Controlling light from nanoparticles

October 3, 2011
Nanoparticle Images

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 (credit Link Lab/Rice University)

Polarized light may make it possible to actively control optical antennas and other plasmonic elements, according to new research at Rice University.

A laboratory led by Stephan Link, an assistant chemistry professor, has discovered a way to use polarized liquid crystals to control light scattered from gold nanorods. Nanorods can function as efficient optical antennas for use in improving optical switches for faster data communications, for example.

The researchers used voltage to sensitively manipulate the alignment of liquid-crystal molecules that alternately block and reveal light from the gold nanorods, which collect and retransmit light in a specific direction.

The polarization secret

Confining visible light to nanoscale dimensions has become possible with surface plasmons, and many plasmonic elements have already been realized.  However, active control of the plasmonic response remains a roadblock for building optical analogs of electronic circuits.

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, who studies the plasmonic properties of nanoparticles.

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 because when illuminated, their surface plasmons re-emit light in a specific direction.

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

Nematic Twist

Applied voltage creates a nematic twist in liquid crystals (blue) around a nanorod (red) between two electrodes in an experiment at Rice University. 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. (Credit: Rice University)

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

The researchers said that what happened then was remarkable. When the team applied as little as four volts 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 cover slip 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.

“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 microns — between the top of the electrodes and the bottom of the cover slip. “The thickness of this gap determines the amount of rotation,” Link said. “Because we created the twisted nematic in-plane and have a certain thickness, we always get 90-degree rotation.”

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.

Ref.: Saumyakanti Khatua, et al., Active Modulation of Nanorod Plasmons, Nano Letters, 2011; [DOI: 10.1021/nl201876r]