Two-Beam Technique Stores Slow-Light Images in Atomic Vapor

If you shine a flashlight’s beam into a fog, you will see the light reflect softly as it disperses through the water particles. Shut the beam off, and the light fades away. Researchers in Israel have found, however, that laser beams and clouds of atoms can provide quite different results.

The group — with members from Technion-Israel Institute of Technology in Haifa and from Weizmann Institute of Science in Rehovot — devised a method to store and display images inside clouds of rubidium atoms. The technique takes advantage of the principles of electromagnetically induced transparency to slow the light to a group velocity of several thousand meters per second, which permitted storage of images for up to 30 μs.

The scientists accomplished their feat by splitting the 795-nm beam from a laboratory-built external cavity diode laser into probe and pump beams. Electromagnetically induced transparency takes place when the light from the probe beam is strongly absorbed by the rubidium atoms, and the pump beam adds a second state of excitation to the atoms, permitting the probe beam to pass through the vapor.

Images of various digits were stored and displayed in a vapor of rubidium atoms using slowed light. Diffusion of the particles reduced the resolution as the storage time increased. Reprinted courtesy of Physical Review Letters.

The beams were aimed toward a cell containing the rubidium atoms as well as a buffer composed of neon. The cell was held at about 52 °C, which provided a vapor density of approximately 1.3 × 10¹¹ rubidium atoms per cubic centimeter.

The researchers stored images by turning off the pump beam after about half of the probe pulse had left the cell. After an arbitrary duration length — the investigators used 2, 6 or 9 μs in one set of experiments — they turned the pump beam back on, and the stored image appeared once again. They used a CCD camera to acquire images of the probe beam after it exited the vapor cell.

Reporting their work in the June 6, 2008, issue of Physical Review Letters, they note that the main limitation of the technique’s spatial resolution is the diffusion of the atoms that occurs during image storage. They have had some success reducing this effect by alternating the phase in adjoining features in each image, and they think that more complex shapes ultimately may be stored using the technique.