A periodic dielectric stack creates a photonic band structure analogous to electrons in crystals. At certain frequencies, Bragg reflections from all layers add constructively — creating a photonic bandgap where light cannot propagate.
Quarter-wave stack (n₁d₁ = n₂d₂ = λ/4) maximizes the bandgap. Omnidirectional reflection arises when the bandgap exists for all angles and polarizations.
Applications: laser mirrors, VCSEL cavities, optical fiber coatings, structural color in butterfly wings (no pigment — just photonic crystals).