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Photonic crystals are periodic optical nanostructures that can control light, specifically photons. Such crystals occur in nature in the form of structural coloration – like the natural microstructures that give opal its iridescent color – or animal reflectors, like the wings of certain butterflies. Their synthetic counterparts have a wide range of potential applications, from reflection coatings to optical computers.
Also known as photonic band gap materials, photonic crystals work on a principle similar to the way semiconductors enable the creation of electronic devices. The crystals have band gaps that permit some wavelengths of light to pass but not others, allowing the unparalleled control of the behavior of light. They are composed of periodic dielectric, metallo-dielectric or superconductor microstructures or nanostructures. Dielectric materials are essentially electrical insulators that are polarizable with an electric field and can affect electromagnetic wave propagation by defining ‘allowed’ and ‘forbidden’ electronic energy bands.
Photonic crystals are regularly repeating regions of high and low dielectric constant. Photons behaving as waves proliferate through a structure or not depending on their wavelength; those that propagate are called modes and groups of allowed modes form bands, disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission (where a quantum mechanical system transitions from an excited to lower state an emits a photon), high-reflection omnidirectional mirrors and low-loss-waveguiding.
Band gaps in photonic crystals are thought to arise from the destructive interference of multiple reflections of light propagating in the crystal at the interfaces of high- and low-dielectric constant regions, much like the band gaps of electrons in solids. The periodicity of the crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. In the visible part of the spectrum, this is around 350nm to 650 nm i.e. blue to red light.
Photonic crystals can be useful in a range of applications – in principle anywhere light must be controlled or manipulated - and can be fabricated in one-, two- or three-dimensions. One-dimensional materials can be made of layers deposited or stuck together, and are already in widespread use in the form of thin-film optics with applications from high and low reflection coatings on lenses and mirrors to color changing inks and paints.
Two-dimensional crystals can be fabricated by photolithography or drilling holes in a suitable substrate. These materials are also widely used as photonic-crystal fibers, where microscale structures are used to confine light with radically different characteristics compared to conventional optical fiber. They use the structural properties of their construction materials to exercise greater control over light and are in applications for nonlinear devices and to guide exotic wavelengths, in high-speed communications, fiber lasers and power transmissions.
Three-dimensional materials can be made by drilling under different angles, stacking multiple two-dimensional layers on top of each other, direct laser writing or by instigating self-assembly of spheres in a matrix before dissolving said spheres. However, three-dimensional photonic crystals are far from being ready for commercialization due to difficulties in their manufacture. Fabricating repeating regions of high and low dielectric constant is problematic, and there are several major challenges that must be overcome to create high-dimension photonic crystals. They need to be designed with enough precision to prevent scattering losses blurring crystal properties and fabrication processes need to be robust enough to withstand the mass production of crystals.
Once these issues are overcome, such materials could offer additional features such as optical nonlinearity which is required for optical transistors, a breakthrough that could help optical computers run up to 10 times faster than their regular counterparts. They could also lead to more efficient photovoltaic cells as a source of power for electronics, thus lessening the requirement for electrical input for power.
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