Energy-efficient way to build a better laser

PRINCETON (US)—Scientists have discovered a more efficient way to produce a high-performing laser. The finding could lead to lasers that operate at higher temperatures than existing devices, making them ideally suited for applications in air quality monitoring, medical diagnostics, and even homeland security.

The phenomenon was discovered by a team of Princeton University researchers using in a type of device called a quantum cascade laser, in which an electric current flowing through a specially designed material produces a laser beam. Unlike other lasers, quantum cascade lasers operate in the mid- and far-infrared range, and can be used to detect even minute traces of water vapor, ammonia, nitrogen oxides, and other gases that absorb infrared light.
Built at Princeton’s nanofabrication facility, the device is about one-tenth as thick as a human hair and 3 millimeters long. Despite its tiny size, it is made of hundreds of layers of different semiconductor materials. Each layer is only a few atoms thick. In this device, electrons “cascade” through the layers as they lose energy and give off synchronized photons.

The group discovered that their device could generate a second beam with very unusual properties, including the need for less electrical power than the conventional beam.

“If we can turn off the conventional beam, we will end up with a better laser, which makes more efficient use of electrical power,” says lead researcher Claire Gmachl, a professor of electrical engineering and director of the Mid-Infrared Technologies for Health and the Environment (MIRTHE) Center at Princeton.

The new laser phenomenon has some interesting features. For instance, in a conventional laser, electrons often reabsorb the emitted photons, and this reduces overall efficiency. In the new type of laser, however, this absorption is reduced by 90 percent, says Kale Franz, a graduate student who built the laser. This could potentially allow the device to run at lower currents, and also make it less vulnerable to temperature changes. “It should let us dramatically improve laser performance,” he explains.

The light emitted by a laser differs fundamentally from light produced by common sources such as the sun, fire or electric lamps. According to the field of physics called quantum electrodynamics, light is made up of particles called photons. Common sources of light emit photons that are in a random order, like crowds milling about a busy marketplace. In contrast, photons in a laser are “in sync” with each other, like a band marching in formation. This property, called coherence, allows laser light to shine in an intense, narrow beam of a single, very pure color.

One way to produce a laser beam is to pass an electric current through a semiconductor such as gallium arsenide. The electric current pumps energy into the material, forcing a large number of its electrons to a higher energy level than normal. Under certain conditions, these electrons drop to a lower level of energy, and emit the extra energy in the form of synchronized photons of light. This is the mechanism underlying lasers used in CD writers, laser pointers, and other common electronic devices.

In an earlier study, Franz, Gmachl, and others reported that a quantum cascade laser they had built unexpectedly emitted a second laser beam of slightly smaller wavelength than the main one. Unlike a conventional semiconductor laser, the second beam grew stronger as the temperature increased, up to a point. Further, it seemed to compete with the “normal” laser, growing weaker as the latter strengthened when more electric current was supplied. “It’s a new mechanism of light emission from semiconductor lasers,” says Franz.

The device used in the most recent study does not fully attain its efficiency potential,  because the conventional, low-efficiency laser mechanism dominates. To take full advantage of the new discovery, therefore, the conventional mechanism would need to be turned off. Franz says the team is working on methods to achieve this outcome.

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