Surface plasmon polaritons (SPPs) help transmit light through tiny openings in metal films via an effect called enhanced optical transmission (EOT). SPPs are electron density waves coupled to light that propagate along a metal-dielectric interface, and their electromagnetic fields can extend hundreds of nanometres from the metal surface. These excitations are critical for guiding and manipulating light in integrated photonic devices with structural elements smaller than the wavelength of light.

Most work to date on making such devices has focused on controlling SPPs by tailoring the 2D structure of the hole arrays. However, localized surface plasmons (LSPs) – which are like SPPs but confined to much smaller distances – cannot be easily tuned in this way. Exciting LSPs in hole arrays, rather than just SPPs, is important because LSPs generate electric field intensities that are orders of magnitude higher than that produced by SPPs, explains team leader Teri Odom. However, how LSPs might contribute to EOT is still a mystery.

3D architecture of holes
The Northwestern researchers have now discovered that under certain excitation conditions, LSPs dominate the EOT more than SPPs on the same substrate – provided that a 3D architecture of holes is employed. Moreover, optical transmission from 3D hole arrays is an order of magnitude higher than that from 2D planar hole arrays. "This result will have a significant impact on plasmonics design in the future," Odom told nanotechweb.org.

Creating 3D architecture at the nanoscale is no easy task – even with sophisticated fabrication tools like electron beam or ion beam lithographies. However, Odom and colleagues have developed a new way to make wafer-scale arrays of 3D nanoholes that combines moulding, template stripping and "oblique angle deposition". The large-area samples produced are also easy to characterize, using optical transmission spectra, for example.

"The other neat thing about our 3D hole arrays is that they support both types of plasmons, LSPs and SPPs. And we can tune which ones we want by tuning the 3D structure."

According to the researchers, the new plasmonic structures could be ideal for cutting-edge applications in broadband photovoltaics, plasmon-enhanced white light-emitting diodes and multichannel biosensors with multiple surface sensitivities. "For example, the 3D hole arrays can be engineered to exhibit both narrowband (<15 nm) and broadband (>100 nm) transmission so biosensing could be carried out with different plasmonic probes with varying surface sensitivities on the same substrate," said Odom.

The team is now busy designing new types of nanoscale 3D architectures and will fabricate them using its moulding and soft nanolithography techniques. These substrates will also be tested for EOT and polarization-dependent light transmission.

The work was reported in Nano Letters.