Opening a window of transparency.

There's no invisible man lurking in the laboratory, but researchers have now discovered a way to make atoms of a gas transparent to light at wavelengths the gas would normally absorb. They achieve this surprising effect by using an intense laser beam to interfere with the usual, quantum-mechanical process by which an atom absorbs light.

"We face the really exciting prospect of making opaque materials transparent at particular wavelengths," says physicist Stephen E. Harris of Stanford University.

Harris, Klaus-Jochen Boller and Atac Imamoglu report the first observation of electromagnetically induced transparency, involving strontium vapor, in the May 20 PHYSICAL REVIEW LETTERS.

Atoms typically absorb light of wavelengths that correspond to the energies required for electrons to jump from one atomic energy level to another. To prevent absorption and induce transparency at one of these wavelengths (corresponding to the transition from energy level 1 to energy level 3 in the accompanying diagram), the idea is to apply an intense laser beam of a wavelength that would cause a transition between level 3 and another state, 2. Because of quantum-mechanical interference, a second laser beam normally capable of exciting the atom from level 1 to level 3 and applied at the same time as the first beam would pass right through without being absorbed.

Although the concept, in the guise of "population trapping," has been around for more than a decade, theorists failed to appreciate its significance when applied to a gas or a vapor instead of just to individual atoms. Harris changed that perception. "What we said is that you can use it to make an opaque material transparent," he says.

"All this had always been available to people, but we somehow never thought of it," adds laser expert Boris P. Stoicheff of the University of Toronto in Ontario.

To demonstrate the technique for inducing transparency, the Stanford team used a heated cell containing strontium vapor. Green laser light at 570 nanometers created the transparency, preventing the vapor from absorbing ultraviolet light at 337 nanometers.

Encouraged by their success with strontium, the researchers tried another atomic gas, using a commercially available laser to induce transparency in lead vapor contained in a molybdenum cell heated to 1,150[degrees]C. They reported their positive result last month at the Quantum Electronics and Laser Science Conference, held in Baltimore.

The choice of strontium and lead for the initial experiments was largely a matter of convenience rather than necessity. The same technique for inducing transparency should work for any atomic gas.

"We have a formula for how to make atoms transparent," Harris says. "If you name a particular absorption line in an atom, I can devise a way to make that absorption line from reasonably to very transparent."

Indeed, Stoicheff and his collaborators have in the last few weeks managed to induce transparency in atomic hydrogen. "We're sticking with hydrogen because you can calculate all its properties," Stoicheff says. Such theoretical calculations afford useful insights into the experimental results.

Whether the technique could be applied to molecular gases, liquids and solids remains far from settled. "We are going to learn in the future how to extend transparency to more complicated things, but I don't know to what extent," Harris says. "I certainly don't want to say we know how to make light go through walls."

"There's no apparent reason it should be limited to gases," Stoicheff says. That leaves lots of room for exploration.