Although research laboratories around the world have studied photonic crystals intensively, few efficient photonic devices have been built from the structures. Scientists at Kyoto University in Japan report that this is due largely to a lack of general rules governing design over broad wavelength ranges, and they have developed an in-plane hetero photonic crystal "channel add-drop" device to demonstrate the effectiveness of general design rules.Central to the concept, originated by Susumu Noda and his colleagues in the department of electronic science and engineering, is that the characteristics of a photonic crystal depend on the structure of the unit cell, so a crystal has allowed and forbidden propagation bands based on the device geometry. In the planar photonic crystal that they built, the spacing of airholes in a silicon slab governs the shape of the band structure.The device comprises seven photonic crystals built with airholes in a triangular lattice. The lattice constant in the first crystal is 418.75 nm, and the holes are 1.25 nm closer together in each of the successive crystals until the spacing is 411.25 nm in the final one. The researchers designed identical point defects of three missing airholes into each crystal that cause radiation at a specific wavelength to be trapped and emitted into free space. Because the lattice constants vary, the trapped wavelength also varies.Experiments demonstrated that each successive crystal splits off radiation from the initial broadband signal in roughly 6-nm increments from about 1515 to 1550 nm with a bandwidth of 0.4 nm. The design rules resulted in good drop efficiencies for all point defects, and the device is 10,000 times smaller than a comparable conventional one.