Photonic Band Gaps
Photonic crystals are periodic arrangements of dielectric materials that create band gaps — frequency ranges where electromagnetic wave propagation is forbidden. First proposed by Yablonovitch and John in 1987, these structures provide unprecedented control over light, enabling waveguides, filters, and cavities at the wavelength scale.
1D Bragg Stacks
The simplest photonic crystal is a one-dimensional stack of alternating high- and low-index layers, known as a distributed Bragg reflector. Constructive interference of reflections from each interface produces a stop band whose width depends on the index contrast and whose center wavelength scales with the optical path length per unit cell.
Gap Engineering
The band gap can be tuned by adjusting the lattice period (shifting center wavelength), index contrast (widening or narrowing the gap), and fill fraction (optimizing gap width). For a 1D crystal, maximum gap-midgap ratio occurs near 50% fill. Two-dimensional and three-dimensional photonic crystals offer omnidirectional gaps but require higher index contrasts.
Applications in Photonics
Photonic crystals enable vertical-cavity surface-emitting lasers (VCSELs), wavelength-division multiplexing filters, photonic crystal fibers with engineered dispersion, and slow-light devices for optical buffering. Their ability to confine light to volumes smaller than a cubic wavelength makes them essential for integrated photonic circuits and quantum optical devices.