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Purple Platinum, Blue Gold?

Photonics.com
Feb 2008
ROCHESTER, N.Y., Feb. 1, 2008 -- A tabletop femtosecond laser has been used to change the surface properties of metals to reflect a specific color or combination of colors. Silver, platinum, gold, and other metals have been turned colors such as blue, gray, black, and purple.

Chunlei Guo, the University of Rochester optical scientist who a year ago used intense laser light to alter the properties of a variety of metals to render them pitch black (See Ultraintense Laser Blast Creates True 'Black Metal' ), has pushed the same process further. He now believes it's possible to alter the properties of any metal to turn it any color -- and even achieve multicolored iridescence like a butterfly's wings.
GuoSample.jpg
Chunlei Guo, associate professor of optics at the University of Rochester, holds a piece of iridescent platinum he created in his lab. He used a tabletop femtosecond laser to form nanoscale and microscale structures on the surface of metals to change the color or colors they reflect. (Images courtesy Richard Baker, University of Rochester)
Since the process changes the intrinsic surface properties of the metal itself and is not just a coating, the color won't fade or peel, said Guo, associate professor of optics at the university's Institute of Optics. He suggests the possibilities are endless -- a cycle factory using a single laser to produce bicycles of different colors, etching a full-color photograph of a family into the refrigerator door, or proposing with a gold engagement ring that matches your fiancée's blue eyes.

"Since the discovery of the black metal we've been determined to get full control on getting metals to reflect only a certain color and absorb the rest, and now we finally can make a metal reflect almost any color we wish," said Guo. "When we first found the process that produced a gold color, we couldn't believe it. We worked in the lab until midnight trying to figure out what other colors we could make."
BlackTitanium.jpg
Black titanium created using an ultra-intense laser beam. The laser changes the surface properties of the metal and is not just a coating, meaning the color won't fade or peel.
Guo and his assistant, Anatoliy Vorobeyv, use an incredibly brief but incredibly intense laser burst that changes the surface of a metal, forming nanoscale and microscale structures that selectively reflect a certain color to give the appearance of a specific color or combinations of colors.

The metal-coloring research follows up on Guo's breakthrough "black metal" discovery in late 2006, when his research team was able to create nanostructures on metal surfaces that absorbed virtually all light, making something as simple as regular aluminum into one of the darkest materials ever created.
GuoLaser.jpg
Guo in the lab at the Institute of Optics at the University of Rochester.
Guo's black metal, with its very high absorption properties, is ideal for any application where capturing light is desirable. The potential applications range from making better solar energy collectors, to more advanced stealth technology, he said.

The ultrabrief/ultra-intense light Guo uses is produced by a femtosecond laser, which produces pulses lasting only a few quadrillionths of a second. 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 electric grid of North America does, all focused onto a spot the size of a needlepoint.

The intense blast forces the surface of the metal to form nanostructures -- pits, globules and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures. Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, Guo said. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what color the metal reflects.

Guo and Vorobyev also achieve the iridescent coloring by creating microscale lines covered with nanostructures. The lines, arranged in regular rows, cause reflected light of different wavelengths to interfere differently in different directions. The result is a piece of metal that can appear solid purple from one direction, and gray from another, or multiple colors all at once.
coloredmetals.jpg
New colors of metals created by Guo and his team include (l-r) gold aluminum, blue titanium, and gold platinum. 
To alter an area of metal the size of a dime currently takes 30 minutes or more, but the researchers are working on refining the technique. 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 should be relatively simple.

The new process has worked on every metal Guo has tried, and the results are so consistent that he believes it will work for every metal known. His team is currently working to find the right tuning to create the rest of the rainbow for the solid-colored metal, including red and green.

A paper on Guo's research appears today in Applied Physics Letters.

For more information, visit: www.rochester.edu


GLOSSARY
absorption
The transfer of energy from an incident electromagnetic energy field with wavelength or frequency to an atomic or molecular medium.
femtosecond laser
A type of ultrafast laser that creates a minimal amount of heat-affected zones by having a pulse duration below the picosecond level, making the technology ideal for micromachining, medical device fabrication, scientific research, eye surgery and bioimaging.
iridescence
The rainbow exhibition of colors, usually caused by interference of light of different wavelengths reflected from superficial layers in the surface of a material.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
stealth
That characteristic which makes a weapon system less visible to radar, optical, acousto-optic, infrared and other military sensors.
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