An ultra-powerful laser can turn regular incandescent light bulbs into power-sippers,
say optics researchers at the University
of Rochester. The process could make a light as bright as a 100-watt bulb
consume less electricity than a 60-watt bulb while remaining far cheaper and
radiating a more pleasant light than a fluorescent bulb can.
Chunlei Guo stands in front of his femtosecond laser, which can double the efficiency of a regular incandescent light bulb. Credit: University of Rochester
The laser process creates a unique array of nano- and micro-scale structures
on the surface of a regular tungsten filament—the tiny wire inside a light
bulb—and theses structures make the tungsten become far more effective
at radiating light.
The findings will be published in an upcoming issue of the journal Physical
Review Letters.
"We've been experimenting with the way ultra-fast lasers change metals,
and we wondered what would happen if we trained the laser on a filament,"
says Chunlei Guo, associate professor of optics at the University of Rochester.
"We fired the laser beam right through the glass of the bulb and altered
a small area on the filament. When we lit the bulb, we could actually see this
one patch was clearly brighter than the rest of the filament, but there was
no change in the bulb's energy usage."
The key to creating the super-filament is an ultra-brief, ultra-intense beam
of light called a femtosecond laser pulse. The laser burst lasts only a few
quadrillionths of a second. To get a grasp of that kind of speed, consider that
a femtosecond is to a second what a second is to about 32 million years. During
its brief burst, Guo's laser unleashes as much power as the entire grid of North
America onto a spot the size of a needle point. That intense blast forces the
surface of the metal to form nanostructures and microstructures that dramatically
alter how efficiently can radiate from the filament.
In 2006, Guo and his assistant, Anatoliy Vorobeyv, used a similar laser process
to turn any metal pitch black. The surface structures created on the metal were
incredibly effective at capturing incoming radiation, such as light.
"There is a very interesting 'take more, give more' law in nature governing
the amount of light going in and coming out of a material," says Guo. Since
the black metal was extremely good at absorbing light, he and Vorobyev set out
to study the reverse process—that the blackened filament would radiate
light more effectively as well.
"We knew it should work in theory," says Guo, "but we were still
surprised when we turned up the power on this bulb and saw just how much brighter
the processed spot was."
In addition to increasing the brightness of a bulb, Guo's process can be used
to tune the color of the light as well. In 2008, his team used a similar process
to change the color of nearly any metal to blue, golden, and gray, in addition
to the black he'd already accomplished. Guo and Vorobeyv used that knowledge
of how to control the size and shape of the nanostructures—and thus what
colors of light those structures absorb and radiate—to change the amount
of each wavelength of light the tungsten filament radiates. Though Guo cannot
yet make a simple bulb shine pure blue, for instance, he can change the overall
radiated spectrum so that the tungsten, which normally radiates a yellowish
light, could radiate a more purely white light.
Guo's team has even been able to make a filament radiate partially polarized
light, which until now has been impossible to do without special filters that
reduce the bulb's efficiency. By creating nanostructures in tight, parallel
rows, some light that emits from the filament becomes polarized.
The team is now working to discover what other aspects of a common light bulb
they might be able to control. Fortunately, despite the incredible intensity
involved, the femtosecond laser can be powered by a simple wall outlet, meaning
that when the process is refined, implementing it to augment regular light bulbs
should be relatively simple.
Guo is also announcing this month in Applied Physics Letters a technique using
a similar femtosecond laser process to make a piece of metal automatically move
liquid around its surface, even lifting a liquid up against gravity.