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Darker = Brighter?

Here’s a University of Rochester press release from 2006:

Scientists at the University of Rochester have created a way to change the properties of almost any metal to render it, literally, black.
The process, using an incredibly intense burst of laser light, holds the promise of making everything from fuel cells to a space telescope’s detectors more efficient—not to mention turning your car into the blackest black around.
“We’ve been surprised by the number of possible applications for this,” says Chunlei Guo, assistant professor of optics at the University of Rochester. “We wanted to see what would happen to a metal’s properties under different laser conditions and we stumbled on this way to completely alter the reflective properties of metals.”
During its brief burst, Guo’s laser … forces the surface of the metal to form and nanostructures—pits, globules, and strands that both dramatically increase the area of the surface and capture radiation.

Metals are normally reflective, particularly when polished, because their smooth surfaces allow electrons to move in a coordinated way in response to the incoming electromagnetic waves. The nanostructure induced by Guo’s laser prevents this, and thus doesn’t reflect.

Pretty much the first thing you learn in the physics of radiation is the notion of a “blackbody” — an object that is a perfect absorber of radiation. It turns out that blackbodies are also, theoretically, perfect emitters of radiation as well. Any incoming light is turned directly into heat, and any heat in the body turns directly into light in accordance with a simple equation.

Well, you can pretty much guess what happens next: another University of Rochester press release, this one from last week.

Laser Process Doubles Brightness for the Same Amount of Energy
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.
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.

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 Vorobyev 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.

When we become able to engineer the nanoscale conducting structures in atomic detail, we’ll build nantenna-based solar collectors, phased-array optics, and the like — but it’s quite interesting to see what can be done with current techniques — and to have the nanostructures to study.

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