||In this thesis, we propose and demonstrate laterally waveguide-coupled hexagonal micro-pillar resonator add-drop filters for wavelength-division multiplexing communications. As compared with conventional circular micro-disk and micro-ring resonators, hexagonal micro-pillar resonators have the key merit of a long lateral interaction length along the entire flat resonator sidewalls. The long interaction length potentially eases the tight constraint on the sub-micrometer separation for evanescent coupling between the resonator and the waveguide. Using ray optics and the concept of wavefront-matching, we reveal that hexagonal micro-resonator modes have 6-bounce ray orbits with either closed or open loop. Such open-loop ray orbits of the same modes can have a distribution of cavity lifetimes before the light ray refractively escapes from near the cavity corners. In order to experimentally and numerically study the hexagonal micro-pillar resonances and coupling for add-drop applications, we employed two lateral coupling techniques: (1) Gaussian beam coupling, and (2) waveguide coupling. Using Gaussian beam coupling to commercially available hexagonal glass optical fibers, we observed in elastic scattering spectra the multimodes that correspond to 6-bouce ray orbits in a 125-μm-sized hexagonal micro-pillar resonator. Using standard microelectronic fabrication technologies, we demonstrate the first laterally waveguide-coupled hexagonal micro-pillar resonator add-drop filters on silicon-nitride-on-silica substrates. We obtained a coupling efficiency exceeding 98%, and a resonance quality factor about 1,400, by using an optimum waveguide width of about 0.50 μm for TM polarization. Similar device optimization was demonstrated for TE polarization with an optimum waveguide width of about 0.65 μm. By means of two dimensional finite-difference-time-domain simulations, we demonstrate that the add-drop filters characteristics can be substantially improved by rounding the hexagonal micro-resonator corners. Double-cavity round-corner hexagonal micro-pillar resonator add-drop filters are also numerically studied.