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When excitons exit an electrical circuit and recombine, they create a flash of light which could be harnessed for optical communications.  (Source: Leonid Butov, University of California at San Diego)
Quasiparticle based circuits could break down the electro-optical communication barrier.

One of the bottlenecks in current electro-optical communication systems is the need to convert electrons into photons. While optical interconnects maybe be amazingly fast and efficient, the conversion process still chews up precious time.

This May, Harvard researchers showed a new technology that could be used to build LEDs directly into an integrated circuit. Last week, University of California at San Diego scientists published work in the journal Science using a more direct approach at converting electricity to light on the fly.

Excitons are an interesting type of particle. They are created when photons enter a semiconductor, exciting the electrons it contains. An excited electron forms an electron-hole pair, which in this case, is called an exciton. What makes excitons useful for optical ICs is that when the electron-hole pair recombines, they emit a flash of light.

The key to creating an electro-optical IC in this case is the ability to control the exciton, preventing it from recombining too early. To accomplish this, the UCSD scientists used a special semiconductor made of gallium arsenide, very low temperatures (less than 40 degrees Kelvin), and a special type of exciton that separates the electron and hole pair by several nanometers, confining them to their own quantum wells.

Using voltage to control the excitons, they can be held in place or allowed to flow. Once they flow to the end of the circuit, the electrons and holes recombine, creating photons that can be captured by optical circuitry for use in interconnects or other communication devices.

The group, led by Leonid Butov, a professor of physics at UCSD, has already created several types of exciton-based transistor switches. The switches are quite fast and are able to be manipulated at about 200 picoseconds so far. The exciton circuits are no faster than standard electrical circuits, but removing the clumsy electro-optical conversion allows a much greater data transmission rate between optically connected devices, thus speeding up the process on the whole.

The circuits, operating at 40 degree Kelvin, are far from ready to be used in mainstream applications. Further work will be necessary with other types of semiconductor materials to bring the operational conditions of the exciton circuits to a usable level.





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