Advanced chemical sensors for detecting explosives and security systems for scanning through clothing are just some of the new technologies that may blossom thanks to new research that promises a cheaper, easier way to change the color of laser light.
Not all lasers are created equal. Red lasers found in CD and DVD players are common, easy to work with, and relatively cheap, but colors from other parts of the spectrum--such as the midinfrared, which is useful for chemical sensing--are harder to produce. To get around that limitation, scientists often employ materials called optical frequency converters. When laser light shines onto crystals of the semiconductor zinc selenide or specially layered materials, for example, the crystal's electrons rattle in such a way that they emit light of the incoming color plus an additional color, which can serve as the source for another laser. But for a crystal to perform well as a converter, researchers thought it had to be large and pure and therefore expensive.
Now a group from the French National Aerospace Research Establishment in Palaiseau has shown that a highly disordered jumble of crystals, which are relatively cheap to produce, can also do the trick. The idea is that light from each crystal domain emits a photon of random phase, so the photons don't interfere with one another, allowing light of a different color to be produced. When the researchers shined two high-frequency infrared lasers onto grainy samples of zinc selenide, the laser light was converted to midfrequency infrared light. Although the light produced using this method is about 10 times weaker than would be seen with a pure crystal, the low cost and ease of use of the disordered crystals make it a viable alternative. The team reports its findings in the 18 November issue of Nature.
The new method "is very straightforward and needs practically no fine-tuning or control," says Claire Gmachl, an experimental physicist at Princeton University. "It is definitely good news." Researchers exploiting optical frequency conversion have tended to avoid disorder like the plague, says Sergey Skipetrov, a theoretical physicist at the French National Center for Scientific Research in Grenoble, but this result should change their minds.