Sound conveyor belt for delaying photons.

To store or process information encoded as light signals, it's often necessary to slow down or temporarily hold up strings of photons. Light travels so quickly, however, that such delays demand unusual efforts.

One approach already in use shunts signals into a kilometer or more of optical fiber, where photons can circulate until needed. Now, a team of researchers has developed a considerably more compact method of accomplishing this goal. In effect, the signal is converted from a wave that travels at the speed of light into one that travels at the speed of sound in a semiconducting material, then back into a speed-of-light wave.

Carsten Rocke, Achim Wixforth, and their colleagues at the University of Munich describe the technique in the May 26 Physical Review Letters.

The device constructed by the group consists of an extremely thin film of indium gallium arsenide, 10 nanometers across, sandwiched between layers of gallium arsenide. The film serves as a quantum well, confining mobile electrons to motion in two dimensions.

The researchers can produce high-frequency acoustic pulses--physical vibrations--that travel across the surface of the quantum well from one end of the sample to the other. Shining laser light onto the quantum well displaces electrons, leaving behind holes. Such holes can be thought of as the positively charged counterparts of electrons in a semiconductor, and like electrons, they can move from place to place in a crystal.

Normally, electrons and holes recombine almost immediately, releasing energy in the form of photons. However, the propagating sound wave induces varying electric fields in the material, and these trap electrons and holes in separate phases of the pulse.

The separated electrons and holes are like objects interspersed on a conveyor belt, with adjacent positive and negative charges 1.5 micrometers apart, Wixforth says. They move along-with the acoustic wave at the speed of sound.

Finally, electrons and holes can be brought together by adjusting the sound wave, and light is emitted. In effect, the photon conveyor belt extends the life of an electron-hole pair from mere nanoseconds to microseconds.

The researchers performed their experiments at 4.2 kelvins. "We hope to introduce a new, room-temperature version of our device in the very near future," Rocke says. Eventually, the researchers want to incorporate methods for processing the "stored light."