In 2D photonic crystals, cavities having a heightened Q factor are made
available, wherein combining the high Q cavities with waveguides affords
channel add/drop filters having high resolution. In a cavity constituted
by a point defect within a 2D photonic crystal, the 2D photonic crystal
is configured by an arrangement, in a two-dimensional lattice of points
defined in a slab (1), of low-refractive-index substances (2) having a
low refractive index relative to the slab (1) and being of identical
dimension and shape. The point defect (4) contains a plurality of three
or more lattice points that neighbor one another, and in these lattice
points no low-refractive-index substances (2) are arranged; therein the
dimension of the low-refractive-index substance (2) that should be
arranged to correspond to at least one of the lattice points nearest the
point defect (4) is dimensionally altered from a predetermined dimension.