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

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Laser filamentation is a highly complex and dynamic nonlinear process that is sensitive to many physical parameters. The basic properties that define a filament consist of (i) a narrow, high intensity core that persists for distances much greater than the Rayleigh distance, (ii) a low density plasma channel existing within the filament core, and (iii) a supercontinuum generated over the course of filamentation. However, there remain many questions pertaining to how these basic properties are affected by changes in the conditions in which the filaments are formed; that is the premise of the work presented in this dissertation. To examine the effects of anomalous dispersion and of different multi-photon ionization regimes, filaments were formed in solids with different laser wavelengths. The results provided a better understanding of supercontinuum generation in the anomalous dispersion regime, and of how multi-photon ionization can affect the formation of filaments. Three different experiments were carried out on filamentation in air. The first was an investigation into the effects of geometrical focusing. A simplified theoretical model was derived to determine the transition of filamentation in the linear-focusing and nonlinear- focusing regimes. The second examined the effects of polarization on supercontinuum generation, where a polarization-dependent anomalous spectral broadening phenomenon due to molecular effects was identified. The third involved the characterization of filaments in the ultraviolet. The combination of physical mechanisms responsible for filamentation in the ultraviolet was found to be different from that in the near infrared.

The main goal of this dissertation is to investigate the third-order nonlinearity of organic molecules. This topic contains two aspects: two-photon absorption (2PA) and nonlinear refraction (NLR), which are associated with the imaginary and real part of the third-order nonlinearity (χ (3)) of the material, respectively. With the optical properties tailored through meticulous molecular structure engineering, organic molecules are promising candidates to exhibit large third-order nonlinearities. Both linear (absorption, fluorescence, fluorescence excitation anisotropy) and nonlinear (Z-scan, two-photon fluorescence, pump-probe) techniques are described and utilized to fully characterize the spectroscopic properties of organic molecules in solution or solid-state form. These properties are then analyzed by quantum chemical calculations or other specific quantum mechanical model to understand the origins of the nonlinearities as well as the correlations with their unique molecular structural features. These calculations are performed by collaborators. The 2PA study of organic materials is focused on the structure-2PA property relationships of four groups of dyes with specific molecular design approaches as the following: (1) Acceptor-π-Acceptor dyes for large 2PA cross section, (2) Donor-π-Acceptor dyes for strong solvatochromic effects upon the 2PA spectra, (3) Near-infrared polymethine dyes for a symmetry breaking effect, (4) Sulfur-squaraines vs. oxygen-squaraines to study the role of sulfur atom replacement upon their 2PA spectra. Additionally, the 2PA spectrum of a solid-state single crystal made from a Donor-π-Acceptor dye is measured, and the anisotropic nonlinearity is studied with respect to different incident polarizations. These studies further advance our iv understanding towards an ultimate goal to a predictive capability for the 2PA properties of organic molecules. The NLR study on molecules is focused on the temporal and spectral dispersion of the nonlinear refraction index, n2, of the molecules. Complicated physical mechanisms, originating from either electronic transitions or nuclei movement, are introduced in general. By adopting a prism compressor / stretcher to control the pulsewidth, an evolution of n2 with respect to incident pulsewidth is measured on a simple inorganic molecule –carbon disulfide (CS2) in neat liquid at 700 nm and 1064 nm to demonstrate the pulsewidth dependent nonlinear refraction. The n2 spectra of CS2 and certain organic molecules are measured by femtosecond pulses, which are then analyzed by a 3-level model, a simplified "Sum-over-states" quantum mechanical model. These studies can serve as a precursor for future NLR investigations.

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A complete study was conducted on optical limiting characterization in samples of carbon black microparticles in a mixture of deionized water and ethylene glyccol using nanosecodn and picosecond later pulses at 532 nm and 1064 nm.

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The research described in this thesis relates to the design, fabrication and testing of novel semiconductor-based light sources that have been designed for the generation of infra-red light. The thesis is formatted to account for two distinct components of my work, where the first part concerns sources producing coherent light by direct laser emission, notably, ultrashort-pulse quantum-dot lasers. These types of lasers continue to show considerable promise as efficient, compact sources of ultrashort pulses with durations of hundreds of femtoseconds, while giving rise to unique and interesting electronic properties such as low lasing thresholds through the quantum nature of their density of states. At the outset a study of the most relevant aspects of the lasing dynamics of an optically pumped quantum-dot laser is outlined. Pumping of the device with intense discrete optical pulses leads to output from multiple electronic states, each having a characteristic wavelength and temporal properties. I show that pulses produced by excited-state emission have shorter durations (24 ps) and arrive earlier in time than those due to transitions from the ground state, which themselves have durations of around 180 ps. Investigations are then made on two different mode-locked quantum-dot laser systems. One is an all-quantum-dot external-cavity laser that is mode locked using a quantum-dot SESAM device at a repetition frequency of 860 MHz with output power approaching 20 mW. This is followed by a study of a monolithic two-section quantum-dot laser that is mode locked stably in a wide temperature range of 20°C to 70°C. The excellent performance characteristics presented serve to demonstrate both the versatility of quantum-dot material as components in mode-locked laser systems and the temperature stability of such laser devices. The second part of the thesis relates to structures that are designed to take advantage of nonlinear frequency conversion in GaAs-based semiconductors. This material system possesses a nonlinear coefficient of ~170 pm/V and is transparent from around 0.9 μm through to 17 μm, making it attractive for the realisation of a new class of efficient, integrable, quasi-phase-matched, optical parametric oscillator devices. Initially, ion implantation is utilised as a vector to create a periodically-switched nonlinear ridge waveguided device. The observation is made that in the course of implantation the transmissive properties of the device are severely degraded. Unfortunately, the high losses incurred, which reached 250 dB/cm, could not be removed without also destroying the modulation in nonlinearity. During the course of this investigation, significant technological advances were made in the production of orientation-patterned GaAs structures. By recognising the elegance and potential of this new orientation-patterned (OP) methodology, a study of its implications and applicability in the context of my project is initiated.

"Molecular engineering of some 2,4,6-(substituted biarylamino)-1,3,5-triazines and crystal data belonging to the products 2,4,6-(m,m’-ditolylamino)-1,3,5-triazine, 2,4,6-(p,p’-ditolylamino)-1,3,5-triazine and 2,4,6-(phenyl-p-tolylamino)-1,3,5-triazine were reported. Retrosynthetic analysis of trigonal octupolar networks led to the identification of tris-substituted diarylamino-triazines as molecular analogs of Piedfort units formed by cofacial dimers of 2,4,6-triaryloxy-1,3,5-triazine molecules. Synthesis of mono and diarylamiono triazines is achieved by coupling of the corresponding anilines with cyanuric chloride. Synthesis of diarylamines exhibiting different functional groups on two phenyl rings is attempted; the successful attempt in the case of phenyl-p-tolylamine is described. All the crystals obtained so far belong to centrosymmetric space group P21/c. Though none of the molecules retain trigonal symmetry in the crystal structures, pseudo-trigonal assembly of molecules is identified in some cases. The assembly of molecules within the crystals results in columnar structures formed by C-H..N and C-H..pi interactions."