Photon crystals refer to artificial periodic dielectric structures with the characteristic of photonic bandgap (PBG), sometimes also referred to as PBG photon crystal structures. The photonic bandgap refers to a frequency range where waves cannot propagate within this periodic structure, essentially creating a "forbidden band" within the structure itself.

This concept was proposed in the optical field, and its research scope has expanded to microwave and acoustic wave bands. Due to the comparable wavelength of this structure's periodic size to the central frequency of the "bandgap," it is easier to realize in the microwave band than in the optical band.

Fundamentals of Photonic Crystals

The generation of photonic crystals is similar, emerging from scientists' hypothesis that photons can also exhibit patterns of propagation akin to electrons in conventional crystals.

From the crystal structure diagram, it is evident that the atoms within the crystal are arranged in a periodic and orderly manner. It is the presence of this periodic potential field that causes moving electrons to undergo Bragg scattering within the periodic potential field, thus forming band structures. There may be gaps between the bands. If the energy of an electron wave falls within a bandgap, it cannot continue to propagate. In fact, whether it's electromagnetic waves or other waves like light waves, any wave subjected to periodic modulation will have band structures and may also experience bandgaps. Waves whose energy falls within the bandgap are also unable to propagate.

In summary, the periodic arrangement of ions in semiconductors gives rise to band structures, which in turn govern the movement of carriers (electrons or holes) within the semiconductor. Similarly, in photonic crystals, it is the periodic variation of the refractive index that creates a photonic bandgap structure, which then controls the movement of light within the photonic crystal.