“To the best of our knowledge, the optical device we designed also appears to have a much higher colour-conversion efficiency than other semiconductors, dielectrics and nonlinear optical crystals,” team member Yu Zhang told nanotechweb.org. “The device works using a process called ‘four-wave mixing’ (an important nonlinear optical process where light can be controlled with light) and its improved colour-conversion efficiency comes thanks to coherent ‘Fano resonances’.”

Nanoplasmonics is a new and upcoming field of research on tailored metallic nanostructures that could be used for making tiny optoelectronics devices. Metallic nanoparticles interact strongly with light via localized surface plasmons, which are collective oscillations of electrons on a metal's surface.

Zhang and colleagues’ device is made up of a cluster of 13 closely packed gold nanodisks that are 120 nm wide and 50 nm thick, spaced 18 nm apart and arranged in a semicircle on a transparent fused silica substrate. This particular pattern enhances nonlinear optical efficiencies by creating intense electric fields in the gaps between the nanodisks. Indeed, the Fano resonances responsible for the improved efficiencies come about thanks to near-field coupling between collective “bright” and “dark” plasmon modes of the cluster. “These resonances boost the efficiency of the relatively weak nonlinear effect underlying four-wave mixing,” explained team member Peter Nordlander, “and the result is a boost in the intensity of the third colour of light that the device produces.”

Nonlinear medium

Normally, in a classic medium, one beam of light does not interact with another beam, he continues. However, the situation changes if the light beam is travelling in a nonlinear medium with electromagnetic properties that do allow for interactions between two light beams. For example, the intensity of a light beam fired through such a medium will be reduced proportionally to the intensity of another beam fired through at the same time.

“The automatic numerical calculations and electron-beam lithography methods developed by team members Yu-Rong Zhen and Fangfang Wen used to design and create our new nonlinear optical device could easily be applied to making a wide range of other similar, nonlinear materials that can mix and produce light beams of many different colours,” said Zhang. “What is more, the nanoclusters making up the device could serve as building blocks for highly efficient optical information processing.”

The team says that it has already succeeded in designing a similar nanocluster that can act as a molecular sensor thanks to surface-enhanced coherent anti-Stokes Raman scattering (SECARS), which is a specific four-wave mixing process. “We also plan to use these nonlinear devices for photonic logic gate studies,” revealed Zhang.

The current work is detailed in PNAS.