In a photonic crystal, the arrangement of material matters as much as its composition. Periodic changes in refractive index determine how light waves propagate. Under suitable conditions, a frequency range cannot travel through the structure in particular directions. This is called a photonic band gap.
Engineers can introduce deliberate defects into the periodic pattern. An altered row may guide light, while a small cavity can confine it in a resonance. Applications and research devices include filters, resonators and structures used in specialty optical fibers. A photonic crystal does not necessarily block every direction: geometry, materials and wavelength determine its behavior.
Confining light in small volumes is useful for sensing, communications and studying interactions between light and matter. Photonic crystals are related to metamaterials but are not a synonym: periodicity and interference play a central role here. Established physical structures support both practical components and ongoing research into integrated photonics.