A large, three-dimensional photonic bandgap

is predicted to occur in an amorphous structure. Photonic crystals are useful because they block the transmission of light in a narrow wave-band. For some applications, the bandgap should be 3D and as isotropic as possible. Achieving that desirable combination has proved difficult. Most crystal structures don’t yield 3D photonic bandgaps. Diamond is an exception, but its bandgap isn’t isotropic. Now, Keiichi Edagawa and Satoshi Kanoko of Tokyo University and Masaya Notomi of NTT Corp have shown that a structure with no long-range order can have a large, 3D bandgap. And because the structure is amorphous, its bandgap is guaranteed isotropic. The new structure is based on a model for amorphous silicon developed in 1971 by Donald Polk. To create it on a computer, the three researchers started with a random assembly of 1000 particles. They let small groups of particles form tetragonal bonds. Then they removed the particles and turned the bonds into dielectric cylinders. The figure on the left shows a diamond lattice; the figure on the right shows the amorphous structure. Using their computer, the researchers compared the optical properties of the two structures. The diamond structure had the wider bandgap, but not by much. The researchers hope to make a real version of their structure, but their simulation has already proven one important result: Contrary to textbook explanations, Bragg scattering off crystal planes is not necessary to form a 3D bandgap. (K. Edagawa, S. Kanoko, M. Notomi, Phys. Rev. Lett. , in press.)