By Andrew Grant
A low-energy alternative to traditional lasers is finally available in plug-in form, a crucial step toward developing a practical alternative to the comparatively inefficient devices in use today. These so-called polariton lasers could soon find a niche in telecommunications and medical applications.
“This is a really important result,” says Alexey Kavokin, a physicist at Saint Petersburg State University in Russia, who wasn’t involved in the research. “It won’t be long before a new generation of lasers based on this new physics will come to the market.”
The lasers that are used in telecom, medicine, manufacturing and consumer electronics all function through a process called stimulated emission. A burst of light or electricity injects energy into a sea of atoms, causing the atoms’ electrons to jump up in energy as they swirl around the nucleus. When those electrons drop back to lower energies, they release photons. Those photons interact with and stimulate other atoms, causing them to emit more photons. All these photons have the same energy and direction, which creates a nice, clean monochromatic beam of laser light.
Since 1996, some physicists have studied a different lasing technique that makes use of semiconductors such as gallium arsenide, which have a special arrangement of electrons. When electrons within the semiconductor jump to a higher energy level, they leave behind a positively charged hole. The holes and the energetic, negatively charged electrons get attracted to one another, creating particle-like units called excitons. Excitons then interact with photons to form exotic hybrids of light and matter called polaritons, which in turn decay and release photons with the same energy and direction – the recipe for laser light.
The big advantage of polariton lasers is that they require far less energy than conventional lasers to boost their electrons to higher energy levels and start the laser process rolling.
Unfortunately, previous polariton lasers required researchers to shine light on the semiconductor to inject energy and create the electron-hole pairs. A laser that requires another laser to function is pretty useless, says physicist Sven Höfling at the University of Würzburg in Germany. So he and his team developed a polariton laser that runs on electricity.
Their device, detailed in the May 16 Nature, has titanium and gold electrodes that send current through the semiconductor and spark the formation of polaritons that emit a beam of infrared light.
The next step toward a practical device is to raise the operating temperature of Höfling’s laser from near absolute zero to around room temperature. That challenge is actually minor, Kavokin says, compared with getting a polariton laser to run on electricity. Kavokin expects the demonstration of an electrical polariton laser functioning at room temperature within a year or two, with commercial applications soon after.
