Lasing turned upside down.

There's a new kind of excitement in the laser world.

Getting a beam of coherent light out of a conventional laser generally requires a significant input of energy. Much of this energy goes into maintaining a majority of the atoms or molecules of the lasing material in an excited state. The result is a population inversion in which there are more particles in the higher-energy state than in the ground state.

Now, a team of physicists from the United States, Russia, and Germany have for the first time experimentally demonstrated laser oscillation without population inversion. The researchers take advantage of a quantum interference effect to generate a sustained laser beam of infrared light in a vapor of rubidium atoms.

Marlan O. Scully of Texas A&M University in College Station and his collaborators report their achievement in the Aug. 21 Physical Review Letters. More recently, a second team has observed lasing without inversion in a sodium atomic beam.

In experiments performed at the National Institute of Standards and Technology in Boulder, Colo., Scully's team used external laser beams to produce what are, in effect, two different paths from the ground state to a particular excited state of a rubidium atom. Under the right conditions, these two quantum paths interfere with each other to block absorption but not emission of photons of the proper energy. This allows lasing to occur without having large numbers of atoms in the excited state.

Such a technique offers the possibility of circumventing some of the difficulties researchers have faced in the past in producing laser light at ultraviolet and X-ray wavelengths.