When Claire Gmachl worked at Bell Labs, she had a parlor trick that never failed to delight visitors. She would place a lens a few inches from a laser and clamp a matchstick six inches in front of the lens. Then she’d use the lens to focus the laser’s invisible infrared beam onto the matchtip, and voil! She could light a match without touching it. You might think a scientist would be glad to have an entertaining way to show off the power of her devices. But to tell the truth, Gmachl found the exercise a bit “dorky.” She has little patience for distractions.
Gmachl, 37, has plenty to focus her laserlike intensity on these days. Once perfected, her tiny, ingenious devices—quantum-cascade lasers—could be used to detect biological and chemical weapons or to sniff airports for explosives. Bell Labs investigators invented quantum-cascade lasers before Gmachl arrived in 1996 from her native Austria, but theirs had a big shortcoming—one she figured out how to overcome.
Every gas molecule absorbs unique frequencies of light, creating a distinctive chemical fingerprint. Lasers are the classic tool for detecting these fingerprints, but until recently they consisted of bulky gas-powered ones or unstable solid-state ones. Quantum-cascade lasers, in contrast, are small, rugged semiconductor pastries whose layers form an
“energy staircase”—a series of progressively lower energy states. An excited electron cascades from layer to layer, shedding energy in the form of a photon with each jump. The beauty is that simply changing the thickness of the layers changes the laser’s frequency. Initially, though, the Bell Labs quantum-cascade lasers emitted a messy, uneven spectrum of light, making them incapable of identifying the telltale absorption frequencies of various chemicals. Gmachl’s feat was to etch a grating onto the laser that precisely amplifies one frequency and filters out the rest. She later designed lasers that emit multiple frequencies, making it possible for a single instrument to detect many different chemicals. Shining one of Gmachl’s lasers across a highway to a sensor on the other side would create a detection system for noxious car emissions; installed in an airport walkway, such a laser could sense trace amounts of explosives. Gmachl, now at Princeton, is trying to make her lasers, which function best at -200