What is a photonic crystal?

A photonic crystal is a periodic structure of materials with differing refractive indices. Therefore, the optical multilayer films widely used as anti-reflection films etc. for the surface of television monitors, glasses and other optical components can be considered to be one dimensional photonic crystals. However, generally the term photonic crystal refers to two dimensional and three dimensional structures. The pitch of the structure is usually chosen such that it is half the wavelength of the light for which the element is designed for. Typically, photonic crystals used in the visible optical region are designed and fabricated so that the period is about 300nm.

Like optical multilayer films, photonic crystals are used to control optical transmission, reflection and refraction characteristics. Using 2D and 3D photonic crystals it is possible to control the propagation of light at arbitrary angles of incidence and not only the light normally incident as is the case for conventional optical films. Further, using photonic crystals, it is possible to achieve optical functionality not possible using conventional optical materials.

Various photonic Crystals

Two different optical materials (or one optical material and air) periodically arranged as above constitute a photonic crystal. A periodic one dimensional structure can easily be fabricated by sequential deposition of two materials. Though, various structures and fabrication methods have been proposed for photonic crystals of two dimensions and three dimensions, these are considerably more difficult to fabricate.

The wood pile photonic crystal was one of the first three dimensional photonic crystals to be proposed. The origin of the name is the manner in which the optical material is piled up to form a support structure similiar to that of a well. The fabrication process of piling up the "logs" is complex and the use of semiconductor fabrication processes is necessary. The opal type photonic crystal stacks beads. These techniques generally require a large amount of time, though there are techniques such as self-assembly; where grains of an optical material are placed in a liquid. Our autocloned photonic crystal is composed of a patterned multilayer film, and is the only, multi-dimensional, photonic crystal that can be fabricated by conventional sputtering deposition, and established mass production technologies

Photonic Crystal Essentials

The key to understanding photonic crystals is the dependancy of the interaction between light waves and the structure on the period of the structure. The most widely know example of a multilayer optical film is the Bragg reflector. The Bragg reflector employs the phenomenon of total reflection, which occurs as a result of the intensity of the reflected light increasing at each layer as shown in the figure below.

Research on the electron transmitted as (an electromagnetic wave) in periodic atomic structures of the atom is established as band theory. A similiar model is applicable to the interaction of a photonic crystal and light. Despite the periodicity of photonic crystals being on the order of 100nm, they are referred to as crystals by analogy to molecular structures with a periodicty on the order of angstroms (1 Angstrom=0.1nm). The wavelength range over which light is transmitted through the crystal is called the "Photonic Band" and the range over which light is not transmitted the "Photonic Bandgap".

Functionality

Optical functionality that cannot be achieved by conventional materials are possible because of phenomena that occur naturally in photonic crystals. For instance, two such phenomena are negative refraction and anomalous dispersion. Such phenomena occur in the narrow region of the band edge, the wavelength range between the photonic band (transmission) and the photonic bandgap (reflection). The technology of controlling the behaviour in such a boundary area is know as photonic band edge engineering. Much photonic crystal research in recent years has focused on the fabrication of photonic crystal light pipes (waveguides). Low loss photonic crystal optical waveguides capable of guiding light through abrupt changes in direction with neglibile loss have been realized, and it is anticipated that the realization of optical circuits will be a revolutionary technology.

The main concern in such research was overcoming the difficulty of the process of manufacture. Until now, because of the extremely intensive processing technology required to fabricate photonic crystals of two or more dimensions, and despite the wide adoption of one dimensional multilayer optical films, higher dimentional photonic crystals have not been commercialized. Our company has succeeded in the establishment of a mass production technology for photonic crystals. We have focused on establishing markets suited to the current level of technology rather than advancing our processing technology. Our stragtegy is to promtly commercialize photonic crystals.