Photonic crystal

A material with a periodic variation in refractive index that controls light propagation. Its structure can create photonic band gaps that prevent selected wavelengths from propagating in particular directions.

Photonic band gap: A frequency range in which a periodic optical structure forbids light propagation in specified directions. The gap is the defining effect of the crystal’s periodic refractive-index pattern.
Refractive index: A dimensionless measure of how much light slows and bends in a material. Its spatial variation supplies the optical contrast that forms the crystal.
Photonic crystal cavity: A resonant optical cavity that confines light using a photonic band gap or a defect in a periodic structure. A defect can trap selected wavelengths in a volume far smaller than many conventional cavities.
Metamaterial: An engineered material whose effective properties arise from structures smaller than the wavelengths being controlled. Metamaterials often rely on subwavelength effective response, whereas photonic crystals rely on periodic scattering.
Eli Yablonovitch: An American physicist who helped establish photonic crystals and proposed using them to control spontaneous emission. His 1987 proposal connected photonic band gaps with suppression of spontaneous emission.
Bragg scattering: Coherent scattering from periodically spaced structures that reinforces waves at selected wavelengths and directions. Repeated scattering from the crystal’s lattice produces its wavelength-selective optical response.
Bravais lattice: An infinite array of points generated by translations along a set of lattice vectors. Lattice symmetry organizes the repeated refractive-index pattern.
Photonic crystal laser: A laser that uses a photonic crystal to control optical feedback, emission, or confinement. Periodic optical structure can shape the laser mode and reduce its required footprint.
Bragg mirror: A layered reflector that uses alternating refractive indices to reflect a designed wavelength range. It is a one-dimensional periodic optical structure, unlike crystals with periodicity in multiple directions.
Sajeev John: A physicist whose 1987 work proposed localization of light in disordered and periodic dielectric structures. His contemporaneous proposal helped launch the modern study of photonic band gaps.

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