Light localizations in photonic crystal line defect waveguides

In this paper, we discuss unique light localizations in photonic crystal line defect waveguides based on two different concepts. The first concept is an additional defect doping that breaks the symmetry of the line defect. Even though such a defect is open to the line defect, the optical field is well confined around the defect at cutoff frequencies of the line defect. This expands the design flexibility of microcavities, and allows effective mode controls such as the single-mode operation. The lasing action of such cavities in a GaInAsP photonic crystal slab was experimentally observed by photopumping at room temperature. The second concept is a chirping of the waveguide structure. The photonic band of a waveguide mode has a band edge, at which the group velocity becomes zero. The band-edge condition shifts in a chirped line defect waveguide, so guided light reaches a zero group velocity point and is localized. A macroscopic behavior of this phenomenon was experimentally observed in a waveguide fabricated into a silicon-on-insulator substrate. In addition, a microscopic behavior was theoretically investigated, which suggested its applicability to a group delay device.