Image Storage, but Not Without Delay

Slow and steady really does win, according to University of Rochester researchers. The group slowed images carried on photon pulses in a hot gas of atoms and then showed that the image information was preserved. As described in the Jan. 26 issue of Applied Physics Letters, this go-slow, all-optical image delay could find use in image processing, holography, optical pattern correlation, remote sensing and quantum information.

For example, image processing requires a buffer. One solution is to convert the image to its electronic equivalent, but because that demands lots of light, information is lost. Another solution is a long delay line, but that suffers from diffraction and space limitations. A small all-optical buffer, which the investigators’ technique enables, would solve these problems.

The researchers made use of the atomic resonances of cesium to slow light of a particular frequency. Such slow-light materials have a slow group velocity, the speed at which energy travels, because they are highly dispersive due to frequency-dependent absorption. An advantage to the cesium-based approach is that it requires only a single laser, unlike the multiple sources needed for other slow-light methods.

In one experiment, the scientists sent 2-ns light pulses with, on average, less than one photon each through a mask and a lens-based imaging system. Between the lenses sat a 10-cm cell full of hot cesium vapor. By changing the cesium’s temperature, the researchers produced a variable delay of up to 10 ns.

After the light pulses traversed the setup, the group captured them with a scanning optical fiber and a camera, taking many seconds to move across the entire array of positions. They did this for a delayed and nondelayed two-dimensional image of a pair of letters, demonstrating similar image fidelity and resolution no matter the delay.

So far, the group has achieved delays of up to 100 ns. Immediate plans are to slow dozens of pulses several milliseconds, but the eventual goal is to achieve far longer delays.