Michal Lipson, an assistant professor at Cornell, in Ithaca, N.Y., described recent research by the Nanophotonics Group in Cornell's School of Electrical and Computer Engineering at the annual meeting of the American Association for the Advancement of Science (AAAS) in Seattle on Sunday, Feb. 15. Her talk was part of a symposium on "21st Century Photonics."
Lipson suggested that one of the first applications of nanophotonic circuits might be as routers and repeaters for fiber-optic communication systems. Such technology, she added, could speed the day when home use of fiber-optic lines becomes practical.
Researchers already have built nanoscale photonic devices in which wires are replaced by square waveguides that confine light by total internal reflection. This works only in materials with a high index of refraction, such as silicon, where there is a loss of light intensity and sometimes distortion of pulses. Lipson described a way to guide and bend light in low-index materials, including air or a vacuum. "In addition to reducing losses, this opens the door to using a wide variety of low-index materials, including polymers, which have interesting optical properties," Lipson said.
Using equipment at the National Science Foundation-supported Cornell Nanoscale Facility, Lipson's group has manufactured waveguides consisting of two parallel strips of a material with a high refractive index placed about 50 to 200 nanometers apart, with a slot containing a material of much lower refractive index. (A nanometer is about the width of three silicon atoms.) In some
devices the walls are made of silicon with an air gap, and others have silicon dioxide walls with a silicon gap. In both cases, the index of refraction of the medium in the gap is much lower than that of the wall, up to a ratio of about four to one.
When a wavefront crosses two materials of very different refractive indices and the low-index space is very narrow in proportion to the wavelength, nearly all of the light is confined in the "slot waveguide." Theory predicts that straight slots will have virtually no loss of light, and smooth curves will have only a small loss. This has been verified by experiments, Lipson reported.
Slot waveguides can be used to make ring resonators, already familiar to nanophotonics researchers. When a circular waveguide is placed very close to a straight one, some of the light can jump from the straight to the circular waveguide, depending on its wavelength. "In this way we can choose the wavelength we want to transmit," Lipson said. In fiber-optic communications, signals often are multiplexed, with several different wavelengths traveling together in the same fiber, each wavelength carrying a different signal. Ring resonators can be used as filters to separate these signals, she suggested.
Like the transistor switches in conventional electronic chips, light-beam switches would be the basic components of photonic computers. Lipson's group has made switches in which light is passed in a straight line through a cavity with reflectors at each end, causing the light to bounce back and forth many times before passing through. The refractive index of the cavity is varied by applying an electric field; because of the repeated reflections, the light remains in the waveguide long enough to be affected by this small change. Lipson is working on devices in which the same effect is induced directly by another beam of light.
Connecting photonic chips to optical fibers can be a challenge because the typical fiber is vastly larger than the waveguide. It's like connecting a garden hose to a hypodermic needle. Most researchers have used waveguides that taper from large to small, but the tapers typically have to be very long and introduce losses. Instead, Lipson's group has made waveguides that narrow almost to a point. When light passes through the point, the waveform is deformed as if it were passing through a lens, spreading out to match the larger fiber. Conversely, the "lens" collects light from the fiber and focuses it into the waveguide. Lipson calls this coupling device "optical solder." Based on experiments at Cornell, the device could couple 200-nanometer waveguides to 5-micron fibers with 95 per cent efficiency, she reported. It can also be used to couple waveguides of different dimensions.
The method of coupling nanoscale waveguides to optical fiber is described in a paper, "Nano-taper for Compact Mode Conversion," published in Optics Letters ( August 2003). Slot waveguides are described in "Guiding and Confining Light in Void Nanostructures," accepted for publication in Optics Letters . Some of the work has been done in collaboration with researchers working under Alexander Gaeta, Cornell associate professor of applied and engineering physics. The Cornell nanophotonics group web site is http://nanophotonics.