Scientists at Massachusetts Institute of Technology in Cambridge have described a three-dimensional photonic crystal in silicon, with deliberately introduced point-defects, that displays resonant signatures around telecommunications wavelengths. They reported the development in the June 3 issue of Nature.In the layer-by-layer fabrication technique, scanning-electron-beam lithography and spin-on dielectric planarization produce a hole and a rod layer in each process cycle. After four cycles, a hydrofluoric acid solution removes the spin-on dielectric, creating seven functional layers. The researchers suggest that the process lends itself to use with alternative material systems and to scaling for the production of large-area photonic crystals on the scale of centimeters.Optical characterization of the photonic crystal revealed a large, 3-D bandgap from 1.15 to 1.45 µm, in good agreement with theoretical predictions. Photonic crystals with different-size holes displayed different bandgaps, confirming that the method may be used to fabricate crystals with the desired optical properties.