PHOTONIC-CRYSTAL FILTERS. Just as semiconductors exclude the movement of electrons in certain energy bands (with important implications for useful devices), photonic crystals (also called photonic bandgap materials) exclude the passage of photons in certain wavelength bands. Developed first for microwaves, photonic crystals for infrared light have also become available. In fact, MIT scientists have now fashioned a silicon structure (a strip of silicon .5 microns wide drilled with .4 micron holes) that forbids light over a wide range---roughly 1300 to 1700 nm. Light at those wavelengths is defeated by multiple reflections from the holes. The photonic crystal has one additional feature, made possible by deliberately staggering slightly the spacing of the holes in the middle of the strip: in the middle of the forbidden band is a small island of wavelength (at 1.54 microns) where light is actually encouraged. In other words, the crystal acts as a filter allowing the transmission of 1.54 micron light (a crucial wavelength favored by fiber optics) but cutting out light at surrounding wavelengths. The tiny zone of silicon in the middle of the lightguide is in effect a tiny optical microcavity with a volume of only 0.055 cubic microns. (Foresi et al., Nature, 13 Nov.)