Researchers at the Mid-Infrared Technologies for Health and the Environment (MIRTHE) center at Princeton University (Princeton, NJ) have discovered what they say is a new type of lasing mechanism, which they came across while experimenting with quantum-cascade (QC) lasers. In essence, a QC laser they had fabricated emitted, along with the expected beam, a second beam with unusual properties that they then spent some time studying. The finding could lead to lasers that operate more efficiently and at higher temperatures than existing devices, and find applications in environmental monitoring and medical diagnostics, say the researchers.
"This discovery provides a new insight into the physics of lasers," says Claire Gmachl, who led the study. Gmachl, an electrical engineer, is the director of MIRTHE. "If we can turn off the conventional beam, we will end up with a better laser, which makes more efficient use of electrical power," notes Gmachl.
The team that conducted the study includes Gmachl's graduate student Kale Franz, who built the laser that revealed the new phenomenon, and Stefan Menzel, a graduate student from the University of Sheffield (Sheffield, England), who unearthed the unique properties of the phenomenon during an internship at Princeton University last summer. The study was published online in Nature Photonics on Dec. 14.
Fabricated at Princeton University's nanofabrication facility, the QC laser is about one-tenth as thick as a human hair and 3 millimeters long. In an earlier study published in Applied Physics Letters in June 2007, Franz, Gmachl and others had reported that a QC laser they had built unexpectedly emitted a second laser beam with a slightly shorter wavelength than that of the main beam. Further studies by Menzel and others revealed that the second beam could not be explained by any existing theory of QC lasers. Unlike in 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 beam, 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.
To explain this mechanism, the researchers invoked a quantum property of electrons called momentum. In the conventional view of QC lasers, only electrons of nearly zero momentum participate in lasing. Further, a substantial number of electrons has to attain the same level of energy and momentum--existing in a so-called "quasi-equilibrium" condition--before they can participate in laser action. In contrast, studies by Gmachl's group showed that 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 nonequilibrium states," says Franz.
The new laser phenomenon has some interesting features. For instance, in a conventional laser relying on low-momentum electrons, electrons often reabsorb the emitted photons, and this reduces overall efficiency. In the new type of laser, however, this absorption is reduced by 90%, says Franz. This could potentially allow the device to run at lower currents, and also makes it less vulnerable to temperature changes. "It should let us dramatically improve laser performance," he says.
The device used in the study does not fully attain this level of performance, 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. The researchers have started to work on methods to achieve this outcome, notes Franz.
Quantum-cascade lasers are made to operate in the mid-IR and far-IR range, and can thus be used to detect even minute traces of water vapor, ammonia, nitrogen oxides, and other gases that absorb IR light. As a result, these devices are finding applications in air-quality monitoring, medical diagnostics, homeland security, and other areas that require extremely sensitive detection of different chemicals. The new discovery should help make these devices smaller, more efficient, and more sensitive, says Gmachl.
The research was partly sponsored by the MIRTHE center, which is funded by the National Science Foundation and directed by Gmachl. MIRTHE is a multi-institutional research collaboration for developing compact sensors to detect trace amounts of gases in the atmosphere and in human breath. Partial support was also provided by the European Union's Marie Curie Research Training Network and its Physics of Intersubband Semiconductor Emitters (POISE) program, which sponsored Stefan Menzel's visit to Princeton University. Kale Franz was supported by the NSF Graduate Fellowship Program.
