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Electrically Switchable Nanoantennas Developed for Holographic Video Technology

Electrically switchable nano antennas

Researchers are developing electrically switchable nano antennas as the basis for holographic video technology. Credit: University of Stuttgart / PI4, Julian Karst

Real-life feeling with online video conferences

Video conferencing played a key role during the Covid-19 pandemic and will dominate many meetings in the future. To achieve the true feeling of face-to-face dialogue, three-dimensional video is required and yet the holographic technology is absent. Researchers at the University of Stuttgart in Germany have now presented a completely new approach for realizing such dynamic holographic displays, based on electrically switchable plasmonic nano-antennas made of conductive metallic polymers. This key element provides the lack of technology to enable video rate holographic displays that would enable virtual conferencing with a “real” feel. The paper detailing this work has been published in the leading journal science on October 28, 2021.

Virtual meeting hologram

Virtual meeting in the future: The conference participant on the right is wearing VR / AR glasses that show a hologram of the lady on the left. Credit: University of Stuttgart / PI4, Julian Karst

Holograms that produce stunning three-dimensional static images are well known. Dynamic holograms that can be switched to video rates with data from a high-speed Internet connection have not yet been possible. So far, the limiting factor has been the display resolution. Holographic images require a resolution of 50,000 dpi (pixels per inch), which is 100x more than that of the best smartphone displays. For such a resolution you have to reduce the pixel size to half a micrometer (one thousandth of a millimeter). However, current liquid crystal technology does not allow such small pixels because it is limited to a pixel size of a few micrometers.

Researchers at the University of Stuttgart have succeeded in breaking this fundamental barrier. In an interdisciplinary collaboration between physics and chemistry, they developed the idea of using electrically switchable plasmonic nano-antennas with dimensions of just a few hundred nanometers made of conductive polymers.

Metallic polymer meta-surface nano-antenna

Scanning electron microscope (SEM) image of the metallic polymer meta-surface that can be used to switch electrical nano-antennas. Credit: University of Stuttgart / PI4, Julian Karst

Conductive functional polymers as a suitable switchable material

For several years, researchers had created metasurfaces that create static three-dimensional holograms. However, their components or nano-antennas were made of metals such as gold or aluminum, which could not be switched like conventional liquid crystal materials. After several years of searching for the right material, doctoral student Julian Karst and nanophotonics expert Dr. Mario Hentschel from the group of Prof. Harald Gießen together with the polymer chemist Prof. Sabine Ludwigs and her team electrically conductive polymers as possible candidates for switchable plasmonics. Sabine Ludwigs contributed her expertise in the electrochemical switching of such functional polymers, which was the focus of the Nobel Prize in Chemistry in 2000.

So far, such materials have mainly been used to transport electricity in flexible displays and solar cells. In cooperation with the clean room manager Monika Ubl, Karst and Hentschel developed a process for the nanostructuring of the metallic polymers through a combination of electron beam lithography and etching, which creates the plasmonic nano antennas. The team showed that the visual appearance of the nano antennas can be switched between a shiny metal and a transparent material by applying a voltage between minus and plus one volt. This switching effect even works at video rates of 30 Hertz. Although only a few tens of nanometers thick and less than 400 nanometers in size, the nano-antennas do the same job as the much larger and thicker liquid crystals used in current technology. These new devices achieve the required pixel densities of approx. 50,000 dpi.

Plasmonic polymer nano-antenna

Left: Image showing the plasmonic polymer nano-antenna switched to the dielectric (glass-like) state. The light beam from below only penetrates without being deflected upwards. Right: Image of the plasmonic polymer nano-antenna, switched to the metallic state. The light beam from below is deflected to the side as it passes through the sample. The light also changes its handedness (see the different directions of rotation of the spiral light) when it travels upwards. Credit: University of Stuttgart / PI4, Julian Karst

Karst created a simple hologram meta-surface from the nano antennas, which could deflect an infrared laser beam by 10 degrees to one side by applying a voltage. He is currently working to make this multi-angle deflection available for applications in LIDAR devices in autonomous vehicles, which are of great interest to the automotive industry. In addition, Karst has developed a hologram that behaves like an optical lens that can be switched on and off by simply applying ± 1 volt. This technology is crucial for future smartphone cameras or optical sensors that could be zoomed from wide-angle to telephoto by switching the applied voltage. Currently, up to four lenses are required for this functionality.

In the future, Prof. Harald Giessen and his team want to address each individual pixel individually in order to dynamically change the holograms at video rates. In addition, the optical properties of the polymer nano-antennas have to be shifted into the visible wavelength range, which requires collaboration with chemists and materials scientists. Together with engineers, it was possible to integrate integrated and dynamically switchable optical displays and the first movable holograms in AR / VR glasses and finally on smartphone screens and even televisions.

If you take Moore’s law for display technology, this advance by a factor of 100 could take place commercially by the year 2035.

Reference: “Electrically switchable metallic polymer nano antennas” by Julian Karst, Moritz Floess, Monika Ubl, Carsten Dingler, Claudia Malacrida, Tobias Steinle, Sabine Ludwigs, Mario Hentschel and Harald Giessen, October 28, 2021, Science.
DOI: 10.1126 / science.abj3433