Semiconductor lasers are a critical component to sensors of all sorts, including chemical microsensors and labs-on-a-chip. They are also critical to many kinds of medical scanners.
Thus the discovery of a new kind of laser beam may be critical to a great many fields, including air quality monitoring, medical diagnostics, homeland security, and other chemical applications. The new type of laser was discovered at Princeton University by a pair of graduate students; Kale Franz, graduate student of Princeton's Claire Gmachl at the Mid-Infrared Technologies for Health and the Environment (MIRTHE) center and Stefan Metzel, a University of Sheffield, UK visiting grad student.
The pair, under the guidance of Professor Gmachl, built a small metallic laser device called a quantum cascade laser, only to find that it unexpectedly emitted not one, but two laser beams. While the first beam was a typical laser beam, the second had unusual properties, including the fact that it required less power to create. Kale Franz describes, "This discovery provides a new insight into the physics of lasers. If we can turn off the conventional beam, we will end up with a better laser, which makes more efficient use of electrical power."
Metzel, an intern at Princeton, was intrigued by the second beam's unique properties as he dug into the phenomenon. The beam, like all lasers consisted of coherent or in ordered photons. In a laser beam photons move in an ordered fashion, lending lasers their distinctive color, beam, and properties. Normal light from sun, typical chemical reactions, or electric lamps are disordered.
It is commonplace to create lasers from gallium arsenide or other semiconductors by passing an electronic current through specially manipulated circuit causing electrons to jump in energy levels, then fall, emitting synchronized photon emissions in the process. This kind of laser is used in media, laser pointers and other devices. The Princeton device, the quantum cascade laser, is a specialized form of semiconductor laser built at a nanoscale. It is one-tenth as thick as a human hair and 3 millimeters long and consists of atom-thick layers of different semiconductors. These layers emit sequentially synchronized photons.
The second beam on the quantum cascade lasers was identified due to its shorter wavelength than the main beam. Unlike normal lasers, which weaken at higher temperatures, this laser increased in strength up to a point. This behavior could not be explained by conventional theory.
The pair of grad students explained the phenomenon via the quantum mechanics concept of electron momentum. Traditional lasers are produced by electrons in equilibrium, where most have a high energy and almost zero momentum. The new laser results from non-equilibrium, lower-energy electrons with more momentum. Explains Franz, "It showed, contrary to what was believed, that electrons are useful for laser emission even when they are in highly non-equilibrium states."
In traditional lasers the low momentum of lasing electrons causes a large amount of photons to be reabsorbed, decreasing efficiency. The new laser cuts this phenomenon by 90 percent, allowing the possibility of a low current laser. It also improves performance by increasing its emissions strength with higher temperatures.
Quantum cascade lasers emit in the mid- and far-infrared range, unlike visible light lasers. These IR beams are perfect for chemical detection.
Additional research is now being performed into how to isolate and optimize the new kind of laser, and how to perhaps extend it to visible light lasers.
The research is funded by the National Science Foundation and reported in the journal Nature Photonics.