Princeton researchers discover new typ...(e allows laser light to shine in an inte...)

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.

The laser used in the Princeton study is a special type called a quantum cascade laser. Built at Princeton University'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" down through the layers as they lose energy and give off synchronized photons.

In an earlier study published in Applied Physics Letters in June 2007, Franz, Gmachl and others had reported that a quantum cascade laser they had built unexpectedly emitted a second laser beam of slightly smaller wavelength than the main one. Further studies by Menzel and others revealed that the second beam could not be explained by any existing theory of quantum cascade lasers. 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," said Franz.

To explain this mechanism, the researchers invoked a quantum property of electrons called momentum. In the conventional view of quantum cascade lasers, only electrons of nearly zer
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