Cascades of light shine from a new laser.

Lasers--those finicky light-shining devices that have yielded better long-distance communications, eye surgery, Star Wars weapons, and audio equipment -- come in many forms.

Essentially, they all emit light based on the same principle. When a charged particle, such as an electron, is driven to a higher energy state and then allowed to settle back down, a photon, or particle of light, pops out along the way. A laser--or Light Amplification by Stimulated Emission of Radiation--enhances that process to produce intense light with a uniform wavelength.

The medium used to stimulate the light emission, however, can vary. There are gas lasers, liquid lasers, and semiconductor lasers, to name but a few. Now, a report in the April 22 SCIENCE heralds another type of laser. Jerome Faist and Federico Capasso, both physicists at AT&T Bell Laboratories in Murray Hill, N.J., and their colleagues describe a new variety of semiconductor laser that they call a "quantum cascade laser."

Twenty-five years in the making, since physicists Rudy Kazarinov and Robert Suris first proposed the concept, the quantum cascade laser promises to be smaller, more flexible, and less costly to make than similar lasers, the Bell Lab team says.

"It works quite differently from other semiconductor and gas lasers," Faist says. In this laser, electrons "cascade down an energy staircase."

Most gas and semiconductor lasers capture a charged particle's energy by recoupling it with an oppositely charged particle. The two annihilate each other's charge, releasing a photon in the process. In the quantum cascade laser, charged particles first move up several energy levels, then cascade down step by step, releasing photons at each plateau.

To accomplish this, Faist's group tailored a new semiconductor, a wafer-thin sandwich of 500 layers clustered in sets of 20 layers. Each set has 10 wells and 10 barriers. The light emission occurs, says Faist, as electrons move among these wells and barriers, first accumulating and then releasing energy. Metaphorically speaking, they leap up and then bump down the energy staircase.

To make the semiconductor sandwich, the scientists had to first master the technique of molecular beam epitaxy to "spray on" 500 layers, a single layer at a time.

One of this laser's unique advantages is its ability to emit light over a relatively wide range of wavelengths. The secret lies in varying the thickness of each layer in the semiconductor sandwich, Faist says. The first quantum cascade laser emits light with a 4.25 micron wavelength. But that wavelength could vary from 2 microns to 100 microns in size, he adds, using the same semiconductor material.

The new laser's compact size and flexibility suit it to a wide variety of applications, the scientists contend. It should be useful for monitoring air quality, controlling pollution, and improving collision-avoidance radar. Another possibility is better "free space" point-to-point communication -- "like fiber-optic communication," Faist says, "but without all the fibers."