A photonic crystal is a nanostructure with a periodically changing refractive index. The propagation of light (electromagnetic waves with wavelengths on the scale of hundreds to thousands of nanometers) within the crystal can be controlled through this nanostructure. An example is a nanoperiodic structure in which, through the periodic arrangement of air holes less than 1 μm in size, the refractive index changes at the same period as the wavelength of light. By confining light inside the structure or by blocking the penetration of light, this structure yields optical phenomena. To take examples from nature, butterfly wings or gemstones such as opals have sparkling structural colors and a luster that changes with viewing angle. This effect is due to the blocking of light penetration by naturally formed nanoperiodic structures. It is possible to create a frequency domain (photonic band gap) with no electromagnetic mode by using a perfect periodic array. Moreover, by intentionally introducing structural disorder into part of the periodic array, it is possible to generate an electromagnetic mode (localized mode) that is isolated spatially and isolated in terms of frequency. As light can be confined locally by using a full band gap, photonic crystals are expected to find application in quantum optics tools for freely controlling the spontaneous emission rate of light, and in future quantum computers.

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Methods used to fabricate photonic crystals include (1) three-dimensional deposition of microspheres of polystyrene or silica (artificial opal), (2) two-dimensional and three-dimensional processing of semiconductors through lithography, (3) dielectric multilayer films through sputtering, (4) cutting, and (5) stereolithography. The adoption of light-emitting materials such as semiconductor quantum wells and quantum dots is important for the future development of photonic crystal slabs. In particular, the use of quantum dots with discretized electronic energy levels is expected to enable the single-photon illuminants required to achieve quantum communication, the quantum bits that form basic computation elements in quantum computing, optical switches that make use of non-linear properties, and optical logical computation elements, as well as conventional illuminants such as semiconductor lasers.