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
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.