Lower-power laser is unveiled

Researchers discover entirely new mechanism for making materials emit laser beams

A Princeton University-led team of researchers has discovered an entirely new mechanism for making common electronic materials emit laser beams. Claire Gmachl, an electrical engineer and director of Princeton’s Mid-Infrared Technologies for Health and the Environment (MIRTHE) center, led the study. The phenomenon was discovered in a type of device called a quantum cascade laser. Gmachl’s group discovered that a unit they had built generated a second beam with very unusual properties, including the need for less power than the conventional beam. Turning off the conventional beam yielded a more efficient laser.

Graduate student Kale Franz built the laser that revealed the new phenomenon, and Stefan Menzel, a graduate student from the University of Sheffield, U.K., unearthed the unique properties of the phenomenon during an internship at Princeton. The study was published online in Nature Photonics .

The quantum cascade laser is about one-tenth as thick as a human hair and 3 mm long and is made of hundreds of layers of different semiconductor materials. Each layer is only a few atoms thick and electrons “cascade” down through the layers as they lose energy and give off synchronized photons.

Studies by Menzel and others revealed that the second beam could not be explained by any existing theory.

A quantum cascade laser generated a second beam with very unusual properties, including the need for less power than the conventional beam.

There is a quantum property of electrons called momentum. In the conventional view of quantum cascade lasers, only electrons of nearly zero momentum participate in lasing. Further, a substantial number of electrons have to attain the same level of energy and momentum be in a so-called “quasi-equilibrium” condition before they can participate in laser action. But, the second laser beam originated from electrons of lower energy, but higher momentum, that were not in equilibrium. “It showed, contrary to what was believed, that electrons are useful for laser emission even when they are in highly non-equilibrium states,” said Franz.

The MIRTHE center is funded by the National Science Foundation. For more on the quantum cascade laser, visit http://www.mirthecenter.org.

Jim Harrison

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