Photonic crystals are periodic dielectric or metallo-dielectric (nano)structures that are designed to affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. The absence of allowed propagating EM modes inside the structures, in a range of wavelengths called a photonic band gap, gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting omnidirectional mirrors and low-loss-waveguiding among others.
Since the basic physical phenomenon is based on diffraction, the periodicity of the photonic crystal structure has to be in the same length-scale as the wavelength of the EM waves i.e. ~300 nm for photonic crystals operating in the visible part of the spectrum. This makes the synthesis cumbersome and complex. To circumvent nanotechnological methods with their big and complex machinery, different approaches have been followed to grow photonic crystals as self-assembled structures from colloidal crystals.
A prominent example of a photonic crystal is the naturally occurring gemstone opal. Its opalescence is essentially a photonic crystal phenomenon based on Bragg diffraction of light on the crystal's lattice planes. Another well-known photonic crystal is found on the wings of some butterflies such as the blue Morpho (Morpho granadensis).
Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. They are of great interest for both fundamental and applied research, and are expected to find commercial applications soon. Two-dimensionally periodic photonic crystals already have reached a level where integrated-device applications are in sight, whereas their three-dimensional counterparts are still far from commercialization but will offer additional advantages possibly leading to new device concepts, when some technological aspects such as manufacturability and principal difficulties such as disorder are under control. The first commercial products involving two-dimensionally periodic photonic crystals are already available in the form of photonic-crystal fibers, which use a nanoscale structure to confine light with radically different characteristics compared to conventional optical fiber for applications in nonlinear devices, guiding exotic wavelengths, and so on.
The simplest form of a photonic crystal is a one-dimensionally periodic structure, such as a multilayer film (a Bragg mirror); electromagnetic wave propagation in such systems was first studied by Lord Rayleigh in 1887, who showed that any such one-dimensional system has a band gap. 1d-periodic systems continued to be studied extensively, and appeared in applications from reflective coatings to distributed feedback (DFB) lasers. 2d-periodic optical structures, without band gaps, received limited study in the 1970s and 1980s. The possibility of two- and three-dimensionally periodic crystals with corresponding two- and three-dimensional band gaps was not suggested until 100 years after Rayleigh, by Eli Yablonovitch and Sajeev John in 1987, and such structures have since seen growing interest by a number of research groups around the world. With applications including LEDs, optical fiber, nanoscopic lasers, ultrawhite pigment, radio frequency antennas and reflectors, and photonic integrated circuits.
- Yablonovitch, Eli, "Inhibited Spontaneous Emission in Solid-State Physics and Electronics". Bell Communications Research, Navesink Research Center, Red Bank, New Jersey.
- John, Sajeev, "Strong localization of photons in certain disordered dielectric superlattices". Department of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey.
- Photonic crystals tutorials by Steven G. Johnson (MIT)