Self-assembly over larger areas brightens outlook for metamaterial manufacture

Oct. 28, 2010

Optical metamaterials can be fabricated in two ways: from the top down, using lithographic or other large-scale patterning techniques; or from the bottom up, using self-assembly. Self-assembly, if made practical, is clearly the cheaper approach.

John Wallace

Leuven, Belgium--Optical metamaterials can be fabricated in two ways: from the top down, using lithographic or other large-scale patterning techniques; or from the bottom up, using self-assembly. Self-assembly, if made practical, is clearly the cheaper approach.

Near-IR metamaterial
Self-assembly of small numbers of metamaterial unit cells is easier than self-assembly of very large numbers. However, progress is being made with the latter: Researchers at Imec have come up with a way to make self-assembled nanomaterials on an industrial scale.¹ They spread polystyrene balls all with a unit-cell-sized diameter of several hundred nanometers onto a gold surface; the balls naturally self-assembled into a hexagonal array. After a succession of coatings and etchings, the result was a layer of silicon dioxide sandwiched between two layers of gold, all containing holes with the same period as the etched-away plastic balls (the array is called a double fishnet). The resulting metamaterial had a negative refractive index in the near-IR region.

Arrays were made up to 100 x 100 microns in size with a small number of defects, and--even more important--a single lattice orientation. (Self-assembly techniques commonly create areas that are patchworks of smaller areas, each with its own lattice orientation--destroying the coherent properties of the larger area.)

Self-assembly versus lithography
What are the remaining advantages of a top-down fabrication method such as photolithography? There are two:

1. With lithography, it is easier to create complex unit-cell structures--for example, metal C rings--which greatly expand the possible types of metamaterial that can be made

2. And also with lithography, if one were willing to spend the very large sum of money it would take to buy or rent a full-fledged wafer fab (a "fab" is a factory used to fabricate the patterns on computer chips), one could create single-lattice metamaterials up to 40 x 40 mm in size with the unit cells all accurately positioned to within a few nanometers, and with unit cells feasible down to below 200 nm in size. But most research labs don't have this sort of money.

REFERENCE:

1. arxiv.org/abs/1010.5138: "Self-Assembled Hexagonal Double Fishnets As Negative Index Materials"

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About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.