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Photonic crystals. Computer artwork of assorted photonic crystals, materials that can trap and direct light (photons). Photonic crystals could be used in optical computers that use photons, not electrons, to carry information. This allows them to be smaller and more efficient. Several types of photonic crystal are seen here. The tall orange blocks are three-dimensional periodic photonic crystals. The arrays of blue balls are metallo- dielectric crystals, which trap microwave radiation instead of light. The red paths are one-dimensionally periodic crystals, while the green rods are two-dimensionally periodic crystals incorporating 90 degree waveguide bends. (KEYSTONE/SCIENCE PHOTO LIBRARY/J. JOANNOPOULOS/MIT)