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11 Jan 2006
Looking ahead over the next 12 months, the optics.org team forecasts that slow light and polymer solar cells are two key technology areas that are likely to be regularly making the headlines. Here's an update on their status.
Slow light
Slowing light down was big news in 2005, with researchers around the world reporting key breakthroughs in the field. Putting the brakes on light is important because it could accelerate the development of all-optical memory chips and biosensors.
The main motivation is the construction of all-optical buffers, which can delay and temporarily store light pulses. Such devices could have important consequences for telecommunications networks, optical computing and optical phased-array antennae.
What's more, a practical solution to controlling the timing of light pulses could be on the horizon sooner than scientists first thought, thanks to the shrewd modification of some well known semiconductor devices.
Scientists at the COM Research Center, Denmark, have recently shown that two popular telecoms components - a quantum-dot semiconductor optical amplifier and an electro-absorption modulator - can both slow the propagation of light pulses. At the same time a team from the University of California at Berkeley and Texas A&M University, US, has demonstrated that, when configured as an amplifier, a vertical-cavity surface-emitting laser can also perform the feat. All three approaches work by carefully controlling the electrical bias of the custom-built devices.
However, perhaps the most exciting recent development in the field came from researchers based at the IBM T J Watson Research Centre, US. They fabricated a silicon chip that combined miniature heaters with photonic crystal technology to control the speed of light pulses and reduce the group velocity of light by a factor of up to 300. Their design features a 250 μm long silicon waveguide that is patterned with 109 nm diameter holes and placed in close proximity to an electrical contact. Applying an electrical signal to the contact heats up the waveguide and allows the team to tune the speed of the infrared pulses, thanks to the corresponding change in refractive index.
As IBM's Yurii Vlaslov explains, the semiconductor-chip approach to slowing light down has many benefits. "It has definite advantages over doing it in [gas] vapour and other media - it is self contained and you don't need high-intensity lasers or a whole room full of equipment," he said. "The problem, though, is that measurements are challenging because the pulses are spread enormously and then it is difficult to assign any velocity to such a distorted pulse."
To make measurements of group velocity, the scientists placed two such waveguides and electronic heaters side by side on the device to create a tiny (0.04 mm2 footprint) Mach Zehnder interferometer. One arm of the interferometer acted as a reference and the other as a variable speed arm.
Another approach to pulse measurement is to make a "slow-light" movie. Using a phase-sensitive near-field scanning optical microscope (PS-NSOM), the Ultrafast Photonics Collaboration imaged light travelling through a photonic crystal riddled with 260 nm diameter holes. The team effort involves scientists from the University of Twente and FOM Institute in the Netherlands, Ghent University and IMEC in Belgium, and the University of St Andrews, Scotland.
"The big thing here is that you can actually see what is happening," commented Thomas Krauss of the University of St Andrews. "We can now produce images and movies with nanoscale resolution that show the propagation of slowed light pulses in the waveguide." A valuable development tool, the PS-NSOM's images revealed that 120 fs duration pulses launched into the collaboration's waveguide were travelling at about 1/1000th of the speed of light in vacuum.
Polymer solar cells
Flexible, thin-film polymer photovoltaics, which can be made using a convenient roll-to-roll process, could soon be challenging traditional silicon solar cells. Last year, US firm Konarka gave itself a European base by acquiring Siemens' organic photovoltaic research activities, and now it believes that its development partners could be manufacturing their first products within 12-36 months.
"On the commercial product side we have been able to make material that is hundreds to thousands of feet long," Daniel McGahn of Konarka told OLE. "What has changed on the technology side is that we've raised the bar on where we see the ultimate efficiency."
The firm is busy optimizing the chemical make-up of its so-called Power Plastic and is experimenting with the addition of sensitizers that would make the material responsive in the near infrared. "We now have a roadmap and want to be able to get from, say, 7% [efficiency] up into the high teens and potentially over 20% in the laboratory," said McGahn. "We've commissioned a pilot coating line for the laboratory development of a finished product and have entered into a relationship with German printing company Kurz to look at very-large-scale production."
Applications
The first beneficiaries of Power Plastic are likely to be portable consumer electronics and sensors that are usually plugged in to the grid to be recharged. Pushing the concept much further foward and assuming further reductions in cost and gains in conversion efficiency, polymer photovoltaics could one day become an important source of green power generation. "Instead of a coal-fired plant you could have a vineyard with a plastic film that has dual functionality and a very large area to generate power," added McGahn.
Such a vision of alternative energy is increasingly in the news, but McGahn downplays the recent surge of attention. "It allows for things such as greater desire of federal governments to invest in technology and a business climate that is more amenable to looking at renewable-energy technology, but at the end of the day it is consumption that winds up driving change," he said. "We can look at macro trends, carbon credits, NOx credits and political pressures, but what matters is the value proposition to the consumer."
Although ordinary solar cells generally work very well in full sun, this means that conventionally powered devices have a restricted window of 4-6 h to harvest their energy. Power Plastic technology is better suited to lower light levels and can actually outperform traditional solar technology under these conditions.
"There is a bit of education and understanding that needs to go on in the marketplace," said McGahn. "The concept is that devices will be self-recharging and simply placed in an environment that is lit."
Exciting possibilities for products using the technology include power-generating windows, doors, awnings and roof tiles. In the future, these items could all contain light-activated material and would be direct replacements for current building components.
According to McGahn, a hurdle facing the solar industry today is what he dubs the last mile problem - how to apply the technology. "[With integrated products] you are simplifying the user experience and are able to get people more excited, more familiar and allow the products to be more useful," he said. "At the end of the day you have actually solved the problem."
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