Microlight: lasing with single atoms - microlaser developed - Brief Article

Science News, Dec 24, 1994 by Ivars Peterson

The process of generating a laser beam typically involves great crowds of atoms or molecules. Now, researchers have developed a microlaser that produces light from interactions between a mirrored cavity and atoms passing through that cavity one at a time.

Kyungwon An, Michael S. Feld, and their coworkers at the Massachusetts Institute of Technology report their results in the Dec. 19 PHYSICAL REVIEW LETTERS.

In a conventional laser, an electrical jolt or a flash of light excites atoms or molecules of a lasing material, such as a ruby crystal or a mixture of helium and neon gas. The photons emitted by excited atoms bounce back and forth between two mirrors, inducing additional atoms to emit light of the same wavelength. These photons move in step to create a coherent light beam.

To construct a microlaser, the researchers had to create a mirrored cavity, or resonator, of sufficiently high quality to strictly limit the number of photons that could escape. They did this by fabricating a pair of precisely aligned, highly reflective mirrors for the project. The resulting resonator was about 10,000 times more capable of storing photons than a resonator in an ordinary laser.

A "pump" laser was used to excite barium atoms from their ground state to a higher energy level just before the atoms entered the 1-millimeter gap between the two curved mirrors (see diagram). Interactions between the empty cavity and the first atom entering the gap induce the atom to emit a photon. The next excited atom traversing the cavity interacts with this photon, emitting a photon of its own, and so on.

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The number of photons present in the cavity quickly builds to a certain value, and a portion of the light can then emerge as a laser beam. Storing about 11 photons at a time in their resonator, the researchers generated a detectable laser beam having a wavelength of 791 nanometers.

Such a microlaser may prove a useful tool for investigating how photons couple with individual particles. "This development has been long sought, and it is expected to lead to further fundamental advances in our knowledge of light and its interaction with atoms," Feld says.