Written by Yuhang Jin, AP225 2011 Fall.
Photonic crystals are periodic dielectric nanostructures that can modulate the propagation of light. Usually a photonic crystal contains periodically repeating nano-sized areas of materials with sufficiently different dielectric constants, as displayed in Figure 1. If the absorption of light by these components is small, light can be prevented from propagating in certain directions with certain frequencies due to the Bragg-like reflection. In other words, photonic band gaps are created in photonic crystals, similar to the energy gaps in the motion of electrons in solid state crystals. Here, the macroscopic media with distinct dielectric constants are analogous to the atoms or molecules in a crystal, and the periodic refraction index experienced by the photons is analogous to the periodic Coulomb potential.
 J. D. Joannopoulos, S. G. Johnson, J. N. Winn, & R. D. Meade, "Photonic Crystals: Molding the Flow of Light", Princeton University Press, 2008.