Light can propagate much slower in photonic crystal waveguides and plasmonic waveguides than in vacuum. Slow light propagation in waveguides shows broad prospects in the terabit communication systems. However, it causes severe signal distortions and displays large propagation loss. Moreover it is vulnerable to manufacturing disorders. This thesis aims to design novel waveguides to alleviate signal distortions and propagation loss using optimization methodologies, and to explore the design robustness with respect to manufacturing imperfections. To alleviate the signal distortions in waveguides, an optimization formulation is presented to tailor the slope of the dispersion curve. The design robustness is enforced by considering different manufacturing realizations in the optimization procedure. Both free- and fixed-topology (circular-hole based) slow light photonic crystal waveguides are obtained using two different parameterizations. Detailed comparisons show that the bandwidth of slow light propagation can be significantly enhanced by allowing irregular geometries in the waveguides. To mitigate the propagation loss due to scattering in the photonic crystal waveg- uides, an optimization problem is formulated to minimize the average propagation loss of the designed modes. The presented approach is employed to design a free-topology slow light waveguide. Numerical result illustrates that slow light propagation in the optimized waveguide displays significantly suppressed propagation loss while keeping the same bandwidth. The first optimization formulation is further employed to design slow light metal- dielectric-metal plasmonic waveguides. It is shown that dispersionless slow light propagation is achieved in the optimized plasmonic waveguide. Further study reveals that the loss in metal can be compensated by integrating gain media in the optimized waveguide, while keeping negligible signal distortions.
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Mørk, Jesper, Sigmund, Ole, Jensen, Jakob Søndergaard