Cambridge, Mass. - March 8, 2012 - Researchers in applied physics have cleared an important hurdle in the development of advanced materials, called metamaterials, that bend light in unusual ways.
Working at a scale applicable to infrared light, the Harvard team has used extremely short and powerful laser pulses to create three-dimensional patterns of tiny silver dots within a material. Those suspended metal dots are essential for building futuristic devices like invisibility cloaks.
The new fabrication process, described in the journal Applied Physics Letters, advances nanoscale metal lithography into three dimensions—and does it at a resolution high enough to be practical for metamaterials.
"If you want a bulk metamaterial for visible and infrared light, you need to embed particles of silver or gold inside a dielectric, and you need to do it in 3D, with high resolution," says lead author Kevin Vora, a graduate student at the Harvard School of Engineering and Applied Sciences (SEAS).
"This work demonstrates that we can create silver dots that are disconnected in x, y, and z," Vora says. "There’s no other technique that feasibly allows you to do that. Being able to make patterns of nanostructures in 3D is a very big step towards the goal of making bulk metamaterials."
Vora works in the laboratory of Eric Mazur, Balkanski Professor of Physics and Applied Physics at SEAS. For decades, Mazur has been using a piece of equipment called a femtosecond laser to investigate how very tightly focused, powerful bursts of light can change the electrical, optical, and physical properties of a material.
The need for this particular combination of chemicals, at the right concentrations, was not obvious in prior work. Researchers sometimes combine silver nitrate with water in order to create silver nanostructures, but that process provides no structural support for a 3D pattern. Another process combines silver nitrate, water, PVP, and ethanol, but the samples darken and degrade very quickly by producing silver crystals throughout the polymer.
With ethanol, the reaction happens too quickly and uncontrollably. Mazur's team needed nanoscale crystals, precisely distributed and isolated in 3D.
"It was just a question of removing that reagent, and we got lucky," Vora says. "What was most surprising about it was how simple it is. It was a matter of using less."
SeungYeon Kang, a graduate student at SEAS, and Shobha Shukla, a former postdoctoral fellow, coauthored the paper. The work was supported by the Air Force Office of Scientific Research.
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