Cloud of bloated atoms takes hot shots.

Using drifting, highly excited atoms as light-sensitive film, researchers have created a new type of camera for making images from infrared light.

Although the prototype device misses much of the light from its subject, its potential advantages over current technology include a faster shutter speed and the ability to be tuned to a particular wavelength. The camera also needs no cooling and works over an exceptionally large range of wavelengths--from 1 to 100 micrometers, its developers say.

"What we present here is a completely different approach" to infrared imaging, says Bart Noordam of the FOM Institute for Atomic and Molecular Physics in Amsterdam. He and his colleagues describe their camera in the March 29 APPLIED PHYSICS LETTERS.

Infrared pictures have proved useful in a wide range of applications, such as night vision, analysis of the composition of surfaces, and observations of celestial bodies enshrouded in dust that blocks visible light (SN: 3/13/99, p. 172). Infrared radiation, beginning just beyond the red end of the optical spectrum, has longer wavelengths and lower energy than visible light.

Because of their low energy, infrared photons pose a challenge for camera designers. Standard semiconductor detectors have limited sensitivity in the infrared because the photons have too little oomph to dislodge electrons and so create a telltale current. Moreover, all materials--including camera parts--at room temperature give off their own infrared radiation, usually making cooling necessary to suppress those emissions.

Noordam and Marcel Drabbels, also of FOM, have devised a way to use a cloud of atoms in an unusual state in order to detect infrared photons. A Rydberg atom is an atom having one highly excited electron orbiting far beyond the usual electron paths. Physicists have known about Rydberg atoms since the start of the century, but in the last 2 decades, lasers have made them easy to generate.

The Dutch team beamed an ultraviolet laser into clouds of potassium or cesium atoms, in each case creating a half-millimeter-thick sheet of Rydberg atoms. Because the far-flung electrons are weakly bound to the rest of the atom, low-energy photons from an infrared source can knock them loose. This creates electrically charged ions. An electric field accelerates the ions into an amplifier, which in turn excites a phosphor on a screen, creating an image of the infrared light.

By adjusting the ultraviolet laser and the electric field, the researchers can select which infrared wavelengths the Rydberg atoms most readily absorb. Having a tunable camera makes cooling unnecessary because background radiation can be screened out, Noordam says.

"It's a very clever idea," says Thomas F. Gallagher of the University of Virginia in Charlottesville. On the other hand, "it's not spectacularly efficient," he notes.

Indeed, the camera catches only 1 in 100 million infrared photons that pass through it. The inventors are working on a new version, with a thousand-fold greater efficiency, but the ultimate performance of the camera remains uncertain, they say.