Theory of light propagation in strongly modulated photonic crystals: Refractionlike behavior in the vicinity of the photonic band gap

Although light propagation in weakly modulated photonic crystals is basical ly similar to propagation in a diffraction grating in which conventional re fractive index loses its meaning, we demonstrate that light propagation in strongly modulated two-dimensional (2D)/3D photonic crystals becomes refrac tionlike in the vicinity of the photonic bandgap. Such a crystal behaves as a material having an effective refractive index controllable by the band s tructure. This situation is analogous to the effective-mass approximation i d electron-band theory. By utilizing this phenomenon, negatively refractive material can be realized, which has interesting optical properties such as mirror-image refraction.