Photonic Crystals from Colloids

3 May 2007

A perfect photonic crystal could reflect 100 percent of all the incident light at a wavelength in particular band gap and would transmit almost all the light at other wavelengths. In practice, however it is very difficult to get a perfect or infinite photonic crystal and therefore the transmission rarely is 0 or 1. Actually numerical calculations for transmission or reflection from these photonic crystals show various data and especially in thin crystals the transmission inside a band gap does not always drop entirely to zero and it is also not easy to determine the band edges.

Mathematical calculations can identify three dimensional crystal structures that can have photonic band gaps, however fabrication of these photonic crystals in small nanometer length range is a difficult task because the material used for fabrication of these photonic crystals have high refractive index. The material used is generally semiconductor and the general nanofabrication techniques include lithography, holography and etching. However, new treat methods such as colloidal self-assembly method to fabricate the desired structure have many advantages over the nanofabrication methods.

Monodisperse colloidal particles can form three dimensionally periodic crystals and the lattice periodicity can also be easily adjusted from a few nanometers to micrometer by varying the particle size. Driving forces such as high concentration of particles favors the formation of three-dimensional lattice. In most cases the crystal structure obtained is fcc (face centered cubic), however depending on repulsive forces other types of lattice crystals can also be formed. Self-assembly methods can be combined with templating methods to produce high refractive index contrasts needed for photonic applications.

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