Just a few weeks ago, two physicists won the Nobel Prize for figuring out how to make lasers out of semiconductors. Now another team of researchers announced that they have made those lasers much more useful. A new technique permits the lasers to shine in previously inaccessible infrared wavelengths. The advance may open the door to cheap devices that can sniff explosives.
The work extends the range of so-called quantum cascade lasers, which are made of many layers of semiconducting materials. Such lasers use devices called dielectric waveguides that confine and direct light by refracting it and bouncing it around inside them. Unfortunately, the longer the wavelength--as blue light becomes red light becomes infrared--the thicker the waveguides have to be and the harder it gets to make.
A team of physicists led by Federico Capasso at Bell Labs in Murray Hill, New Jersey, tackled this problem by exploiting a property of electrons, called surface plasmons, that reside at the interface between a semiconductor and a conductor. Plasmons are waves of electrons that slosh back and forth when excited by, say, an incoming photon, but they remain trapped between the semiconductor and the conductor--much like a waveguide traps light in a laser. By building a sandwich of conductors and semiconductors in a chip, the team created a plasmon that focuses 80% of the light right where the laser needs it. That is more efficient than a traditional waveguide's 50% focusing power, the team reports in the 9 October issue of Applied Physics Letters. And the plasma waveguide is thinner than traditional models even when tuned to infrared wavelengths.
One potential application is sensors that detect faint whiffs of chemicals. A battery of specially tuned long-wavelength lasers might be able to detect the spectral fingerprints of different molecules, says Richard Zare, a laser chemist at Stanford University. "There is a potential here to produce robot laser sensors," he says. "They don't exist today."