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.

The plasmonic Fano switch, developed by Naomi Halas’ and Stephan Link’s teams at Rice, consists of a specifically designed cluster of gold nanoparticles fabricated using electron-beam lithography. The cluster comprises a large hemi-circular disk surrounded by seven smaller nanodisks. Interactions between localized surface plasmon resonances of the individual nanoparticles within a cluster leads to a so-called Fano resonance, which comes about thanks to near-field coupling between collective “bright” and “dark” plasmon modes of the cluster.

Symmetry breaking

“By breaking the symmetry of the nanoparticle cluster through the hemi-circular centre disk, the Fano resonance is polarized and can only be observed for one polarization of incident light,” explained Link. “For orthogonally polarized light, for example, no Fano resonance appears in the light spectrum.”

The device works because the nanoparticle clusters are incorporated into liquid crystals in which the molecules at the device interface can be rotated in plane by 90° when an AC voltage of around 6 V is applied. The field creates a twist in the overall alignment direction of the crystals, which leads to a “homogenous nematic” (voltage off) to a “twisted nematic” (voltage on) phase transition.

Fano resonance switching

“Thanks to the birefringence of the liquid crystal, the voltage-induced phase transition causes an orthogonal rotation of the scattered light from the plasmonic clusters as it travels through the device,” Link told nanotechweb.org. “This results in a switching between the optical response with and without the Fano resonance, so we are thus able to switch the Fano resonance on and off in a voltage dependent manner.”

The device might be used as an active filter that reflects/transmits light of a certain wavelength and that could then be turned on or off by applying an external voltage, he adds. It could be ideal for use in colour displays because plasmonic nanostructures are much more stable than the organic chromophores typically employed as colour pigments today. Replacing these organic molecules, which photobleach over time, with plasmonic nanostructures could thus dramatically increase the lifetime and brightness of colour displays.

The Rice team says that it is now working on Fano switch devices that operate at lower voltages. “We are also further optimizing the cluster geometry to manipulate the polarized Fano resonances so that we can achieve a larger contrast of the on-off modulation for a narrow spectral range,” said Link.

The current work is detailed in Nano Letters.