When a point defect is created in a photonic crystal, it is possible for that defect to pull a light mode into the band gap. Because such a state is forbidden from propagating in the bulk crystal, it is trapped. The mode decays exponentially into the bulk. Such a point defect, or resonant cavity, can be utilized to produce many important effects.
Point defects are at the heart of many other photonic crystal devices, such as channel drop filters.
The picture at right depicts a resonant cavity in our three-dimensional structure, where the colors correspond to the value of the magnetic field component perpendicular to the plane of the image. By changing the size or the shape of the defect, its frequency can easily be tuned to any value within the band gap. Moreover, the symmetry of the defect can also be tuned. By increasing the amount of dielectric in the defect, one can pull down higher-order modes, corresponding to the s, p, d, etcetera states in atoms.