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On 21 October 2014, Yannick Lefevre defended his PhD thesis titled: “New Perspectives on Four-Wave Mixing in Silicon Waveguide Structures.” Optical four-wave-mixing processes are nonlinear mixing interactions that enable a wide variety of photonic functionalities, including wavelength conversion, parametric amplification, all-optical switching, and signal regeneration. In silicon waveguide structures, these interactions can be triggered at low optical powers due to the tight optical confinement offered by these structures. The goal of this PhD work is to optimize various aspects of the four-wave-mixing processes in such structures, and achieve more efficient and robust devices as well as novel four-wave-mixing applications. Yannick did so by developing new schemes for realizing efficient four-wave-mixing interactions in silicon-on-insulator nanowires. The resulting schemes of ‘phase-mismatch switching’ and ‘double quasi-phase-matching’ are applicable in a wider variety of conditions than conventional methods, and also enable novel applications. In addition, Yannick developed a new, computationally-efficient optimization method to design optical devices based on coupled-mode equations that depend on a large number of design parameters. This ‘adjoint-enabled optimization’ allows for improving the performance and robustness of four-wave-mixing-based devices, and for acquiring additional physical insight into the mechanisms leading to optimal performance. Finally, Yannick discussed in his PhD work the prospects for four-wave-mixing in silicon slow-light photonic crystal waveguides. He investigated the limitations of the conventional first-order nonlinear Schrödinger equation for modeling such devices, and introduced a novel higher-order formalism to overcome these limitations.