If small and identical particles are regularly arranged like a crystal

as in Fig. 5.1c, whose lattice size lies on the order of the wavelength

of light, light scattered from each particle will interfere with each

other and will be reflected to a regular direction. Such a structure

is normally called photonic crystal and has attracted particular attention in the fields of science and technology. Although this type

of structure seems to be a simple extension of 2D diffraction grating

to 3D system, the importance of photonic crystal lies in the fact that

light scattered at each particle interferes in a complicated way to

produce unique features in an optical region.