Electronic devices based on the crystalline properties of semiconductors—transistors and integrated circuits—have unleashed immense computing power and transformed disciplines across science and industry. This success has spurred development of optical analogs. The fundamental principles of transistors can be traced to the interaction of the wavelike properties of electrons with the repetitive spacing of atoms found in semiconductor crystals. An optical device would rely instead on the interaction of the wavelike properties of photons with a periodic spacing of dielectric constants. Devices based on this principle are known as photonic crystal (PhC) devices. They offer for optics the same potential for miniaturization that has occurred for electronic integrated circuits.