By carefully placing nanostructures on a flat surface, researchers at Linköping University, Sweden, have significantly improved the performance of so-called optical metasurfaces in conductive plastics. This is a major step for controllable flat optics, with future applications such as video holograms, invisibility materials, and sensors, as well as in biomedical imaging. The study has been published in the journal Nature Communications.
To control light, curved lenses are used today, that are often made of glass that is either concave or convex, which refracts the light in different ways. These types of lenses can be found in everything from high-tech equipment such as space telescopes and radar systems to everyday items including camera lenses and spectacles. But the glass lenses take up space and it is difficult to make them smaller without compromising their function.
With flat lenses, however, it may be possible to make very small optics and also find new areas of application. They are known as metalenses and are examples of optical metasurfaces that form a rapidly growing field of research with great potential, though at present the technology has its limitations.
"Metasurfaces work in a way that nanostructures are placed in patterns on a flat surface and become receivers for light. Each receiver, or antenna, captures the light in a certain way and together these nanostructures can allow the light to be controlled as you desire," says Magnus Jonsson, professor of applied physics at Linköping University.
Today there are optical metasurfaces made of, for example, gold or titanium dioxide. But a major challenge has been that the function of the metasurfaces cannot be adjusted after manufacture. Both researchers and industry have requested features such as being able to turn metasurfaces on and off or dynamically change the focal point of a metalens.
But, in 2019, Magnus Jonsson's research group at the Laboratory of Organic Electronics showed that conductive plastics (conducting polymers) can crack that nut. They showed that the plastic could function optically as a metal and thus be used as a material for antennas that build a metasurface. Thanks to the ability of the polymers to oxidize and reduce, the nanoantennas were able to be switched on and off. However, the performance of metasurfaces built from conductive polymers has been limited and not comparable to metasurfaces made from traditional materials.
Now, the same research team has managed to improve performance up to tenfold. By precisely controlling the distance between the antennas, these can help each other thanks to a kind of resonance that amplifies the light interaction, called collective lattice resonance.
"We show that metasurfaces made of conducting polymers seem to be able to provide sufficiently high performance to be relevant for practical applications," says Dongqing Lin who is principal author of the study and postdoc in the research group.
So far, the researchers have been able to manufacture controllable antennas from conducting polymers for infrared light, but not for visible light. The next step is to develop the material to be functional also in the visible light spectrum.
