Video conferencing has played a key role during the COVID-19 pandemic and will dominate many meetings in the future. In order to achieve the true feeling of face-to-face dialogue, three-dimensional video is required, but holographic technology is still missing so far. Researchers at the University of Stuttgart in Germany have now introduced a completely new way to achieve this dynamic holographic display, based on electrically switchable plasma nano-antennas made of conductive metal polymers.
This key factor provides the missing technology for holographic display of video rates, which will give virtual meetings a "real life" feel. A paper detailing the work was published in the authoritative journal Science on October 28, 2021.
Virtual meetings of the future. The conference member on the right wears VR/AR goggles that show a hologram of the lady on the left.
Holographic technology is well known for its ability to create impressive three-dimensional still images. Until now, it was not possible to use a high-speed internet connection to switch to a dynamic hologram at the video rate. Prior to this, the limiting factor was the resolution of the display. Holographic images require a resolution of 50,000 dpi (pixels per inch), which is 100 times higher than the best smartphone displays. In order to achieve such a resolution, we must reduce the size of the pixel to half a micron (one thousandth of a millimeter). However, current liquid crystal technology does not allow for such small pixels, which are limited to a pixel size of a few microns.
Researchers at the University of Stuttgart have successfully broken down this fundamental obstacle. In an interdisciplinary collaboration in physics and chemistry, they came up with the idea of using electrically switchable plasma nano-antennas that are only a few hundred nanometers in size and made of conductive polymers.
Scanning electron microscopy (SEM) images of metal polymer meta surfaces that can be used for motorized nano-antenna switching.
Over the years, researchers have created metasurfaces that produce static 3D holograms. However, their components or nano-antennas are made up of metals such as gold or aluminum and cannot be switched like ordinary liquid crystal materials. After years of searching for the right material, Dr. Mario Hentschel, a nanophotonic expert in the group of PhD students Julian Karst and Professor Harald Giessen, worked with polymer chemist Professor Sabine Ludwigs and his team to identify conductive polymers as potential candidates for switchable plasmas. Sabine Ludwigs contributed her expertise in electrochemical switches for such functional polymers, which was the focus of the 2000 Nobel Prize in Chemistry.
Until now, this material has been mainly used for current transmission in flexible displays and solar cells. In collaboration with Cleanroom Head Monika Ubl, Karst and Hentschel developed a process that uses a combination of electron beam lithography and etching to nanostructure metal polymers to create plasma nano-antennas. The team showed that by applying a voltage between negative and positive one volt, the optical appearance of the nano antenna can be switched between shiny metal and transparent materials. This transition even works at video rates of 30 Hz. Although only a few tens of nanometers thick and less than 400 nanometers in size, nano-antennas do the same job as larger and thicker liquid crystals currently used in state-of-the-art technology. These new devices achieve the desired pixel density, which is about 50,000 dpi.
Left: Image showing a proton polymer nano antenna, switching to the dielectric (vitreous) state. The beam from the bottom just passed through the top and was not deflected. Right: An image showing a proton polymer nano antenna, switching to a metallic state. The beam from the bottom is deflected to the side as it passes through the sample.
Custer created a simple hologram element surface with a nano-antenna that could deflect the infrared laser beam 10 degrees to one side by applying a voltage. Currently, he is working to make this deflection available for many angles for applications to lidar equipment in autonomous vehicles, which has a keen interest in the automotive industry. In addition, Custer created a kind of hologram that behaves like an optical lens that can be turned on and off.
This technology is essential for future smartphone cameras or optical sensors, where the voltage applied by switching can be amplified from a wide angle to telephoto. Currently, up to four lenses are required to achieve this functionality.
In the future, Professor Harald Gissen and his team aim to solve each pixel individually, dynamically changing the content of the hologram at the video rate. In addition, the optical properties of polymer nano-antennas must be transferred to the visible wavelength range, which requires collaboration with chemists and materials scientists. Together with engineers, the integrated, dynamically switchable optical display and the first moving hologram could be integrated into ar/VR goggles and eventually integrated into smartphone screens and even TVs.
According to Moore's Law for display technology, the technology could be commercialized around 2035.