Nanoplasmonics is a new and upcoming field of research on tailored metallic nanostructures that could be used to making tiny optoelectronics devices. Metallic nanoparticles interact strongly with light via localized surface plasmons (collective oscillations of electrons on a metal's surface) and so act as efficient optical nanoantennas. They can focus light to wavelengths dramatically below the diffraction limit.

The new work, by Hatice Altug's team, shows that nanostencil lithography can be used to fabricate these arrays quickly and on a large scale. Such a technique could significantly speed up the widespread use of plasmonic devices for real-world applications, say the researchers.

Although existing top-down nanopatterning processes already exist for making a large variety of nanostructures, their major drawback is low throughput. Each nanostructure has to be created one at time, something that is both slow and expensive. Moreover, electron-beam lithography – a widely used technique for fabricating nanostructures – is also limited by the choice of substrates that can survive exposure to an electron beam.

Everyone is familiar with stencilling in arts and crafts. Nanostencil lithography is basically the same but on a much smaller scale. Altug and colleagues created gold nanorod antenna arrays by first fabricating a nanostencil with rectangular-shaped nanoholes placed on a substrate. The researchers then deposited gold over the apertures. "When the stencil is removed, we get gold nanorods particles on our substrate with shapes complementary to the patterns used in the stencil," explained Altug.

The main advantage of the technique is that can transfer any pattern to any surface in a single step. This is in contrast to existing lithography that relies on resist base patterns, which require an extra chemical lift-off step. More importantly, like ordinary stencils, nanostencils can be reused over and over again to pattern the same nanorods arrays. "This capability could be particularly useful when high-throughput replication of optimised nanoparticle arrays is needed," Altug told "And as we show in our work, a series of plasmonic nanoantenna arrays created from a single nanostencil have nearly identical optical responses."

Collective plasmonic excitations and the strong light-matter interactions in nanoantennas could be important for surface-enhanced spectroscopy. If the matter in question is a biomolecule, for example, then the strong interaction with light could be used to make ultrasensitve biosensors and other bioanalaysis tools, said Altug. "In fact, our group has managed to detect proteins with zeptomole sensitivity using these engineered antenna arrays," she added.

The work was reported in Nano Lett.