Hands on: Crafting ultrathin atoms-thick color coatings on paper

IN HARVARD’S HIGH-TECH CLEANROOM, APPLIED PHYSICISTS PRODUCE VIVID OPTICAL EFFECTS—ON PAPER

In a sub-basement deep below the Laboratory for Integrated Science and Engineering at Harvard University, Mikhail Kats gets dressed. Mesh shoe covers, a face mask, a hair net, a pale gray jumpsuit, knee-high fabric boots, vinyl gloves, safety goggles, and a hood with clasps at the collar—these are not to protect him, Kats explains, but to protect the delicate equipment and materials inside the cleanroom.

While earning his Ph.D. in applied physics at the Harvard School of Engineering and Applied Sciences, Kats has spent countless hours in this cutting-edge facility. With his adviser, Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, Kats has contributed to some stunning advances.

One is a metamaterial that absorbs 99.75 percent of infrared light—very useful for thermal imaging devices. Another is an ultrathin, flat lens that focuses light without imparting the distortions of conventional lenses. And the team has produced vortex beams, light beams that resemble a corkscrew, that could help communications companies transmit more data over limited bandwidth.

Certainly the most colorful advance to emerge from the Capasso lab, however, is a technique that coats a metallic object with an extremely thin layer of semiconductor, just a few nanometers thick. Although the semiconductor is a steely gray color, the object ends up shining in vibrant hues. That’s because the coating exploits interference effects in the thin films; Kats compares it to the iridescent rainbows that are visible when oil floats on water. Carefully tuned in the laboratory, these coatings can produce a bright, solid pink—or, say, a vivid blue—using the same two metals, applied with only a few atoms’ difference in thickness.

Capasso’s research group announced the finding in 2012, but at that time, they had only demonstrated the coating on relatively smooth, flat surfaces like silicon. This fall, the group published a second paper, in the journal Applied Physics Letters, taking the work much further.

“I cut a piece of paper out of my notebook and deposited gold and germanium on it,” Kats says, “and it worked just the same.”

That finding, deceptively simple given the physics involved, now suggests that the ultrathin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics.

“This can be viewed as a way of coloring almost any object while using just a tiny amount of material,” Capasso says.

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