PORTLAND, Ore. — Electric control of the spectrum, direction and efficiency of light-emitting nanotubes (LENs) has been demonstrated by researchers at IBM Corp.'s Thomas J. Watson Research Center, bringing silicon photonics one step closer to reality.

IBM Research (Yorktown Heights, N.Y.) previously demonstrated record-breaking silicon optical waveguides and higher electroluminescent efficiency for LENs compared to LEDs. Now, it has put a LEN inside an optical waveguide to achieve directional surface emission, wavelength selectivity and the potential for ultrahigh efficiency.

"Like most light-emission sources, nanotubes emit light in all directions. Their spectrum was relatively broad and their efficiency was not very high," said Phaedon Avouris, IBM Fellow and manager of Nanometer Scale Science and Technology at IBM Research. "We attacked all these problems, making its light directional so it can be coupled to optical filters or to a device to transport it. We controlled its spectrum with an optical cavity and we have proposed a theory to help us achieve higher efficiency."

By fabricating an optical cavity around light-emitting nanotube mirrors at the bottom and top, wavelengths were confined to the desired 1.55-micron communications frequency.

IBM achieved surface emission by combining a single nanotube-based field-effect-transistor with a pair of metallic mirrors, one above and below the nanotube which lies flat on the silicon chip. The bottom mirror was made from silver, with a top half-mirror made from gold. Light was emitted from the nanotube in the cavity, which was filled with transparent dielectric.

The distance between the top and bottom mirrors was calculated to be half of the desired emission wavelength, which was set to be near a communications wavelength of 1.55 microns. Light was reflected upward off the bottom of the cavity, where half was passed as a surface emission from the LEN while the other half was reflected back down to the bottom mirror to reinforce the desired emission wavelength.

"We confined the emission in an optical cavity with two mirrors, so that light forms a standing wave between the mirrors which enhanced the frequencies, whose wavelength were equal to half the size of the cavity," said Avouris. "We used lithography to form the cavities, which achieved a dramatic enhancement--confining the spectrum to about 10 percent of what it was without the cavity, and giving us an overall enhancement [in the efficiency] of the emission of 400 percent."

Nanotubes have slightly different diameters (in this case, about 2 nanometers). As a result, they have slightly different bandgaps, and thus emit light at slightly different frequencies. However, by integrating the nanotube inside a cavity, physical confinement in the structure "eliminates unwanted frequencies thus [solving] the problem of nanotubes having slightly different diameters," according to Avouris.