Conductive Polymer Could Enable Tunable Nano-Optics

Optical nanoantennas, made from a conducting polymer instead of a traditional metal and developed by scientists at Linköping University, could enable a new type of controllable nano-optical component for use in various applications, such as smart windows and reflective displays. The nontraditional, organic material that was used can support localized surface plasmon resonances in the near-infrared and can function as a dynamic nano-optical antenna, with resonance behavior that is tunable by chemical redox.

Resonant light-matter interaction can be achieved using conventional plasmonics based on metal nanostructures, but the tunability is limited due to a fixed permittivity. The Linköping team wanted to explore materials with switchable states for dynamic control of light-matter interaction at the nanoscale.

Plasmons in plastics. Courtesy of Thor Balkhed.

The scientists used a variant of PEDOT, a widely used polymer in thermoelectrics and bioelectronics, for the optical nanoantennas and showed that light could be converted to plasmons in nanostructures of this organic material. The plasmons in the conducting polymer were created by polarons (not electrons).

Positive charges along the polymer chain created electrical conductivity. Along with associated chain distortions, these positive charges formed polarons that oscillated collectively when light was incident on the nanostructure. It is this collective oscillation that constitutes the plasmon. “Our organic antennas can be transparent to visible light while reacting to light at somewhat longer wavelengths,” professor Magnus Jonsson said.

Billions of nanodisks are deposited onto an area that is 1 sq cm in size. Each one of them responds to the incident light and creates plasmons. Courtesy of Linköping University.

The researchers produced billions of nm-size disks of the organic conducting material on a surface. These disks react to light and act as tiny antennas. The team showed that the diameter and thickness of the disks determine the light frequency to which they react, and thus, that it is possible to control the wavelength by changing the disk’s geometry. The thicker the disk, the higher the frequency. The researchers demonstrated complete, reversible switching of the optical response of the nanoantennas by chemical tuning of their redox state.

The team hopes to increase the range of wavelengths to which its nanoantennas can react by changing the polymer it uses to build them. The researchers are continuing to explore ways to switch the organic nanoantennas on and off. The material manufactured in their laboratory is initially in an oxidized state, and 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,” Jonsson said. “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.”

Shangzhi Chen, doctoral student (left) and Magnus Jonsson, leader of the Organic Photonics and Nano-optics Group at the Laboratory of Organic Electronics at Linköping University. Courtesy of Thor Balkhed.

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

The research was published in Nature Nanotechnology (www. doi.org/10.1038/s41565-019-0583-y).