurements and allow the possibility of exploring schemes hitherto unattainable by crystal-based Optical Parametric Oscillators (OPO) (with gains of 4 %). The OPO has been studied in situations above or below the threshold, but in the transition experimental results are rare. The results we obtained were compared with literature, and the next step is to use them in open cavities to construct an OPO that can be easily operated above or below the threshold, and to study the boundary between entangled vacuum and intense beams. In this system, it is expected to be observed non-Gaussian entangled states, and to be able to study its applications in information processing with continuous variables states of light.

Cette thèse explore certaines possibilités qu’offre l’optique intégrée en silicium pour des applications en ingénierie quantique. Un premier chapitre établi la théorie de la propagation non linéaire scalaire du champ électrique dans des guides d’onde en silicium. La génération de paires dans de tels guides est également présentée. Le second chapitre reprend un travail expérimental de caractérisation des propriétés non linéaires des guides utilisés. Le résultat original principal de ce travail est un montage de caractérisation non linéaire par la méthode D-scan en régime picoseconde. Le coeur du travail est présentée dans le troisième chapitre, il s'agit de la mise en évidence, la caractérisation et de l'étude approfondie de la génération de paires de photons au sein de guides d’ondes. Le dernier chapitre est consacré à l’intégration proprement dite de la source de paires de photons au sein d’un circuit quantique afin de réaliser la majeure partie d’un expérience clé d’optique quantique sur une puce en silicium. Nous présentons deux sources de paires de photons prêtes pour l'intégration avec un circuit optique (paires en cavité et filtration spectrale). Nous présentons ensuite la préparation d'expériences intégrées préliminaires. En particulier, nous montrons l'enchevêtrement en chemin produit dans une structure intégrée. Nous réalisons également l'expérience de Hong-Ou-Mandel.