Replacing electronics with 'photonics' (switches and signalling systems thatoperate using light) may lead to faster computers, reports Dick Ahlstrom
Modern computers are getting so fast that the electronics that make them work are starting to get in the way. Changing to components that work using light is one way to get a bit more pace out of our processors.
Trinity College Dublin and Dublin City University have formed a research collaboration to develop new kinds of semiconductor components that can be used in switching devices for optical communications.
"We are interested in materials that produce light and try to find ways to use the light," explains TCD physics lecturer Dr John Donegan. "We use semiconductors because they are good at producing light."
Donegan is head of the Semiconductor Photonics Group at TCD and works with opposite number Dr Liam Barry of DCU on photonics devices. There are about 16 people working on this technology at TCD and another eight at DCU. There is also a link to the University of Sheffield via Prof John Roberts, whose research group grows the specialised semiconductor materials that are at the heart of optronics. Funding comes from Enterprise Ireland's basic research grant scheme.
These exotic new devices flow from earlier work led by TCD's provost, Prof John Hegarty, who formerly led the Semiconductor Photonics Group. Over the past seven years the group has put a strong research emphasis on a device known as a "microcavity semiconductor", explains Donegan.
"This is a piece of semiconductor material which we are using to try and make an optical switch," he says.
The switches used to shunt data about in communications systems need to work "as fast as possible so you can switch data from one channel to another", says Donegan. Existing electronic switches are getting close to their physical limits, however, "and we need to move beyond that", he adds.
Using light instead of an electric current to control devices presents challenges, however.
"The trick with light is if you cross two beams of light, they pass through one another; they don't interact," says Donegan. The researchers want to put a useful device in the way of the light beam to trigger a switch as quickly as possible, and their microcavity devices do just that.
The silicon in a common microchip is a semiconductor and, depending on the material used, can be made to emit or absorb light. Some semiconductor materials are not very efficient at this, however, unless the light or power inputs are very high.
This is where the microcavity comes in. It is like a three-part sandwich, a gallium aluminium arsenide semiconductor in the middle and, at either end, a mirror-like material that captures incoming photons of light. What is special about it is that it is made to the exact wavelength of the light being emitted or absorbed, a manufacturing challenge given that light wavelengths can be less than one millionth of a metre long.
"We have to be very precise about the length of the material," says Donegan.
Earlier work with light-emitting microcavities showed that if the microcavity length and wavelength matched, resonance would occur. This positive feedback greatly boosted the amount of light given off.
For optical switching, the researchers want microcavities that absorb light, because light photons make them produce a "photocurrent", an electrical flow that can throw a switch or make something happen. The TCD/DCU team has developed just such a microcavity that has some useful characteristics.
Work done 20 years ago showed that semiconductors could absorb one or two photons to make a current, but this wasn't very useful to researchers. Single absorption gave a signal, but it wasn't fast enough. Double absorption was very fast, but the photocurrent wasn't good enough.
The resonance or feedback effects in the team's microcavitiy semiconductors have given new importance to double photon absorption, however. The microcavity feedback boosted the photocurrent by a massive 12,000 times, using low light power. It can therefore work as a very fast switch that produces enough current to be useful in a computer.
The team has patented the device, and results are so promising that a company has already been selected to develop the technology.