April 2001

Nanodot Lasers

Optical Networks

By Peter Fairley

Make particles of semiconductors small enough-just a few nanometers across-and they glow in a dazzling range of colors. These nano particles are known as quantum dots, because quantum effects tune the color of the glow to the size of the particle-a phenomenon that scientists have seized upon to make exquisitely sensitive biomedical assays (see "Quantum Dot Com," TR January/February 2000). In theory, these tiny glowing particles could also be a boon for optical networking by providing lasers and amplifiers that work in a wide range of frequencies. But for over a decade experts have been trying to fashion quantum-dot lasers, with little success.

Now MIT chemist Moungi Bawendi and Victor Klimov, a laser expert at Los Alamos National Lab in New Mexico, may have stumbled upon the solution. Klimov and Bawendi discovered that when the dots are stimulated with a powerful pump of light, most of them fritter away the energy as heat in less than one-millionth of a second. But if the dots are crammed close enough together, photons released by neighboring dots arrive in time to trigger additional photons from a nanodot before the energy dissipates. A dense film of cadmium selenide dots that Bawendi prepared for Los Alamos did the trick, generating a cascade of photons.

Bawendi has since fashioned this cascade of light into a laser and has started to tweak the dots to make the beam more efficient. Success could free optical networks and other laser-dependent technologies from today's limited spectrum of beams. For starters, quantum-dot amplifiers could extend long-range transmission of fiber-optic signals to wavelengths of light outside the narrow band of infrared beams served by today's amplifiers. "The colors are essentially limitless," says Bawendi.

If the scientists are right, the future of quantum dots in expanding the possibilities of optical communication could be bright, indeed.