Already in the medieval times, nano­structures of noble metals were embedded in glass to create beautiful colours. Although not known at the time, the colours appear because light at certain frequencies transforms into plasmons, which are collective charge oscil­lations in the metallic nano­particles. The nanoparticles act as antennas for light, which makes them very versatile. For example, plasmonic nano­antennas are now explored in appli­cations ranging from energy conversion and photo­catalysis to biosensing and flat metaoptics.

A major limitation, however, has been that optical nano­antennas made from conven­tional metals like gold or silver have fixed properties that cannot be tuned after fabrication. This limits their use to static functions in devices. With the prospect of realising dynami­cally tuneable nano­antennas, researchers have therefore begun to explore materials with optical properties that can be changed back and forth. The ideal case would be a material that can be repeatedly switched all the way between being optically metallic and dielectric. Researchers at Linköping University in Sweden have demonstrated such dynamically tuneable plasmonic nano­antennas made from a conducting polymer, which can be switched between metallic and dielectric properties in the near infrared wavelength range.

“While polymers are traditionally known to be non-conducting plastics, conducting polymers cannot only transport electricity but their electrical conductivity can also be modulated via the redox state of the polymer. We explore this behaviour for a new type of dynamic organic nanooptics”, says Magnus Jonsson, who leads the group in which the research has been performed. The same team of researchers already showed that certain conducting polymers can be made suffi­ciently conducting to obtain plasmonic properties. Nano­antennas made from the polymer could then be switched off and on again by exposing the samples to gases and liquids, which changed the polymer’s redox state. One of the key contributions of the new study is that the researchers now demonstrate electrical control of their polymeric plasmonic nanoantennas, forming a critical step towards more useful practical applications.

“By fabricating the conducting polymer nanoantennas on a transparent electrode and coating them with an ion conducting gel, we could control their redox state by an external potential”, says Akchheta Karki. “Besides repeated on/off switching we demons­trate the possi­bility for gradual tuning of the nanoantennas, controlled by the external bias potential”, continues Karki. The researchers compare their results with optical simulations and calcu­lations to better understand the underlying mechanisms of the tuning process. The results indicate that both the density and mobility of the charge carriers in the nano­antennas vary during tuning and that the process is reversible.

“Conducting polymers form an interesting new type of materials for dynamic nano­optics, thanks to the possi­bility to vary their properties between metallic and dielectric. In turn, the transition in the field from static to dynamic nanooptics is important for many future appli­cations, including steerable meta­optics and dynamic smart windows.”, says Magnus Jonsson. (Source: U. Linköping)

Reference: A. Karki et al.: Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas, Adv. Mat., online 21 January 2022; DOI: 10.1002/adma.202107172

Link: Laboratory of Organic Electronics (LOE), Linköping University, Linköping, Sweden