Metasurfaces are potential replacements for conventional optical components. Their working principle is based on the ability of the specially distributed nanoantennas to accurately modulate light with desired properties i.e. polarization, amplitude and phase. Due to the nanoscale tailoring of light, metasurfaces have been used in a variety of flat elements and devices. It has also inspired the improvement of existing optical functionality, thereby expanding the potential application of metasurfaces. Unfortunately, the nanostructures are highly sensitive to dust and other contaminants, which give rise to a crucial challenge of protecting the metasurfaces, thereby limiting their practical applications.
Current research efforts have focused on developing feasible strategies for cleaning and protecting the metasurfaces from dust and other contaminants without damaging their nanostructures or compromising their optical performance. Besides, the design of next-generation metasurfaces must consider enhancing the device’s effectiveness, physical stability and adaptability to complex application conditions. Like metasurfaces, bioinspired surfaces consisting of nano/microsized structures exhibit improved physical characteristics, the most important one being superwettability. As a result, bioinspired surfaces support versatile applications and tend to be smarter and more robust in various harsh environments.
Previous findings revealed several advantages of combining the optical performance of the device with wettability design. This included realizing ultrafast humidity responsive structural colors and self-cleaning antireflective surfaces capable of rolling off water droplets and contamination. This breakthrough design utilized a water-selective metasurface with hydrophobic and hydrophilic to modulate and direct light between wetting and drying states. This design can improve the optical performance of metasurfaces and enhance their functionalities, such as smart and self-cleaning capabilities. However, realizing the desired superwettability for designing optical metasurfaces remains a big challenge.
It is well-known that through spatial arrangement of nanostructures, structural color metasurfaces can produce vivid images, while dynamic color response can be efficiently realized by tuning the refractive index of surrounding the antennas. Inspired by the intelligent bioinspired surfaces exhibiting self-cleaning properties, a team of researchers from Paderborn University: Jinlong Lu, Dr. Basudeb Sain, Philip Georgi, Maximilian Protte, Professor Tim Bartley and Professor Thomas Zentgraf developed a new versatile dielectric metasurface with superwettability for dynamic color response and self-cleaning capabilities. The design was achieved by embedding a structural color metasurface in a large area wettability supporting structures to enhance fabrication efficiency and simultaneously realize the optical and wettability functionality. Their research work is currently published in the journal, Advanced Optical Materials.
The researchers demonstrated the design of a versatile metasurface with two main wettability states: superhydrophilic and quasi-superhydrophobic states. Whereas the superhydrophilic improved the optical response of the device with water, the octadecylphosphonic acid (ODP) modified quasi-superhydrophobic state equipped the antennas with the self-cleaning and contamination removal abilities with water. Moreover, while the superhydrophilic state was highly efficient in obtaining a rapid structural color change, its stability significantly improved once the surface was treated in oxygen plasma. Furthermore, the resulting metasurface could be easily and repeatedly switched between the two functional/wettability states without degrading its optical performance.
In summary, the authors successfully developed a feasible strategy based on the structural color metasurface and superwettability property to simultaneously realize versatile functionality with dynamic color and self-cleaning capabilities. The incorporation of wettability strategy not only improves the functional properties of the metasurfaces but is also an effective and economical solution. In addition, the design strategy is versatile and could be extended to the fabrication of different metasurfaces for application in more complex environments. In a statement to Advances in Engineering, Professor Thomas Zentgraf , the corresponding authors explained the presented design strategy offers new opportunities for fabricating smart metasurfaces with improved functional properties suitable for a variety of application conditions.
Lu, J., Sain, B., Georgi, P., Protte, M., Bartley, T., & Zentgraf, T. (2021). A Versatile Metasurface Enabling Superwettability for Self‐Cleaning and Dynamic Color Response. Advanced Optical Materials, 10(1), 2101781.