Femtolaser Pulse Creates 3-D Nanostructures

A new fabrication process using femtosecond lasers creates 3-D nanostructures in materials, an essential step toward creating invisibility cloaks and other advanced materials that bend light in unusual ways.

The experimental setup in Prof. Eric Mazur's laser laboratory at Harvard. Using femtosecond lasers, Mazur and colleagues have developed a new nanofabrication process for use in creating metamaterials. (Image: Eliza Grinnell, Harvard SEAS)

Researchers in Eric Mazur's laboratory at the Harvard School of Engineering and Science (SEAS) fired a femtosecond laser, which releases incredibly bright flashes of light that last 5 x 10^-14, s, at a glass slide coated with a mixture of silver nitrate, water and PVP, a water-soluble polymer. The laser blast changes the electrical, physical and optical properties of the slide, and photoreduces the silver ions on the slide into nanocrystals of silver metal suspended on the polymer.

Previous attempts to create a 3-D structure failed because the coating was not quite right. When the researchers used only the silver nitrate and water, there was no lattice support for the silver atoms, they said.

A new laser fabrication technique developed at Harvard allows for the creation of precisely arranged silver nanoparticles that are disconnected in 3D and supported by a polymer matrix. The new technique may prove critical in the development of metamaterials. (Image: Kevin Vora)

"Normally, when people use femtosecond lasers in fabrication, they’re creating a woodpile structure: something stacked on something else, being supported by something else. If you want to make an array of silver dots, however, they can’t float in space," Mazur said. Ethanol and the PVP polymer were added to the solution to provide support to the structure, but reactions were fast and uncontrollable. Removing the ethanol solved the problem entirely.

"What was most surprising about it was how simple it is. It was a matter of using less," Mazur said.

Kevin Vora examines a sample in the lab. Vora and colleagues at SEAS led by Prof. Eric Mazur developed a fabrication process that enables the creation of nanostructures patterned in three dimensions. (Image: Eliza Grinnell, Harvard SEAS)

The new fabrication process advances nanoscale metal lithography into three dimensions, and does it at a resolution high enough to be practical for metamaterials.

"This work demonstrates that we can create silver dots that are disconnected in X, Y, and Z," said Kevin Vora, a graduate student working on the project. "There’s no other technique that feasibly allows you to do that."

The work, which was supported by the Air Force Office of Scientific Research, is described in Applied Physics Letters.

For more information, visit: seas.harvard.edu