Beam Team: Unusual laser emits a band of light

Typically, lasers emit light of one pure color, or wavelength. A new little laser breaks that mold by generating a beam containing all the wavelengths in a swath of the electromagnetic spectrum.

SEMICONDUCTOR PHYLLO. A new laser’s 650 microscopic layers (left) include 36 light-emitting regions. Diagrams of sample regions (right) show that indium gallium arsenide (orange) and aluminum indium arsenide (blue) are deposited at different widths to generate wavelengths from 6 micrometers (top) to 8 micrometers (bottom). Gmachl/Transmission electron micrograph by S.-N. George Chu, Agere Systems

This new broadband laser operates in the infrared spectrum, which is invisible to the human eye. The multiwavelength emission makes the laser more suitable for many applications than conventional single-wavelength lasers are, its inventors say. These uses include monitoring air pollution and observing ultrafast reactions in combustion and other chemical processes.

It’s “a breakthrough and a milestone in laser development,” comments Erich Gornik of the Vienna University of Technology in Austria.

An ordinary laser emits only a single color because it’s built with a light-emitting substance that naturally generates one wavelength of light when energized. To create a laser that would emit a broad band of wavelengths, Claire Gmachl, Federico Capasso, and their colleagues at Bell Labs’ Lucent Technologies in Murray Hill, N.J., deposited on a microchip alternating layers of two semiconductors in a configuration known as a quantum cascade.

Like earlier quantum-cascade lasers, the new broadband laser contains hundreds of exquisitely thin semiconductor layers, each one affecting the energies of electrons passing through it, Gmachl says.

In any quantum-cascade laser, a high voltage coerces an electric current to penetrate layer after layer in the stack. The tight physical confinement of many of those stacked layers makes them act as so-called quantum wells, in which electrons can only have certain amounts of energy, Gmachl explains.

Those specific energy levels are determined by the laws of quantum mechanics.

The stack emits a laser beam because many electrons forced into those wells by the high voltage carry more energy than the wells can accept. Those electrons shed their excess energy, sometimes as heat but often as photons. At each end of the stack, partially reflective crystal surfaces cause many of those photons to bounce back and forth. The rebounding photons boost the odds that other electrons in the quantum wells also will convert their energy into photons rather than heat, Gmachl notes. A fast buildup of light intensity permits enough photons to streak beyond the end mirrors to form a beam.

In earlier generations of quantum-cascade lasers, all the quantum wells had the same thickness. In the new version, however, the researchers varied the quantum wells’ thickness from a few atomic layers to a few dozen, causing each well to emit light of a different wavelength. Also, each well emits small amounts of light with wavelengths slightly longer and shorter than its dominant one. The result: a beam of high intensity at every wavelength from 6 to 8 micrometers, in the so-called midinfrared range.

The Lucent researchers made the new laser, which is 2 millimeters long and less than one-tenth the thickness of a human hair, by stacking quantum wells.

Each well is based on a layer of indium gallium arsenide sandwiched between layers of aluminum indium arsenide. The team describes its invention in the Feb. 21 Nature.

By using the same design principle with other materials, the scientists expect to make broadband lasers that work in portions of the electromagnetic spectrum other than the midinfrared. One such device, already in the works, would operate at the shorter infrared wavelengths now widely used for optical-fiber telecommunications, Gmachl says. In principle, the same technique might someday lead to a white-light device for room lighting, computer displays, or video projection, she notes.

Previous attempts to generate broadband laser light have employed exotic crystals or extreme operating conditions. In contrast, the Lucent laser works at room temperature.

“This is really the first time that you’ve had a laser with such a broad spectrum,” comments Jerry R. Meyer of the Naval Research Laboratory in Washington, D.C. “It’s a spectacular demonstration of something new.”

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