| Photonic crystal is an artificially engineering optical material where the index of refraction is periodically varied. It is an optical analogy to the solid state crystal. The scattering at the dielectric interfaces can produce a "photonic" bandgap where the propagation of electromagnetic field is not allowed. Creating a point defect in an otherwise perfect lattice will cause photons to be trapped in the defect leading to a small dimension, high quality factor resonator. On the other hand, creating a linear defect in the photonic crystals will channel the electromagnetic field along the defect, resulting in a tightly confined photonic crystal waveguide.; This work is to provide a comprehensive understanding of photonic crystal waveguides. First, the origin of the defect modes in "photonic" bandgap is analyzed by applying the Slater-Koster model for the electromagnetics. The dispersion relation of the photonic crystal waveguide modes are calculated both from coupled mode theory point of view and with the full vectorial finite difference time-domain method. The transmission properties of photonic crystal waveguides on either a suspended membrane, or an oxidized lower cladding, or a low index contrast epitaxial structure are numerically and experimentally characterized. The mechanism for the vertical radiation loss is identified and an optimization strategy is outlined. It is shown that the out-of-plane radiation loss can be reduced by shifting the photonic crystal cladding half a period with each other along the propagation direction.; An efficient power coupling method is also proposed between single-mode optical fibers and photonic crystal waveguides. Better than 90% transmission can be achieved with a full width half magnitude bandwidth of 12 nm. It is shown that the photonic crystal cladding functions as a grating coupler as well as provides the field confinement. |
