The prevalence of cold, blue energy-saving light bulbs might soon begin to fade. That’s according to researchers in the US, who claim to have discovered how to make traditional incandescent bulbs 100% efficient.
“Many people still love incandescent light bulbs because they create the most pleasant light and are cheaper to buy,” Chunlei Guo of the University of Rochester, New York, told physicsworld.com. “The downside is the low efficiency of the conventional incandescent bulbs. This research addresses that very problem.”
Guo, together with colleague Anatoliy Vorobyev, made the discovery having spent several years investigating how intense laser pulses can affect the structure of metal surfaces. In 2006 the two researchers found that by applying a series of femtosecond (10-15 second) laser bursts to a metal, its surface would become pitch black. In other words, the metal’s ability to absorb light would soar.
But according to Kirchhoff’s law, at thermal equilibrium the absorptivity of a surface equals its emissivity. So Guo and Vorobyev figured the same “blackening” technique could enhance the emission, and hence efficiency, of an incandescent bulb’s metal filament.
Kirchoff’s law stands
To see if this could work, the researchers used an amplified laser to expose part of a tungsten filament to a number of 65-femtosecond pulses at a wavelength of 800 nm and a repetition rate of 1 kHz. Once they had inserted the blackened filament inside a bulb, they pre-heated it to its equilibrium temperature of 900°C before monitoring its emission with a photomultiplier tube.
Guo and Vorobyev found that the number of laser pulses given to the filaments strongly affected the emission. Up to about 500 pulses there was a sharp increase, while towards 4000 pulses the increase levelled out. The researchers also found that the enhancement depended on the wavelength of light emitted from the bulb — at 400 nm the increase was about 25% but at 800 nm the increase rose to 55%.
Scanning-electron-microscopy images showed the laser pulses had caused the tungsten surface to adopt a structure of periodic, nano-sized ridges. The researchers believe these ridges encourage thermally excited surface “plasmons” — quanta of plasma oscillations — to couple with electromagnetic emission in free space, and thereby boost emissivity.
Super efficient
Guo says that, over the entire visible spectrum, the efficiency rose from roughly 50% to 100%. However, Shawn Lin, a photonics researcher at the Rensselaer Polytechnic Institute in Troy, New York, is not so sure. “The higher emissivity is over visible and near-infrared wavelengths, which means it would generate a lot of wasted heat — 80 to 95% — in the infrared regime,” he says. If there is wasted heat, adds Lin, the researchers could reduce it by further nano-structuring.
Nevertheless, according to the researchers, the boosted efficiency is not the only benefit of the technique. Guo and Vorobyev found that the strength and duration of the laser treatment could alter the colour balance of the emission. Moreover, they found that the emission was somewhat polarized, possibly because of the direction of the surface ridges.
“Light polarization has a variety of applications, from liquid crystal displays to polarized sunglasses,” explains Guo. “If we start to illuminate with polarized light, a wide variety of perceptive effects can be achieved, such as brightness and clarity.”
However, the immediate future may not be looking too dark for today’s energy saving bulbs — the possibility of commercial devices is not yet at the front of the researchers’ minds. “The research just came out,” says Guo. “This is too early to say at this point.”
This research will be published tomorrow in Physical Review Letters.
