The attractiveness of a photonic crystal can be attributed to its unique optical characteristics. Specifically, the photonic crystal cavity provides strong light confinement, and enables the switching of light with very low threshold power. In a conventional process, to add this functionality, we have to modify its periodical structure during the CAD data processing step before electron beam lithography. This requirement limits the freedom of fabrication process. We can eliminate this limitation by taking advantage of the mode gap phenomena, with which we can furnish new cavity in the pre-patterned photonic crystal structure. The details of our cavity formation mechanism are described in reference 1). In Fig. 1(b), the pre-patterned structure is a line defect waveguide in a two-dimensional (2D) photonic crystal slab, and we assume that the refractive index is modulated in the yellow shaded region. A very small spatial index modulation (δn/n < 0.1%) changes the mode-gap edge frequency of the modified area to create barrier regions, while the unmodified area retains its original mode-gap edge frequency. As a result, an ultrahigh-Q cavity with a small volume can be created.