Billions of nanodisks can be deposited onto an area of 1cm2; each nanodisk can react to incident light and create plasmons. Image: Conceptualized.
Billions of nanodisks can be deposited onto an area of 1cm2; each nanodisk can react to incident light and create plasmons. Image: Conceptualized.

Researchers at Linköping University in Sweden have developed optical nanoantennas made from a conducting polymer. These antennas can be switched on and off, and could lead to completely new types of controllable nano-optical components.

Plasmons are produced when light interacts with metallic nanoparticles, with the incident light setting off a collective oscillation of the electrons in the particles. It is this collective oscillation that is the plasmon.

Metallic nanostructures and their ability to shape light on a scale of nanometers are studied by many research groups around the world, for potential use in biosensors and energy conversion devices, and to reinforce other optical phenomena. Additional potential fields of use include in miniature medical equipment and windows that control the amount of light and heat admitted to or emitted from a building.

In a paper in Nature Nanotechnology, the Linköping University researchers report optical nanoantennas made from a conducting polymer, rather than from a traditional metal such as gold or silver. In this case, they used a variant of PEDOT, a widely used polymer in many other areas, including thermoelectrics and bioelectronics.

"We show that light can be converted to plasmons in nanostructures of the organic material," says Magnus Jonsson, leader of the Organic Photonics and Nano-optics group at Linköping University’s Laboratory of Organic Electronics.

In the conducting polymer, it is not electrons that create plasmons, but polarons. A polymer consists of a long chain of connected atoms; positive charges along this polymer chain are responsible for the electrical conductivity of PEDOT. Together with associated chain distortions, these positive charges form polarons, which start collective oscillations when the nanostructure is illuminated with light.

"Our organic antennas can be transparent to visible light while reacting to light at somewhat longer wavelengths, making them interesting for applications such as smart windows", says Jonsson.

The researchers initially carried out theoretical calculations and used simulations to design the experiments, which they were subsequently able to conduct. Shangzhi Chen, a doctoral student in the group, has managed to produce billions of tiny nanometer-sized disks of PEDOT on a surface. These small disks react to light and act as tiny antennas.

The researchers have shown that both the diameter and the thickness of the disks determine the frequency of light to which they react, meaning this frequency can be controlled by simply changing the geometry of the disk. The thicker the disk, the higher the frequency. The researchers are also hoping they can increase the range of frequencies to which the nanoantennas react by using different conducting polymers.

Another innovation they have explored is the ability to switch the organic nanoantennas on and off, which is difficult with conventional metals. When initially manufactured in the laboratory, PEDOT is in an oxidized state, meaning the nanoantennas are switched on.

"We have shown that when we reduce the material by exposing it to a vapor, we can switch off the conduction and, in this way, also the antennas," explains Jonsson. "If we then reoxidize it using, for example, sulfuric acid, it regains its conductivity and the nanoantennas switch on again. This is a relatively slow process at the moment, but we have taken the first steps and shown that it is possible.

"While this is basic research, our results make possible a new type of controllable nano-optical components that we believe can to be used for many applications."

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.