All that glitters golden is not gold. It could be aluminum. Or tungsten. Or another metal of Chunlei Guo’s choosing. In a feat of optical alchemy, Dr. Guo, a professor of optics at the University of Rochester,
and Anatoliy Y. Vorobyev, a postdoctoral researcher, use ultrashort
laser bursts to pockmark the surface of a metal in a way that is not
perceptible to the touch — it still feels smooth to the finger — but
that alters how the metal absorbs and reflects light.

From left, aluminum turned a gold color, titanium turned to blue, and platinum turned gold.

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.

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.

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.

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

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.

Via NY Times