Photonic crystals (PCs) are extremely small structures, typically no more than a few micrometers on a side, which are made of alternating regions of insulating material and air. One way this can be achieved is by drilling or etching holes in the material at regular intervals in a grid pattern. A beam of photons passing through a PC thus experiences periodic changes in refractive index – high in the insulator, low in the air holes.
This is strikingly analogous to what electrons experience as they move through the geometry of a semiconductor lattice, and it allows researchers to manipulate the passage of light through PCs in much the same way that electrons are controlled in a transistor. For example, creating a defect in the crystal lattice can strongly confine photons much the same way that electrons are confined in lattice defects of a crystal.