CAMBRIDGE, Mass. -- Researchers at the Massachusetts Institute of Technology have designed and tested a silicon device that some view as an important step toward creating fiber optic communication systems with dramatically increased efficiency and carrying capacity. The device incorporates a microcavity within a waveguide. The researchers deposited a thin strip of silicon, which serves as the conduit for light, onto a silicon dioxide wafer. They etched eight holes into the silicon strip, each measuring only 0.2 µm in diameter. They spaced the holes at 0.22-µm intervals, except for the gap between the fourth and fifth holes, which was slightly larger. This gap -- essentially an intentional defect in design -- proved crucial to the unique function of the microcavity. Mirroring effect White light enters the waveguide, propagating down the strip of silicon. The selected wavelength is trapped in the gap between the fourth and fifth holes, undergoing internal reflection -- a "mirroring effect." As it bounces back and forth, the light builds up optical power and emerges from the other end of the waveguide as a strong signal. A very narrow spectrum of light centered at 1.54 µm emerges. The device thus can transmit light at a chosen frequency or act as a switch, picking out one frequency from a number of incoming wavelengths. The researchers said their project demonstrates that multiple reflections caused by the very high index of refraction contrast in the silicon device prevent light from propagating over a wide band of wavelengths. They also noted that the device operates between 1.3 and 1.7 µm, the wavelengths used in fiber optic communications.