Nanoscale metallic gratings are important in optical phenomena such as enhanced optical transmission (EOT), light beaming and negative refraction. Surface plasmon polaritons (SPPs) – quasiparticles that arise from the interaction of light with a metal's conduction electrons – play a major role in these processes. SPPs are part light and part electron wave and possess new properties not seen in either photons or excitons. They are alternatives to planar waveguides and photonic crystal structures for strongly guiding and manipulating light.

Teri Odom and colleagues have now shown that the spectral features of quasi-1D arrays (made from patterned gold films perforated with 2D rows of finite-length slits) can be characterized by high-order, 1D SPP modes. The researchers have also found that the multiple modes of these 1D arrays can be used as simple refractometers to determine the refractive index of unknown materials. This is possible thanks to the fact that surface plasmons are sensitive to changes in the surrounding dielectric environment.

"What we did was to calibrate the refractive index sensitivity of the nanoslit arrays," Odom told "Once we performed this calibration, we could then use the arrays to determine the refractive index of non-absorbing materials."

To show that their technique worked, the Northwestern researchers measured the refractive indices of different concentrations of common salt (NaCl) and found values that were in good agreement with reference values. They obtained their results by angle-resolved optical transmission measurements.

The study shows that nanoslit arrays can offer improved sensitivity for monitoring protein-ligand binding events, explains Odom. This is because the nanofabricated substrates exhibit multiple, narrow plasmon modes that have two advantages. The first is that they respond well to a range of targets, such as proteins, and the second is that systematic errors are low compared with conventional, single-mode surface plasmon resonance substrates. What's more, the large-area substrates are able to detect several species at once so can potentially analyse a high throughput of proteins.

Odom and co-workers are now carrying out real-time, ligand-protein sensing experiments. "We would like to determine how these new nanofabricated gold surfaces compete with traditional sensing measurements, such as prism-based, SPR measurements," said Odom. "We are also looking at how to improve sensitivities by changing the nanostructured pattern and by employing other materials in the plasmonic substrates."

The work was published in Nano Lett.