I. Introduction
Photonic crystals (PCs) with photonic bandgap (PBG) [1], [2] have been investigated widely, and may be incorporated into various integrated photonic devices including optical filters, sharp bending light guides, and very low threshold lasers [3]– [5]. (a) Schematic diagram of a self-assembled opal-clad waveguide. (b) Top-view of silica opals with uniform fcc closed-packed structure. (c) Cross-sectional SEM image of a fabricated 10- m-wide opal-clad waveguide, which has self-assembled silica opal cladding with uniformly coated top surface. Using nanofabrication processes, PCs of the two-dimensional (2-D) slab-type and three-dimensional (3-D) layer-by-layer structures can be built [6]–[9]. Simpler and cheaper alternatives are the self-assembled PCs (inverse opals) with full 3-D PBG [10] [11] [12]. Self-assembled opals with pseudo-PBG are used as the template, and are removed after infiltration of high refractive index materials. A defect in a PC, which allows light localization, is required to realize integrated photonic functional devices [13], [14].