The new plasmonic nanostructures are designed to "cascade enhance" electromagnetic fields, says team leader Sasha Grigorenko of the University of Manchester. "It is well known that single noble metal nanoparticles can strongly enhance these fields but we show that adding a second, larger nanoparticle can increase this enhancement even more." In contrast to previous attempts to achieve such cascaded enhancement in hot spots of a random collection of nanoparticles, the new structures provide robust and reliable field enhancement, he explains. This turns out to be a resonant process – the sizes of both the small and large particle need to be tuned to optimum values to achieve the maximal field.

The teams employed electron beam lithography to fabricate the composite structures. Two different designs were studied – tower- and pagoda-type structures (see figures). When illuminated with light, both types of structure generate extremely strong light fields that are around 50 times larger than the original applied fields. This happens because the electromagnetic energy is channelled towards the smaller nanoparticle using a design that looks a lot like an optical Fresnel lens, albeit made of gold.

Local light intensity maps
The researchers probed the enhanced fields around the structures by coating them with a layer of fluorescent dye and measuring the fluorescence observed using a scanning confocal microscope. "This is a nice technique that allows us to extract information about local electromagnetic fields from measurements of radiation from a dye excited by external light," Grigorenko told "By scanning the focal position of the microscope lens, we obtain maps of local light intensity."

The scientists say that they now plan to fabricate three-tier nanostructures (the next obvious step), which theory predicts should enhance light fields even more. "The main difficulty here will be to develop a way to measure local fields reliably that will work on the 10–20 nm scale," said Grigorenko, "but we are working on this."

The work was published in Nano Lett.