The result is that pure aluminum looks like gold, and the appearance is literally skin deep.
“I cannot tell it’s not gold,” Dr. Guo said. “It looks very pretty.”
Dr. Guo and Dr. Vorobyev reported their findings in the journal Applied Physics Letters published online Thursday.
The golden aluminum follows work a little more than year ago where Drs. Guo and Vorobyev reported that they could make gold and other metals look black — indeed a black that is blacker than the usual black, sucking up almost all light that impinged upon it.
The laser bursts — each lasting only about 60 millionths of a billionth of a second — melt and vaporize metal atoms near the surface, which then reassemble in minuscule structures including pits, spheres and rods that are a fraction of a millionth of a meter in size.
By changing the length, strength and number of pulses, the researchers found they could vary the resulting color.
In some cases, the change causes the structures to absorb a range of colors so that they cannot be seen. But the colors that are not absorbed are still reflected, and thus visible, resulting in gold aluminum or dark blue tungsten.
In other cases, the laser pulses create a periodic array of structures that cause the reflected light to interact and interfere with itself, producing an iridescent, shimmering rainbow — much like some butterfly wings, Dr. Guo said.
Dr. Guo imagines a kaleidoscope of potential uses, from the practical (a reflective filter) to the whimsical (etching the family photograph onto a metal refrigerator door, for instance). Another possibility is custom colors for bicycles or cars, without the need for any paint.
“It’s pretty robust, because it’s right on the metal, and it’s not going to peel off,” Dr. Guo said.
He cannot yet make all metals into all colors but says he believes that it is only a matter of trial-and-error to find the right recipe for each permutation.
With his black metal finding, Dr. Guo suggested the possibility of black gold rings. He was surprised when jewelers started calling. “They are actually indeed interested in making colored jewelry,” he said.
In the new article, he suggests a blue gold ring, perhaps a blue to match the eyes of a fiancé.
