Nanodot Lasers

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.