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  • Light from Nanoparticles Controlled
Oct 2011
HOUSTON, Oct. 3, 2011 — Rice University researchers have created a technique to control plasmonic scattering from gold nanoparticles using liquid crystals. The researchers use voltage to sensitively manipulate the alignment of liquid crystal molecules that alternately block and reveal light from the particles; the gold nanorods collect and retransmit light in a specific direction.

“The key to our approach is the in-plane rotation of liquid crystal molecules covering individual gold nanorods that act as optical antennas,” said Stephan Link, a Rice chemist who said the technique took two years to refine to the point where light from the nanoparticles could be completely controlled. “Learning how our devices work was exciting and has provided us with many ideas of how to manipulate light at the nanoscale.”

Link said the device is actually a super half-wave plate, a refined version of a standard device that alters the polarization of light.

Rice University researchers have created a technique to control plasmonic scattering from nanoparticles using liquid crystals. Clockwise from top left are Saumyakanti Khatua, Jana Olson, Wei-Shun Chang, Pattanawit Swanglap and professor Stephan Link. (Image: Jeff Fitlow/Rice University)

With the new device, the team expects to be able to control light from any nanostructure that scatters, absorbs or emits light — even quantum dots or carbon nanotubes.

“The light only has to be polarized for this to work,” said Link.

In polarized light, like sunlight reflecting off water, the light's waves are aligned in a particular plane. By changing the direction of their alignment, liquid crystals can tunably block or filter light.

The Rice team used gold nanorods as their polarized light source. The rods act as optical antennas; when illuminated, their surface plasmons re-emit light in a specific direction.

In their experiment, the Rice team members placed randomly deposited nanorods in an array of alternating electrodes on a glass slide; they added a liquid crystal bath and a coverslip. A polyimide coating on the top coverslip forced the liquid crystals to orient themselves parallel with the electrodes.

In an experiment at Rice University, applied voltage creates a nematic twist in liquid crystals (blue) around a nanorod (red) between two electrodes. This graphic shows liquid crystals in their homogenous phase (left) and twisted nematic phase (right). Depending on the orientation of the nanorods, the liquid crystals will either reveal or mask light when voltage is applied. (Image: Link Lab/Rice University)

Liquid crystals in this homogenous phase blocked light from nanorods turned one way while allowing light from nanorods pointed another way to pass through a polarizer to the detector.

What happened then was remarkable. When the team applied as little as 4 V to the electrodes, liquid crystals floating in the vicinity of the nanorods aligned themselves with the electric field between the electrodes while crystals above the electrodes, still under the influence of the coverslip coating, stayed put.

The new configuration of the crystals, called a twisted nematic phase, acted like a shutter that switched the nanorods' signals like a traffic light.

Polarized dark-field scattering images of single gold nanorods in electrode gaps show them either turned on or off, depending on voltage applied to a swarm of liquid crystals. The arrows indicate the polarization of detected light, either parallel (purple) or perpendicular (green) to the electrode array. (Image: Link Lab/Rice University)

“We don't think this effect depends on the gold nanorods,” Link said. “We could have other nano objects that react with light in a polarized way, and then we could modulate their intensity. It becomes a tunable polarizer.”

Critical to the experiment's success was the gap — in the neighborhood of 14 µm — between the top of the electrodes and the bottom of the coverslip.

“The thickness of this gap determines the amount of rotation,” he said. “Because we created the twisted nematic in-plane and have a certain thickness, we always get 90-degree rotation. That's what makes it a super half-wave plate.”

Link sees great potential for the technique when used with an array of nanoparticles oriented in specific directions, in which each particle would be completely controllable — like a switch.

The research was reported in the American Chemical Society journal Nano Letters.

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quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
twisted nematic phase
A form of liquid crystal widely used in displays, achieved by sandwiching a liquid crystal material between two substrates whose inside surfaces have been prepared with a special coating that aligns the molecular axes. The direction of alignment of one of the substrates is placed perpendicular to that of the second substrate, creating a 90° twist in the liquid crystal molecules. An applied voltage untwists the molecules as they align parallel to the direction of the electrical field,...
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