I was reading a review by Kalele et. al, and found out this very informative segment on photonic band gap materials:
"Photonic band gap materials are optical equivalents of semiconductors.
Behaviour of photons in such materials can be analogically correlated to behaviour of electrons in semiconductors. Colloidal particles can self-assemble to form a three-dimensional crystal having long-range periodicity.
Air gaps between the particles form a region of low refractive index, while the particles form a region of high refractive index. When photons are incident on these materials, they pass through regions of the high and low refractive indices. For photons, this contrast in refractive index is similar to the periodic potential that an electron experiences while passing through a semiconductor. If the contrast in refractive index is large, then the photons are either totally reflected or confined inside the dielectric material. The colloidal crystal blocks wavelengths in the photonic band gap, while allowing other wavelengths to pass through.
The photonic band gap can be tuned by changing the size of the particles.
Similar material can be prepared using core shell particles (120). Core shell particles are better suited for this application, as relative refractive index contrast in core and shell particles is more.
Band gap can be tuned from visible to IR range by changing index contrast (121)."