Nov 4, 2009
Moth's eye inspires multifunctional anti-reflection surface
The reduction of reflection plays an important role in applications ranging from display technologies through to individual optical components. Reporting their results in the journal Nanotechnology, researchers from the University of Joensuu, Finland, have introduced an anti-reflective surface that provides significant reflection suppression below 0.45% for the visible wavelength band from wide angles. The design does not alter visual colour perception and it also repels water. Furthermore, the surface can be easily applied using high-precision, low-cost fabrication techniques.
The anti-reflection (AR) phenomenon has its origins in nature. For example, a sub-micron AR structure in the eyes of a night moth makes it easier for the insect to hide from predators.
In nanotechnology, several dexterous and simple methods exist for producing AR surfaces and the most common way to suppress reflection is to apply thin films to a substrate. Alternative approaches include modifying the surface with a suitable pattern or introducing a porous material to the surface.
In this study, the group modified the substrate surface to form a pyramid-shaped "moth eye"-structure. The multifunctional continuous-shaped nanoscale pattern offers a number of benefits – reflection reduction, wide-angle performance, controlled surface colour perception and hydrophobicity.
To define the optimal shape of the pyramid structure, the researchers used numerical calculations based on rigorous diffraction theory.
Electron beam lithography was applied to make the mask, followed by wet etching of silicon to manufacture the inverted pyramid structures. Finally, the structure was transferred from silicon to transparent silicon dioxide by nanoimprint lithography and proportional reactive ion etching.
The AR properties of the fabricated surfaces were verified optically by ellipsometric measurements. By introducing a silane treatment for producing an anti-adhesion layer, the surface was shown to have hydrophobic attributes with contact angles of more than 140°.
Although electron beam lithography has been used here, the process could be upgraded fully to suit fast large-area fabrication by choosing, for example, interferometric exposure for the mask making phase and roll-to-roll processing for the imprinting stage of either the shallow pyramids or the final structure.
About the author
The work was performed in the Department of Physics and Mathematics at the University of Joensuu, Finland. The work was funded by the Finnish Funding Agency for Technology and Innovation (FinNano project SupiHeli) and the Ministry of Education (Research and Development Project on Nanophotonics). The corresponding author Birgit Päivänranta was a PhD student in the Department of Physics and Mathematics, based in the Micro- and NanoPhotonics (MNP) group. Starting from November 2009 she will be working as a postdoctoral fellow at the Paul Scherrer Institut in Switzerland. Toni Saastamoinen is a postdoctoral fellow working in the MNP group and Prof. Markku Kuittinen is the head of the MNP group.