Metasurfaces are man-made structures that are designed to control light and other forms of radiation using very small elements, typically with sub-micrometer dimensions.
Conventional methods of producing theses advanced material coatings typically require specialist cleanroom facilities capable of etching patterns into materials or deposit separated structures on the surface by a process called lift-off lithography. The resulting surface topography, however, is not always suitable and can strongly limit the device performance when embedded into a stack of many independent layers.
The researchers explored a new technology that locally and selectively modulates optical properties without changing the flatness of the material surface. The research was based on new technique and the discovery that an oxygen plasma can greatly reduce the electron concentration of certain metal oxides, which hold unique properties for integrated circuits, solar cells, chemical sensing and catalysis.
The plasma patterning technique has been demonstrated by fabricating two devices, a planar metasurface-based optical solar reflector for satellite radiant cooling and a multiband metasurface with different operation ranges.
The research involved close collaboration between Southampton University and physicists and Sustainable Electronic Technologies experts in Electronics and Computer Science (ECS). The Southampton team, led by Physics and Astronomy’s Professor Otto Muskens and ECS’s Professor Kees De Groot, have published the research in Advanced Materials.
According to Dr Kai Sun, lead author, “Plasma patterning provides a completely new route to form electrical and optical devices with structures beyond existing manufacturing limitations. Here, we have shown for the first time that a planar metasurface, based on aluminum-doped zinc oxide, can be achieved with an optical metasurface function but physically flat.
“The metasurface devices in this work are only a start of this new technique’s applications and its full potential impact is still to be seen.”
Transparent conductive oxides such as aluminium-doped zinc oxide (Al:ZnO) have a high electron density, making it dielectric in visible range but metallic in infrared (IR) range. This electron density of Al:ZnO is critical for its material electrical and optical properties. To achieve an optical property contrast, which is required for optical metasurface formations, parts of the Al:ZnO film need to be removed, leaving a non-planar structure.
The study found that oxygen plasma can reduce Al:ZnO electron density by up to five orders of magnitude. This led the team to propose the new fabrication technique that forms a metasurface by selectively modulating the electron density using oxygen plasma, made possible by accurate location control through a lithography definition.
The fabricated device achieves its metasurface function for its optical property contrast but maintains a planar topological surface profile. This metasurface is highly desirable for its compatibility with any added functional layers.
Southampton scientists have applied to file the plasma patterning technique as a UK patent. Research was funded by the Dstl MultiMeta project, which has currently entered its second phase of development.
