Photonic Crystal

A material whose refractive index repeats like atoms in a crystal — but for light instead of electrons — carving out colour-specific forbidden zones that photons simply cannot enter.

A photonic crystal redirects or completely blocks chosen wavelengths of light, without absorbing them. This can inhibit spontaneous emission from atoms or quantum dots embedded in the crystal, route light around sharp corners without loss, trap light in tiny cavities with extremely high quality factors, and guide photons in hollow-core fibres without any solid material touching the light at all.

Just as electrons in a semiconductor experience a periodic electric potential from atom positions and develop an electronic bandgap, photons in a periodic dielectric experience a periodic refractive-index potential. Bragg scattering from the lattice planes creates destructive interference for photons at specific frequencies. If the contrast between the two dielectric materials is large enough and the geometry is right, the stop-bands from every lattice direction overlap and form a complete photonic bandgap: a frequency range forbidden in all directions and polarisations. The bandgap scales with the lattice constant — roughly, the bandgap is centred at a wavelength similar to twice the lattice period — so optical bandgaps need sub-micrometre periodicity while microwave-frequency gaps use millimetre-scale structures.

In plain terms

Imagine a wall of perfectly spaced fence posts that stops exactly one note from a speaker — every other sound passes through, but that one frequency bounces back no matter what angle you aim it. A photonic crystal does the same thing to light: its repeating pattern of different materials turns certain colours of light into a permanent no-entry zone.

Also called: PBG material, photonic bandgap material, photonic lattice.

Why it matters

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A short history

The concept of a photonic bandgap was proposed independently in June 1987 by two papers published in the same issue of Physical Review Letters: Eli Yablonovitch at Bellcore (PRL 58, 2059) showed that a three-dimensionally periodic dielectric could exhibit an electromagnetic bandgap that rigorously forbids spontaneous emission, and Sajeev John at Princeton (PRL 58, 2486) showed that photons in disordered dielectric superlattices could undergo Anderson localization. In 1991 Yablonovitch's group physically realised the first complete 3D photonic crystal — the structure now called Yablonovite — by drilling holes in a dielectric block at centimetre scale to open a bandgap at microwave frequencies, demonstrating the theoretical predictions. Photonic crystal fibres emerged in the late 1990s from Philip Russell's group at the University of Bath; the first hollow-core fibre appeared in 1999, enabling light guidance through air. The Joannopoulos group at MIT provided the definitive computational toolkit and, with the textbook Photonic Crystals: Molding the Flow of Light (freely available at ab-initio.mit.edu), shaped how the field is taught and practised worldwide.

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