Researchers develop patterning technology for new optical metasurfaces

Sustainable Electronic Technologies experts from the University of Southampton have demonstrated a new plasma patterning technique that can create high-quality nanostructured surfaces for infrared thermal control coatings and nonlinear optics.

The research was carried out through a close collaboration between physicists and researchers in Electronics and Computer Science (ECS).

Metasurfaces are man-made structures designed to control light and other forms of radiation using very small elements, typically with sub-micrometer dimensions.

Conventional methods of producing the advanced material coatings in specialist cleanroom facilities such as the Southampton Nanofabrication Centre etch patterns into materials or deposit separated structures on the surface by a process called lift-off lithography. However, the resulting surface topography is often undesirable and can strongly limit the device performance when embedded into a stack of many independent layers.

In their study, the researchers explored a completely new technology that locally and selectively modulates optical properties without changing the flatness of the material surface. The research team based their new technique on 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 novel devices, a planar metasurface-based optical solar reflector for satellite radiant cooling and a multiband metasurface with different operation ranges.

Professor Kees de Groot, Head of the Sustainable Electronic Technologies group, says: "The ability to precisely control the local density of electrons in semiconductors is very fundamental and forms the basis of modern nanoelectronic circuits. The fact that we can do this now for other materials such as metal oxides could be a game-changer with much wider impact on both electronic and photonic technologies.

"The collaboration between physics and ECS over the years has been very fruitful in bringing together complementary skills in materials and optical analysis, and this result highlights the importance of such multidisciplinary science, which is a particular strength in Southampton."

The Southampton team, led by Kees and Professor Otto Muskens, have published the research in Advanced Materials.

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.