Linköping University (Sweden) researchers built a dynamically tunable plasmonic nanoantenna using a conductive polymer that can switch between metallic and dielectric properties at frequencies in the near-infrared optical range (see figure). This advance has implications for future dynamic flat metaoptics and tunable smart materials.
Metallic nanostructures interact strongly with light, which transforms the light into collective charge oscillations, or plasmons. “These nanostructures act as tiny antennas for light,” explains Magnus Jonsson, a professor of applied physics and principal investigator of the Organic Photonics and Nanooptics group within the Laboratory of Organic Electronics. “Our organic nanoantennas behave in the same way, but for slightly longer wavelengths at which the conducting polymer is optically metallic.”
Jonsson has worked with conventional inorganic plasmonics for many years and understands their limitations when it comes to active tuning. After first learning about conducting polymers, he wondered whether organic materials could also act as metals within plasmonic nanoantennas. If it worked, it would open up a new type of organic optical nanoantennas that could be turned on or off. His group went on to demonstrate it in 2020.
“By fabricating conducting polymer nanoantennas on a transparent electrode and coating them with ion-conducting gel, we can control their redox state by an external potential,” says Akchheta Karki, one of the researchers working on the project.
Gradual tuning
Beyond repeated on/off switching, the group demonstrated the possibility of gradual tuning of the nanoantennas, controlled by the external bias potential.
To gain a better understanding of the underlying mechanisms of the tuning process, they’re comparing their results with optical simulation and calculations. Results show both density and mobility of charge carriers within the nanoantennas vary during tuning, and the process is reversible.
Many strategies for dynamic nanooptics based on conventional gold-based plasmonics have already been explored, Jonsson points out, such as stretchable systems and tuning by modifying the properties of other materials close to the nanoantennas.
“While interesting, such systems cannot be completely turned off because metallic structures are always present within the device during tuning,” he adds. “An interesting aspect of our concept is we don’t use any conventional metals at all; the nanoantennas are instead made from a tuned material.”
Since the material the researchers are using can be tuned all the way from metal to dielectric and back again, it provides a large tuning range.
“In previous work, we showed polymeric nanoantennas could be tuned by exposure to gases and liquids,” Jonsson says. “With our present work, we extended the concept to tuning by electrical potentials, which will be much more practical for real applications. One challenge was ensuring all other parts of the devices are sufficiently transparent to light within the near-infrared region at which these nanoantennas have their resonances.”
Future uses
Conducting polymers “form an interesting new type of materials for dynamic nano-optics, thanks to the possibility of varying their properties between metallic and dielectric,” adds Jonsson. “The transition in the field from static to dynamic nano-optics is important for many future applications, including steerable metaoptics and dynamic smart windows.”
While the researchers are excited about introducing a new materials platform for dynamic nano-optics, they note that there are still many important studies to perform and applications to develop. “We demonstrated nanoantennas with resonances down to a wavelength of 1270 nm in our current study, and one remaining challenge is to further tune the resonances into the visible region,” says Jonsson. “Exploring other organic conducting materials for dynamic nanoantennas and improving the efficiency of the nanoantennas are important topics for future work.”
The researchers’ latest article about their work, by A. Karki et al., appears in the journal Advanced Materials.
