Drawing a bead on quantum dot lasers.

Tiny clusters of atoms that trap electrons like shipwrecked passengers on an island may hold the key to a new generation of lasers. Scientists want to use these atomic clusters, known as quantum dots, to make more powerful and efficient lasers that would replace the standard variety widely used in compact disc players and other devices.

To date, quantum dot lasers have been made only in infrared wavelengths. Now, scientists at the National Research Council of Canada in Ottawa have demonstrated the first quantum dot laser that emits visible light, thus increasing the potential versatility of these lasers.

In standard lasers, electrons are restricted to a flat semiconducting layer and generate light when an electric current causes them to jump between different energy levels. In a quantum dot laser, electrons are confined to a tiny volume. one only a few nanometers across (SN: 4/4/92, p. 222). Because their movement is so limited, these electrons can't waste energy on random motion, as electrons in standard lasers do.

As they report in the Nov. 22 Science the researchers sandwiched a dense carpet of indium aluminum arsenide clusters between two layers of aluminum gallium arsenide. Each cluster had a diameter of about 20 nanometers, says study coauthor Simon Fafard. Although infrared quantum dot lasers have been made before, the Canadian team tackled the "much more difficult" task of pushing the laser energy into the red part of the visible spectrum. Going from infrared to visible light entails spreading the energy levels of the semiconductor further apart by fine-tuning its chemical composition.

Part of the technical challenge lies in making a large number of quantum dots all of the same size, says Evelyn Hu, director of the Center for Quantized Electronic Structures at the University of California, Santa Barbara. There must be enough dots initially emitting photons to trigger further emissions, leading to the coherent cascade of photons that constitutes laser light. Quantum dots of different sizes emit light of many different wavelengths -- also undesirable in a laser.

It's difficult to "make a million or billion of them and guarantee that the size fluctuation is going to be less than 10 percent," Hu says. In a typical quantum dot, that constraint allows 20 atoms or fewer of leeway.

By choosing the right combination of semiconductors, however, researchers can induce the material they deposit to bead into islands of the correct size, much like water droplets on the hood of a freshly waxed car. Scientists try to avoid this beading when making flat semiconductor layers, but for quantum dots, the effect works to their advantage.

The current that a set of quantum dots needs to produce light compares well with the requirements of a flat system, Fafard says, but he hopes to make the new device more efficient by stacking layers of quantum dots. "Funny things happen when you grow one layer of quantum dots over another," Hu says. We don't yet understand completely the best way to control the growth."